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.

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.

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.

Re: Piston to head clearancing
 

I would agree.

Re: Piston to head clearancing
 

What's stock piston to head clearance?


When in doubt. Mill .050.

Re: Piston to head clearancing
 

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

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.

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.

Re: Piston to head clearancing
 

That was standard practice in the olden days.
The pistons then came to deck height.

Re: Piston to head clearancing
 

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.

Mart.

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?

Re: Piston to head clearancing
 

With a used gasket, torqued down.

Re: Piston to head clearancing
 

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

Re: Piston to head clearancing
 

Yup, your measurements are what I normally find on uncut heads and OE Ford pistons.
Martin.

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

Re: Piston to head clearancing
 

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.

Re: Piston to head clearancing
 

No difference. In either case you put strips of clay or little foil balls on the piston tops.

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.

Re: Piston to head clearancing
 

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.

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 .

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?

Re: Piston to head clearancing
 

What can be used if you don't have any used head gaskets?

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.

Re: Piston to head clearancing
 

lower compression when using a blower also.

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A new one? Can't see why not.

Mart.

Re: Piston to head clearancing
 

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

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.
Martin.

Re: Piston to head clearancing
 

Double post.

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.

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

Re: Piston to head clearancing
 

Won't be much to see.

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)

Re: Piston to head clearancing
 

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).

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.

Re: Piston to head clearancing
 

Dale, excellent point!!

Re: Piston to head clearancing
 

Yes 1mm is around 0.040.

I have not done any other measuring, not anything else than enjoyed the better performance of my flathead:)

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

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.

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

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