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Old 08-07-2018, 04:33 PM   #1
Ol' Ron
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Default Piston to head clearancing

I get this question allot, thought I just post this information for those that might not understand why this is a good practice.

Understanding why this is important will give you the reason to do it. Modern engines have a very hi compression ratio, and still run on regular gas. This is due to the very hi turbulence in the chamber just prior to ignition. This is done by an area called the Quench area. The Flathead doesn't have such an area, so we have to make one. Thus the .045/.050" piston to head clearance. I measure this with little alum balls I make from aluminum foil. A bout 1/2 3/8 in in diameter. I stick them in the area to be measured. Now if your not sure about this, have someone else help you. You must mill the head to get this number, which is usually just an area around the perimeter of the piston. I relieve the valve clearance at to top of the cylinder head with a small die grinder, so as not to remove to much material which would lower the CR. Doing this will not raise the CR very much, but cause a hi degree of turbulence in the chamber condensing the AF for better and complete combustion. Moer power, economy and better throttle responses. And for very little cost.
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Old 08-07-2018, 05:40 PM   #2
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Default Re: Piston to head clearancing

I agree with Ron about this. I spent a lot of time getting .050 clearance on my last two engines. They both run very well and are "crisper" than those I built previously. It is well worth the effort.
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Old 08-07-2018, 06:51 PM   #3
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Default Re: Piston to head clearancing

I've heard about this and when the time came for a rebuild, I "clay'd" the clearance to see what would be required. (see below) The numbers are with a used head gasket torqued down.
I asked my machinist (who has done hundreds of mild-to-wild flatheads) what he thought about milling ~.070" off a head; his jaw literally dropped open! From what I've read here and elsewhere, I think my numbers are pretty common for uncut heads.
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Old 08-07-2018, 09:52 PM   #4
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Default Re: Piston to head clearancing

I would agree.
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Old 08-07-2018, 09:57 PM   #5
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Default Re: Piston to head clearancing

What's stock piston to head clearance?


When in doubt. Mill .050.
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Old 08-07-2018, 10:36 PM   #6
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Default Re: Piston to head clearancing

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I think that measuring is the only way to do this. I only concern myself with the area around the perimeter of the piston. However a check should be made in the center for safety. Just milling off a bunch could be expensive
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Old 08-07-2018, 10:38 PM   #7
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Default Re: Piston to head clearancing

I agree Ron. Was wondering about stock clearance.


Lets say you have stock heads you "think" have never been milled. How do you check this? I don't know.
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Old 08-07-2018, 10:48 PM   #8
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Default Re: Piston to head clearancing

Has anyone used a 4 1/8" stroke crank with stock 4" stroke pistons? This would put the piston .0625" closer to the head, and in the case of post #3 would be close to ideal with a little hand work.
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Old 08-07-2018, 11:38 PM   #9
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Default Re: Piston to head clearancing

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Has anyone used a 4 1/8" stroke crank with stock 4" stroke pistons? This would put the piston .0625" closer to the head, and in the case of post #3 would be close to ideal with a little hand work.
That was standard practice in the olden days.
The pistons then came to deck height.
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Old 08-08-2018, 05:48 AM   #10
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Default Re: Piston to head clearancing

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I think that measuring is the only way to do this. I only concern myself with the area around the perimeter of the piston. However a check should be made in the center for safety. Just milling off a bunch could be expensive
I'd second checking in the centre too. Some pistons (French for one) are more "pointy" than others. I had a problem with stock French pistons hitting an unmilled Offenhasuer head. They only touched in the middle.

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Old 08-08-2018, 06:45 AM   #11
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Default Re: Piston to head clearancing

When measuring for the 45 - 50 thou clearance is it done with a gasket in place or without a gasket?
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Old 08-08-2018, 07:02 AM   #12
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Default Re: Piston to head clearancing

With a used gasket, torqued down.
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Old 08-08-2018, 07:29 AM   #13
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Default Re: Piston to head clearancing

what would be the difference between flat and domed pistons as in the way clearance is measured?
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Old 08-08-2018, 07:51 AM   #14
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Default Re: Piston to head clearancing

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Originally Posted by Ross F-1 View Post
I've heard about this and when the time came for a rebuild, I "clay'd" the clearance to see what would be required. (see below) The numbers are with a used head gasket torqued down.
I asked my machinist (who has done hundreds of mild-to-wild flatheads what he thought about milling ~.070" off a head; his jaw literally dropped open! From what I've read here and elsewhere, I think my numbers are pretty common for uncut heads.
Yup, your measurements are what I normally find on uncut heads and OE Ford pistons.
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Old 08-08-2018, 08:27 AM   #15
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Default Re: Piston to head clearancing

I milled .045 and .055 off a used set of edelbrock heads to get the clearance I wanted. I bought it as a stock unfired rebuild. looked like the deck had been skimmed but there was .010 difference per side.



seems to run good. definitely sounds different than my last motor which had about .070
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Old 08-08-2018, 02:19 PM   #16
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Default Re: Piston to head clearancing

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Originally Posted by 40 Deluxe View Post
Has anyone used a 4 1/8" stroke crank with stock 4" stroke pistons? This would put the piston .0625" closer to the head, and in the case of post #3 would be close to ideal with a little hand work.

I suppose it's too simple an idea to just increase the compression height a little (say, .030" - .040") on an otherwise stock piston blank? That would help a lot.
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Old 08-08-2018, 03:02 PM   #17
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Default Re: Piston to head clearancing

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what would be the difference between flat and domed pistons as in the way clearance is measured?

No difference. In either case you put strips of clay or little foil balls on the piston tops.
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Old 08-08-2018, 03:07 PM   #18
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Default Re: Piston to head clearancing

Thanks for posting this Ron.

I was getting ready to ask questions about this regarding new after-market heads (Offy, Edelbrock, etc.) to verify that these also need to be measured and/or milled.

As I understand what you are saying and what others have posted, I think the answer is yes, regardless of the CR these manufacturers may advertise.
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Old 08-08-2018, 03:17 PM   #19
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Default Re: Piston to head clearancing

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Thanks for posting this Ron.

I was getting ready to ask questions about this regarding new after-market heads (Offy, Edelbrock, etc.) to verify that these also need to be measured and/or milled.

As I understand what you are saying and what others have posted, I think the answer is yes, regardless of the CR these manufacturers may advertise.

the squish has nothing to do with actual compression ratio. the idea is to keep the combustion area as small as possible. if the piston to head clearance is large this becomes another portion of the combustion chamber even when the piston is all the way up. think of a smog head when they used dished pistons to get lower compression. the motors pinged and never ran right. all because of a large chamber volume. in a flathead you actually want the combustion chamber to be the transfer area only.
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Old 08-08-2018, 05:57 PM   #20
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Default Re: Piston to head clearancing

I to followed Ol Ron’s advise last winter when building my 59ab . The machine that
Milled the heads is still talking about my request .
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Old 08-08-2018, 08:11 PM   #21
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Default Re: Piston to head clearancing

Has anyone here ever set the clearance close enough that they hit the heads? .045 seems real conservative on a low rpm engine with a close piston to wall clearance that keeps the pistons from rocking. I've seen .030 and less on OHV engines, and it seems that less is better, right?
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Old 08-08-2018, 08:20 PM   #22
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Default Re: Piston to head clearancing

What can be used if you don't have any used head gaskets?
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Old 08-08-2018, 10:34 PM   #23
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Default Re: Piston to head clearancing

Yes, you can use a tighter piston to head clearance, if you use a lighter piston. The heavy 4 ring pistons have a tendency to stretch the rods a little. Especially is you running a good cam at run over 5000 RPM;s Just remember ONE thing: There;s always a reason for making a modification. Some work, some don't.
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Old 08-08-2018, 10:35 PM   #24
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Default Re: Piston to head clearancing

lower compression when using a blower also.
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Old 08-09-2018, 06:41 AM   #25
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Default Re: Piston to head clearancing

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What can be used if you don't have any used head gaskets?
A new one? Can't see why not.

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Old 08-09-2018, 07:42 AM   #26
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Default Re: Piston to head clearancing

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3739ford>>>What can be used if you don't have any used head gaskets? >>>
Mart>>>A new one? Can't see why not.>>>

Nope. Can't do that. Cuz the new one will then be a used one. 8^) Jack E/nJ
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Old 08-09-2018, 10:13 AM   #27
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Default Re: Piston to head clearancing

Nah, if it's a coper gasket, installing it and toqueing it down for measurement purpose, won't make it a used gasket. Just refit to the same side, it'll be as good as new.
You can also grease the felpro type ones and use them as above.
With the coper gaskets you can use um a good few times without a worry.
I believe ol Ron does this as well.
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Old 08-09-2018, 10:14 AM   #28
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Default Re: Piston to head clearancing

Double post.

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Old 08-09-2018, 11:13 AM   #29
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Default Re: Piston to head clearancing

Using head gaskets over, has been a practice since the stockcar days. I have a stack of old copper gaskets and I can't remember buying a new one. Same with bearings. I use Hi tac or copper coat. In the future I'll use Latex paint on the aluminum side. For the new builders, better check the cam bearing clearance, Some of these are way out. Now I measure them before i remove them. Gota admit, they're well lubricated.
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Old 08-11-2018, 01:33 PM   #30
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Default Re: Piston to head clearancing

Does anyone have a picture of a head before and then a picture after the head has been milled .... thanks
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Old 08-11-2018, 04:50 PM   #31
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Default Re: Piston to head clearancing

Won't be much to see.
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Old 08-12-2018, 05:06 AM   #32
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Default Re: Piston to head clearancing

Well, my 1 millimeter milled 8BA head here.

Got 8,5 bar = 123 psi compression with these (with FelPro gaskets)
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Old 08-12-2018, 05:23 AM   #33
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Default Re: Piston to head clearancing

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Well, my 1 millimeter milled 8BA head here.

Got 8,5 bar = 123 psi compression with these (with FelPro gaskets)
Lemme see, 1 MM is about .040", right? Have you checked the resulting piston clearance? It would be interesting to know. As I said in an earlier post, .050 clearance with Edmunds heads on my 8BA gives 160 lbs. (Fel-Pro gaskets as well).
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Old 08-12-2018, 02:59 PM   #34
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Default Re: Piston to head clearancing

The other thing you need to think about if you're running a tighter squish than about .035 to .040 is piston to cylinder wall clearance and piston rock. When running larger clearances (with forged pistons for example), you will have more potential piston rock (especially when cold). This means that the static dimension of .040 might get to .020 on the 'high side" if the piston rocks (just for discussion sake). There is always a certain amount of rock - though it will vary by piston material, bore clearances, application, ring packs, etc..

To see for yourself, when building a new engine push on the sides of the pistons and put a dial indicator on the edge - you'll get an idea as to how much rock is possible with just hand force.
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Old 08-12-2018, 11:19 PM   #35
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Default Re: Piston to head clearancing

Dale, excellent point!!
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Old 08-13-2018, 02:38 AM   #36
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Default Re: Piston to head clearancing

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Lemme see, 1 MM is about .040", right? Have you checked the resulting piston clearance? It would be interesting to know. As I said in an earlier post, .050 clearance with Edmunds heads on my 8BA gives 160 lbs. (Fel-Pro gaskets as well).
Yes 1mm is around 0.040.

I have not done any other measuring, not anything else than enjoyed the better performance of my flathead
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Old 08-16-2018, 05:07 PM   #37
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Default Re: Piston to head clearancing

Couldn't you measure the squish through the spark plug hole, with the foil ball on the enf of a piece of tie wire. This could be done without pulling the heads, what you say ol' Ron. Al
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Old 08-16-2018, 05:17 PM   #38
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Default Re: Piston to head clearancing

You could, but it would be difficult to know where you were on the piston dome. The clearance will vary around by location. When you do this check, you do it in several positions for each piston. You can attemp to modify the head for more uniform measurements or use the tightest measurement for your reference.
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Old 08-16-2018, 05:40 PM   #39
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Default Re: Piston to head clearancing

Yes, this can be done with rosin core solder. However, it takes practice getting the solder in the right locations. But it does give a very accurate measurement.

I've never had to take more than 25 or 30 tho off, I like to get the clearance down to around 30 tho around the edge of the Distion, then shape the edge down with a Dremel, to 45.

It's importan to realize we're talking street engines here and forged pistons with large wall clearances are a different story. I build my engines with Egge 3 ring and fir the pistons to .002 wall clearance. Seems to work quite well
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Old 08-16-2018, 05:46 PM   #40
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Default Re: Piston to head clearancing

Yes, this can be done with rosin core solder. However, it takes practice getting the solder in the right locations. But it does give a very accurate measurement.

I've never had to take more than 25 or 30 tho off, I like to get the clearance down to around 30 tho around the edge of the Distion, then shape the edge down with a Dremel, to 45.

It's importan to realize we're talking street engines here and forged pistons with large wall clearances are a different story. I build my engines with Egge 3 ring and fir the pistons to .002 wall clearance. Seems to work quite well
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Old 08-16-2018, 07:03 PM   #41
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Default Re: Piston to head clearancing

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You could, but it would be difficult to know where you were on the piston dome. The clearance will vary around by location. When you do this check, you do it in several positions for each piston. You can attemp to modify the head for more uniform measurements or use the tightest measurement for your reference.
correct.. You need to check around the edge of the piston, piston center/ dome area has plenty of clearance.
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Old 08-17-2018, 10:56 AM   #42
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Default Re: Piston to head clearancing

Kenny Kloth told me many years ago, that at .035" his pistons would barely touch when 7K rpm was reached by accident.
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Old 08-17-2018, 12:04 PM   #43
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Default Re: Piston to head clearancing

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Yes, you can use a tighter piston to head clearance, if you use a lighter piston. The heavy 4 ring pistons have a tendency to stretch the rods a little. Especially is you running a good cam at run over 5000 RPM;s Just remember ONE thing: There;s always a reason for making a modification. Some work, some don't.
I sometimes wonder about "rod stretch". Is a rod elastic enough to stretch at high RPM and return to original length? Or could the crankshaft be flexing enough to allow piston contact? Has any research been done on this?
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Old 08-17-2018, 12:07 PM   #44
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Default Re: Piston to head clearancing

I have not seen any test data, but rod stretch is a common concern in high RPM engines.
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Old 08-17-2018, 12:45 PM   #45
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Default Re: Piston to head clearancing

Any metal can be stretched within its limit and go back to what it was from start.
Then it can be plastic deformated getting stretched permanently.
And finally loading it beyond its tensile strenght it will snap.
Terms might be off...pardon my swedish...someone jump in and correct whats needed.
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Old 08-17-2018, 02:46 PM   #46
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You are talking about a stress-strain curve for steel. Here is an example.
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Old 08-17-2018, 03:56 PM   #47
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Default Re: Piston to head clearancing

I've used rosin core solder on industrial engines before to find what the firing order was, some of them can have the flywheel put on in different positions and have timing marks all over the place. I worked for an industrial ignition, hydraulic, and mechanical governor co. and have seen just about everything that can be done wrong. I wish there was a distributor with the point contacts sealed in a glass tube ( one for each cyl.) and closed by a magnet on the rotor like the Altronic ignition system uses. It had a small alternator built in the housing and I never had a problem with them and I converted many a industrial engines over to it, up to 10 cyl. That should get some of you younger guy's thinking about how to do it, I'd by one for my Flat Head 41. Al
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Old 08-17-2018, 05:18 PM   #48
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Kenny Kloth told me many years ago, that at .035" his pistons would barely touch when 7K rpm was reached by accident.

Thanks, that's the sort of info I'm looking for. Do you have any idea what stroke, what rods, what pistons, etc? You know I have to ask, right?
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Old 08-17-2018, 10:05 PM   #49
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Default Re: Piston to head clearancing

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Kenny Kloth told me many years ago, that at .035" his pistons would barely touch when 7K rpm was reached by accident.
That is quite true with stock rods and crank.
We can run a little closer with modern rods, pistons, billet crank and girdle.
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Old 08-17-2018, 10:41 PM   #50
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The rules for sportsman engines back in the late 50's was 1/8 overbore max, stock 3 3/4 crank. ahy cam stock valves, heads and intake..
In an effort to get max acceleration off the corners. WE ran the 38/42 crank, 21A rods and Silverlight pistons. The rich guys tried Thomas mag pistons. I had a set for yhe 4" crank. but they never became league.. Light rear drums and anything else that spun around. This is when we discovered rod stretch.
We also ran alum clutch and flywheels. Cheating was the name of the game, but getting caught could loose your number. The best mod was a hollowed out 33/34 intake and 2GC carb. Now on a short track, non of this was very importan,FI, You didn't have the ***** to use it,
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Old 08-19-2018, 06:00 PM   #51
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Originally Posted by Ol' Ron View Post
The rules for sportsman engines back in the late 50's was 1/8 overbore max, stock 3 3/4 crank. ahy cam stock valves, heads and intake..
In an effort to get max acceleration off the corners. WE ran the 38/42 crank, 21A rods and Silverlight pistons. The rich guys tried Thomas mag pistons. I had a set for yhe 4" crank. but they never became league.. Light rear drums and anything else that spun around. This is when we discovered rod stretch.
We also ran alum clutch and flywheels. Cheating was the name of the game, but getting caught could loose your number. The best mod was a hollowed out 33/34 intake and 2GC carb. Now on a short track, non of this was very importan,FI, You didn't have the ***** to use it,
Ron,
Why did you use a 33-34 intake and anything special about the carb? Would these modifications be useful today?
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Old 08-19-2018, 09:09 PM   #52
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Default Re: Piston to head clearancing

The 33/34 "Camel back" Intake is a large casting, and can be hollowed out, The Bore can be increased to the size of the 2GC carb the carb can have a third bolt added where the PB port is filled with JB weld Noe it's a legule carb because it in the original place. The inside of the intake is formed with epoxy and fiber glass Then filled with foam to shape the runners. After the Epoxy hardens just pore some gas into the intake and lall the foam will melt away. I still have one of the last intakes I made. I;ll take some pics and try and post them. This was one mod that really worked quite well. It's nice when something work better than planned.
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Old 09-17-2021, 07:39 AM   #53
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Default Re: Piston to head clearancing

I am interested in this process , any more detail or pictures?
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Old 09-17-2021, 09:21 AM   #54
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Default Re: Piston to head clearancing

They make some rosin core solder that's about 1/8" in dia. Just feed this thrugh the spark plug hole and turn the engine over and you'll get some accurate piston to head clearance.
Over the years I've done allot of experimenting with the flathead combustion chambers and for street use found the compression and turbulence. In the early 90"s I bout a milling machine and used flathead aluminum heads were a dime a dozen. I made sa fixture that allowed me to rotate the head to any angle I wanted and began machining heads. With a flow bench I could check some of the results. I cabe to the delusion that clearance over the valves should also be .050" at a 4 degree angle. The transfer angle at 8Degrees and the exit at 45 degrees. Angle milling a set of EAB heads .070" and following the above numbers will give yiu a head with a chamber around 60cc. I also made a fixture to relocate the spark plug to clear the exhaust valve, but this isn't working right YET@!! At present I'm working on creating a Vortex/LS intake port. The bowl adapter is finished and a new powered file has bee made to shape the port. Getting old prevents me from getting much done. Right now I've finished the new disk braking system for the roadster, hope it works. Running out of time???
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Old 09-17-2021, 10:29 AM   #55
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Default Re: Piston to head clearancing

Quote:
Originally Posted by Ol' Ron View Post
They make some rosin core solder that's about 1/8" in dia. Just feed this thrugh the spark plug hole and turn the engine over and you'll get some accurate piston to head clearance.
Over the years I've done allot of experimenting with the flathead combustion chambers and for street use found the compression and turbulence. In the early 90"s I bout a milling machine and used flathead aluminum heads were a dime a dozen. I made sa fixture that allowed me to rotate the head to any angle I wanted and began machining heads. With a flow bench I could check some of the results. I cabe to the delusion that clearance over the valves should also be .050" at a 4 degree angle. The transfer angle at 8Degrees and the exit at 45 degrees. Angle milling a set of EAB heads .070" and following the above numbers will give yiu a head with a chamber around 60cc. I also made a fixture to relocate the spark plug to clear the exhaust valve, but this isn't working right YET@!! At present I'm working on creating a Vortex/LS intake port. The bowl adapter is finished and a new powered file has bee made to shape the port. Getting old prevents me from getting much done. Right now I've finished the new disk braking system for the roadster, hope it works. Running out of time???
So if i am understanding you correctly, you are milling the head at a angle taking more off the cylinder side? If so what angle are you doing it at and did you ever do it to cast iron heads?
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Old 09-17-2021, 10:26 PM   #56
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Default Re: Piston to head clearancing

I do this just to see if I cam make it better, and improve preformance and econmy.. Most aftermarket heads add material to the transfer area ti increas compression. Then we relieve the block to improve flow. YET! The transfer area controls flow. I ran a flow bench teat on the angle of the transfer area and found a 10/12 degree angle provided the beat flow. However there isn't enough material there to t remove much. It would also reduce compression.
I only take about .010" over the dome area of the head. The rest comes off the valve area. The clearance the area over the valves at a 4 degree angle with matches the angle of the valves.. Now this depends on the lift of the cam. now removing more material from an *ba style head, material should only be removed from the block sursurface of the head as the uppre portion is very thin..
Last but not least the aftermarket heads are very expensive and only have an aesthetic value.
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Old 09-18-2021, 08:19 AM   #57
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Default Re: Piston to head clearancing

Gramps,
Great post, My head is spinning with information. The last set of heads i did were milled at a shop to get .050. I plan on doing my 59a heads on my mill and have been contemplating how to hold them to the table at an angle. Can you share a photo of the fixture you made to hold the head?
Thank you ya’ll !
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Old 09-18-2021, 05:53 PM   #58
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Default Re: Piston to head clearancing

I loaned the fixture out a few year back, I'll try and get it back, don't think it ever used anymore.
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Old 09-18-2021, 09:43 PM   #59
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Default Re: Piston to head clearancing

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Using head gaskets over, has been a practice since the stockcar days. I have a stack of old copper gaskets and I can't remember buying a new one. Same with bearings. I use Hi tac or copper coat. In the future I'll use Latex paint on the aluminum side. For the new builders, better check the cam bearing clearance, Some of these are way out. Now I measure them before i remove them. Gota admit, they're well lubricated.
OK Ron I'll bite! what's with the latex paint and bearings?
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Old 09-19-2021, 08:52 AM   #60
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Default Re: Piston to head clearancing

Copper and aluminum creat an electrical current which caused the errosion of the aluminum. When you remove the heads after they have been on awhile, you can see where the aluminum has been removed. After beed blasting you can fill the area with Devon F, providing the fire ring is original. I was told by a fellow engine builder that painting the heads with Latex paint will insolate the two materials and prevent erosion. Might work, can't hurt.
As for the cam bearings? I don't usually measure the clearance of the cam bearings , However, back in the 90's someone, might have been Walt said he founf .006" clearance with a new bearing. (max is.003). Since then I've been checking all my cam bearings and have found several loose ones. Now when I receive a block in fair condition I measure the cam bearing before I remove them. If they check OK I leave them in the block if I'm not having it bake cleaned.
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Old 09-19-2021, 09:55 PM   #61
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Default Re: Piston to head clearancing

OK, got it! thanks
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Old 09-23-2021, 06:50 AM   #62
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Default Re: Piston to head clearancing

Interesting thought about using the latex paint. I always assumed the corrosion on the aluminum head was just due to the water and coolant. never thought about electrolysis as being the culprit. I have used aluminum paint on tractor heads before, of coarse they are cast heads and not aluminum. Just goes to show you we can always be learning. Thanks for sharing your knowledge with us.
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