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Old 03-30-2002, 05:12 PM
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Question Piston to bore clearance-How much is too much

I have a buick 455 bored 30 over. Had a pistom crack (valves ate the ring also)and scratch it's cylinder. Used a Hone to remove the scratches. The Bore is now 4.3455, instead of 4.342. Piston are all slightly undersize (3.438 - 3.440). the other bores are all slightly over without any hone at all (from about 4.343 - 4.344). The pistons are sealed power hypereutectics, compression is around 9.5, no more. The engine may have seen 2000 miles since rebuild, did it wear that much or was my machinist overzealous.The pistons are sealed power hypereutectics, compression is around 9.5. Tach will never see the dark side of 6000 rpm, limiter at 5600. Anyway, thanks for looking this over, any help would be sincerly appreciated. other comments are welcome, (I know why would anybody build a buick.. It was there).

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Old 03-30-2002, 06:23 PM
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First, you want all bores the same size. Regardless of what piston clearance you are running, it needs to be consistent.

Hypereutectics run tighter than any other type of piston. Check with the mfg. for their recommended clearance.

You may need to check your ring gaps, as the hypereutectics reflect more heat into the chamber causing the top ring to expand more.

I am envious of your motor! The Buick 455 would work great in my Chevy 4x4

[ March 30, 2002: Message edited by: reddwarf ]</p>
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Old 03-30-2002, 07:59 PM
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Generally 0.001" per inch of diameter is a good rule of thumb, for forged pistons it is more like 0.0015" to 0.002" per inch of dia to accomodate the denser material expansion. More than this and the rings will wear faster due to the piston rocking in the bores. I have successfully run 0.007" clearance in 4" bores with cast pistons with a light knurling on the piston skirt to prevent piston rock. Is this recommended? Not really, but it works.

If you want to go ahead and run it the way it is, she will still run OK but the rings will leak down more in that cylinder and won't last as long as the others. How much I can hear you asking? lets just say the pistons/rings will out live the valves. Personally I would just stick her back together and don't worry about it, too much clearance is good insurance against break-in seizure and most shops put in a little more just to be on the safe side. I always do on my own engines.

It is far more important that the bores are round, a 0.0005" egg shape will slowly kill that cylinder and this is what I would be worried about. Ideally any out of round should be unmeasurable and this is why torque plates are such a good idea. I have seen bores distort 0.0002" without being torque plate honed and this is why the break-in is so important as these high spots are worn away.

I say don't worry about it, but you could contact the manufacturer to get a slightly larger piston that measured on the high side of the tolerance off the piston machining center if you wanted.

Tapered bores stress the rings and will eventually crack them from the constant contraction, this is another important thing to check.

So to summarize out-of-round, tapered bores, lack of a deburr at the bores top edge are all signs of a poor job. A little oversize is OK and acceptable considering that the process is done without the aid of direct measurement until the hone comes out of the bore for sizing. Remember honing 0.001" takes some time with the finer grits so it is possible that your bores were in bad shape and needed to be enlarged a little to make them round or get the taper out.
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