I put a set of forged pistons in my new engine. I don't remember what brand they were, whatever comes with Summit rebuild kits. Anyway, the box the pistons came in said that the clearance should be .001. I measured them and that's what they were. Anyway, I put the engine together like that and haven't had any problems. (800 miles) But I just read something, (then did a search on the bb and found a few other somethings) that say the clearance should be around .0075!! I'm sure I read the box right, the pistons fit perfect to their spec, and I don't appear to have any problems (yet). I'm also sure they were forged, that's what summit told me, and also I inspected the pistons when I got them. They were forged. So what gives? I'm starting to get a little freaked out here about my new engine... someone please tell me this was ok.

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91 Z28
Come see The Vicious...

 

most forged clearances are in the .004 to .005 range. Very few are more than that. If you went .001 and it is running ok at 800 miles I'd say you are ok. Thats a little tight but it can be done. You might be getting a little scuffing on the skirts but I would not worry about it.


You could send in a couple of samples from your next oil changes to see whats showing up in the old oil.


[This message has been edited by jcb999 (edited November 28, 2001).]

 

The pistons that Summit puts in those kits are the TRW/Federal Mogul/Speed-Pro forged pistons. They are at the low end of the forged piston scale for quality and strength but they are still pretty nice for the price.

From what I know/have been taught, the .004 to .005 specs are the ones to shoot for with forged pistons. But the engine in my GTA (which also has TRW forged pistons) was around .0025. I was a little worried myself, but I asked a few of my machine shop buddies about it and they said to let her buck...... Might I add that it took me not one, not two, not three, but four sets of those TRW pistons to piece together one set of eight that were within tolerances for weight, clearances, and non-dinged-upness from shipping for my GTAs engine... . ....won't be using them ever again.......

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

That's really strange. Dpending on the motor (N/A or sprayed) we set it anywhere from .005-.007. One guy at work runs his at .014! With .001 clearence, I'm surprised they didn't seize on break in. Are you positive you didn't get hyperutetic pistons? They run with ALOT less clearance that a true forged piston.

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85 Berlinetta Iroc exterior NOW 454 powered, 4.10 gears, 275/60/15 Hoosier Quicktimes freshly installed th350 3200 stall, shift kit, reworked valvebody, Megashifter, Biondo linelock ..........YES, it's street driven!

check it out at http://www.bsronline.net

 

Don't worry about it. Piston/bore clearances depend on water jacket temperature, piston temperature, piston material, and piston shape. I have also heard of race cars running around 0.015" clearance on forged pistons. Marine engines also tend to use high clearances because the jackets are cooled with sea water. In the case of the race engines some of the clearance is to reduce friction to increase power, not to keep the pistons from corking. I have never heard of 0.001" for forged pistons though. I am not certain how you determined they were forged and not hypereutectic which would easily run at 0.001". I have heard of small block forged pistons, such as SRP pistons, running around 0.002" to 0.0018" on a block that was torque plate honed. This makes certain the bore doesn't get tighter anywhere when the engine is bolted together. They can run this tight because they are made of a different forged aluminum alloy that expands less. On a street engine where you will see higher water jacket temperatures and lower piston temperatures it works out. If the engine has lived this long it will probably last. Now is not a good time to get freaked. If you were doubtful you should have addressed it before the block was bored. Personally I would have overbored the block an extra 0.0005" to make it .0015". Not enough extra to hurt anything but enough to make you sleep better at night. If these are hypereutectic pistons you would want them as tight as you can get without seizing. The expansion on those is practically nil, and when they are too loose they knock like a desil engine.

 

I used TRW'S f/f tops in my .030 over 305 and the box said a minium of .002 which I thougth sounded tight for forged pistons,but I've read that some of the newer forged pistons run tighter clearences.Haven't fired it up yet but feel confident it'll be ok.

 

if thats what the instuctions said then there right, diffrent pistons like a diffrent gap, if it was a problem you would have noticed it a long time ago.

 

I called Summit and verified that they are indeed forged pistons. Matt was right, they're speed-pro/TRW. And the spec is also correct, actually .0015. Whatever... just so long as my engine doesn't sieze.

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91 Z28
Come see The Vicious...