Tight Piston Clearance, Not Model A

[Tom Wesenberg]

On another car site there is talk about modern piston clearance. I haven't rebuilt newer car engines for several years, but am used to piston clearance of .0015" to .0035" for most car engines. Someone said new cars are running with only .0004" to .0005" clearance. That's as snug as wrist pins. Are they really running that tight now?

What are the Model A's with Chevy pistons running for clearance?

[tbirdtbird]

Tom, don't know much about modern stuff.
Allegedly with 'chevy' style pistons in an A you can run .0025, but don't u believe it. much written on this. if the cyls were all the same temp .0025 would work. but you pros know that is not the case. The 20* hotter 3&4 spoils everything
We have had many jobs whereby we had to hone out to .0035 because other shops used .0025.

as you well know these engines will easily tolerate .0035-.0040 clearancing with no issues at all. others on here will vehemently defend .0025. My point is use .0035 and be safe, after all it cannot harm anything. That extra thou never made a piston slap. When at op. temp these clearances are less anyway. dave

[Jim Brierley]

Interesting question, to which I don't know the answer, hopefully someone will and post it here. Sounds awfully close to me.

[Bruce Lancaster]

One thing is cold piston shape...the small-block Chevy at some point in its history went through a lot of development of piston shape so the irregular lump would assume a round shape as it heated. I think Yunick was involved in this. That's ancient history now, so I would imagine a lot more work has gone into that endeavor. Also, top rings have moved upward, presumably influencing the amount of heat reaching lowe part of piston.



I don't own any actual manuals for anything remotely modern...a quick look at the google kicked up this chart of Toyota clearances up to 2005. Will give URL as columns don't behave when copied... http://www.autozone.com/repairguides...96b43f80380186

[J and M Machine]

I am currently working on two Jaguar 4.2 six cylinder engines.
The pistons are made by Mahle which purchased Clevite.

Since the spec was very tight clearance I decided to call and make sure if it was correct for an engine that was 47 years old.

According to the tech on the line that these pistons are designed for .001" "thousandths" clearance .
This is due to the fact he mentioned that they vibrate the mold at high frequency and the material used is very stout. He mentioned that the Aluminum used is very stable.
He also threw in that the new 4.7 Dodge engines pistons have a coating on and the clearance for those is tight enough to scuff off the coating during installation.!

[Patrick L.]

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Some 'newer' engines do run clearances of less than .001". This depends on what the piston material is and the purpose of the engine.

Ford uses several different pistons in basically the same engine depending on how the engine is outfitted. The supercharged units use a forged piston with less than .001" at the 'normal' measuring position but is tapered to allow .0025" at the bottom and at BDC. They are still trying to figure out some failure issues due to detonation, most engines can handle a fair amount of detonation if not supercharged. It appears they are having some ignition timing issues causing failures under high boost.

I've been playing with supercharged engines since I was 15 and have grenaded my share.


I had a Tundra POS which went thru 3 engines [pistons] in less than 30K miles due to coated piston failures and tight clearances. I don't know the clearances because I never got near enough to be able to measure, they got rid of those engines as soon as they could. But they supposedly ran them tight.

[edmondclinton]

KB pistons have recommended their minimum clearance to be .0015 and no more than .002 maximum, for years, on street engines normally aspirated with a 4.100 inch bore or less. My most recent catalog is 15 years old.

A close fitting piston rocks less, supports the rings better and seals the engine for the best power.

[tbirdtbird]

run'em as tight as you want
when they score, seize, shrink and do other bad things i don't wanna hear it.
we fix everyone else's mistakes here.

[edmondclinton]

Originally Posted by tbirdtbird

run'em as tight as you want
when they score, seize, shrink and do other bad things i don't wanna hear it.
we fix everyone else's mistakes here.

So in your opinion the engineers at KB Performance Pistons don't know what they're doing.

[tbirdtbird]

if you make the effort to read carefully you will see that is not what I said at all.
this is what i said:
"when they score, seize, shrink and do other bad things i don't wanna hear it."

so, when they do score, seize, shrink and do other bad things, then each of you can judge for yourselves if the engineers know what they are doing.

[edmondclinton]

Originally Posted by tbirdtbird

if you make the effort to read carefully you will see that is not what I said at all.
this is what i said:
"when they score, seize, shrink and do other bad things i don't wanna hear it."

so, when they do score, seize, shrink and do other bad things, then each of you can judge for yourselves if the engineers know what they are doing.

Yes, I read it and you're exactly right that is not what you said "at all" ... but you most definitely implied that tighter running pistons will give trouble. This you post right after I made statements about .0015 clearance of KB Performance pistons.

Apparently you are not familiar with hypereutectic aluminum. This is what is used in KB Performance pistons. This particular alloy has less thermal expansion than conventional piston alloys. The nature of its major alloying element, silicon, at up to 18% acts to keep the heat in the combustion chamber and prevents heat transfer, thus allowing the rest of the piston to run cooler.

[Ol' Ron]

I haven't worked on anything newer than a Flathead for the past 30 years, but back in the 60's I worked on a number of Chrysler 318 and 383 ngines and the stock clearance was .0005-,oo1. This caused the engines ti seze in very cold weather when using a multi grade oil. Once the engines were warmed up in a garage they started and ran fine, #10 wt was recommended for winter use. After a few years the oil companys fixed this problem.
As for Flatheads I use .002 on Egge pistons over the past 20 years and so far so good.

[tbirdtbird]

Mr Edmund Sir,
Apparently you are not familiar with engines that have scored due to overly tight clearances. In this shop, we are. Perhaps you need to start an overhaul shop and get your hands dirty working on trashed engines that people have ruined with their clearancing methods. Since you are having so much trouble with all this, perhaps it is time to remind you that we are not talking about modern vehicles with modern uniform cooling systems. We are talking about ancient vehicles with less than uniform cooling. Why not interview the man who has had a failed Model A engine rather than sparring with someone who fixes them? And as I also just said, you can set your clearances however tight you wish. No problem with me.

I recently spoke to a rebuilder who actually made this statement: "But everyone is having trouble with scoring in holes 3 & 4"
Then truly 'everyone' must be doing something very wrong since Henry certainly never had this problem. I will leave it to you to solve the problem of why this is so, since according to you my explanation is so utterly useless.

[MikeK]

This discussion is suffering from an 'apples vs. oranges' syndrome.
Two points come to mind:

1) The KB (and certain other new-car pistons) are cast hypereutectic alloy. The expansion rate is significantly less than standard hypoeutectic piston alloys. On the opposite side of the spectrum, forged pistons have a significantly higher expansion coefficient and require more clearance than standard cast alloys.

2) Not everyone uses the same criteria or point to measure clearance. If you look at the KB installation page, HERE you will see their mic point is near the bottom of the tang on the slipper skirts. Those pistons, and all modern new vehicle pistons are cam profiled to an eccentric shape that reduces progressively toward the crown and with a different progression rate in the axis 90 degrees from the pin.

Even if the clearance is ZERO at the designated measure point on some pistons, there is plenty (several thou) clearance if measured higher up and at a different axial point. All well designed modern pistons (not the typical model A junk) also sport reduced land diameters that prevent that area from ever contacting the cylinder wall no matter how hot the piston top gets.

If you were to measure the KB (and many modern) pistons just below the ring lands and above the pin you would find 0.004 or more clearance, usually ~90 degrees from the pin axis, and about 2-3 on the pin axis when the slipper skirt tangs are at the recommended 0.0015-0.0020.

In the 'good old days' of round pistons with full skirts you could just stuff a feeler gauge in the bore next to the piston. You can't do that with cam profiled pistons, either full or slipper (cutback) skirt designs. If you did the results would be disastrous. Apples Vs. Oranges.

[dave in australia]

The last set of pistons I installed were on my Durant D60. I gave the bore diameter to the piston manufacturer, JP Pistons of Adelaide, South Australia, and they machined the pistons to suit. What the clearances were, I have no idea, but they went in, don't have any piston slap, don't seize when warm and running, so all is good. By giving JP the bore size, they can then set the clearance they require for their pistons, and not some mechanics "we used to set xxx clearance way back when", and then have something go wrong.

[colin1928]

Yea 1/2 a thou is common on modern engines with piston squirters thin wall bores and modern materials
You are supposed to measure at 20 deg C and not hold the piston with your hands
Regardless of what others think they know follow the manufactures specs with piston clearances because materials and designs change from piston to piston

[edmondclinton]

Originally Posted by tbirdtbird

Mr Edmund Sir,
Apparently you are not familiar with engines that have scored due to overly tight clearances. In this shop, we are. Perhaps you need to start an overhaul shop and get your hands dirty working on trashed engines that people have ruined with their clearancing methods. Since you are having so much trouble with all this, perhaps it is time to remind you that we are not talking about modern vehicles with modern uniform cooling systems. We are talking about ancient vehicles with less than uniform cooling. Why not interview the man who has had a failed Model A engine rather than sparring with someone who fixes them? And as I also just said, you can set your clearances however tight you wish. No problem with me.

I recently spoke to a rebuilder who actually made this statement: "But everyone is having trouble with scoring in holes 3 & 4"
Then truly 'everyone' must be doing something very wrong since Henry certainly never had this problem. I will leave it to you to solve the problem of why this is so, since according to you my explanation is so utterly useless.

Not familiar???

Mr. Bird, I can assure you when it comes to engines you're telling Noah about the flood !

Moreover, the title of this thread is Tight Piston Clearance, NOT MODEL A

[edmondclinton]

Originally Posted by MikeK

This discussion is suffering from an 'apples vs. oranges' syndrome.
Two points come to mind:

1) The KB (and certain other new-car pistons) are cast hypereutectic alloy. The expansion rate is significantly less than standard hypoeutectic piston alloys. On the opposite side of the spectrum, forged pistons have a significantly higher expansion coefficient and require more clearance than standard cast alloys.

2) Not everyone uses the same criteria or point to measure clearance. If you look at the KB installation page, HERE you will see their mic point is near the bottom of the tang on the slipper skirts. Those pistons, and all modern new vehicle pistons are cam profiled to an eccentric shape that reduces progressively toward the crown and with a different progression rate in the axis 90 degrees from the pin.

Even if the clearance is ZERO at the designated measure point on some pistons, there is plenty (several thou) clearance if measured higher up and at a different axial point. All well designed modern pistons (not the typical model A junk) also sport reduced land diameters that prevent that area from ever contacting the cylinder wall no matter how hot the piston top gets.

If you were to measure the KB (and many modern) pistons just below the ring lands and above the pin you would find 0.004 or more clearance, usually ~90 degrees from the pin axis, and about 2-3 on the pin axis when the slipper skirt tangs are at the recommended 0.0015-0.0020.

In the 'good old days' of round pistons with full skirts you could just stuff a feeler gauge in the bore next to the piston. You can't do that with cam profiled pistons, either full or slipper (cutback) skirt designs. If you did the results would be disastrous. Apples Vs. Oranges.

Regardless of the measure points, KB brags on the fact that their pistons can be installed tighter than other "performance" pistons. It's the first sentence in their catalog.

[Ed Northrop]

I took a big block Mopar to the machine shop along with a set of 30 over TRW forged pistons they refused to use pistons spec as they said the clearance was to tight. They allowed .008 for clearance or they would not do the work. I have to say they must have been right as there is no piston slap.

[Ron W]

We always used about .001/inch for cast pistons. Kieth Black's can run a little tighter, especially in aluminum blocks. Forged pistons got about .0015/in when I did them many years ago in race engines. What we did then was crude compared to what is done today. Ron W
PS Too loose will run, too tight won't!

[edmondclinton]

s

Originally Posted by Ed Northrop

I took a big block Mopar to the machine shop along with a set of 30 over TRW forged pistons they refused to use pistons spec as they said the clearance was to tight. They allowed .008 for clearance or they would not do the work. I have to say they must have been right as there is no piston slap.

Were the specs from TRW for street driving at moderate RPM's on standard pump gas?

[edmondclinton]

Originally Posted by Ron W

Ron W
PS Too loose will run, too tight won't!

Very true, and too loose can also = loss of power...

[Kurt in NJ]

I remember reading --I think Murry Fahnstock --that the model a piston could run at .001 tight, I looked in the service bulletins and found-- on page 455--"In production pistons are now fitted to cylinders with a clearance of .0005 to .0025" --so even the model A was designed to run tight piston clearance

[Ed Northrop]

I would have to look back in my records I do not remember I do remember they were new but around 20 years old. I believe I was told new pistons do not swell as much.

[Ron W]

Page204 of Service Bulletins calls for loose on .002 and tight on .004. Ron W

[Dodge]

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Also remember there are no water jackets in the lower part of a Model A Motor they need
the clearance. Those Keith Black pistons are amazing, they don't grow much.
When they were first coming out with them they gave us pistons to play with. We ran
them all over the board, first loose, they were ok at .003 but started making noise at
.0035 and they didn't quiet down when they warmed up. Then we kept making them
tighter and tighter they really seem to like .001 to .0015 and thats in a race motor.
I took a while to figure out that the top ring needed a lot more clearance and thats
how they run them now.
They are strong also, one motor dropped a valve and it when right through the piston
leaving a round hole, dropping the valve into the crankcase. The rest of the piston was
intact with the rings still on it still going up and down. Another motor lost a radiator
hose and got so hot it seized up when the driver lifted his foot off the gas. After
the car cooled down it was started back up and the pistons didn't make any noise.
When the motor was torn down later there were no scuff marks on the pistons. These
were in at .0025
There is a Keith Black 283 piston that fits and a hemi piston that also fits a Model A Motor.
So have fun and clearance is your friend.......