Broken piston skirt question

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Jun 13, 2004 | 01:27 PM

packrat56

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Fleet Mechanic

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From: Willamette Valley, Oregon

Broken piston skirt question

I have a 302 with piston damage due to a washer in the cylinder(see other posts) I pulled the bad pistons and found a skirt cracked in 2 places.. WHat are the usual causes, what should I check? should I pull all the pistons? Replace them all?

Jun 13, 2004 | 06:06 PM

Pro-Street/StateTK

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From: East Flat Rock NC

It is from over heating it will crack the piston skirt!

P.s. chances are there are more pistons that look just like that one ,you may
look down inside the piston to see it but I dont think that would be good
enuff ! Since you aready have it open.

Last edited by Pro-Street/StateTK; Jun 13, 2004 at 06:17 PM.

Jun 13, 2004 | 06:29 PM

scroob

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If you have a old engine with worn cylinder bores, the pistons will slap against the cylinder walls when cold, eventually cracking and breaking the skirts. This is known as piston slap, and is heard when the engine is cold as a dull, deep knocking sound that goes away when the engine warms up. It's very different from the rap of a lifter or the knock knock sound of a rod.

I would have the cylinders miked to see if they are within tolerances. If not, a rebore is in order. At the very least, I'd replace all the pistons, but it's not much good doing that if the cylinder walls are too worn.

Jun 13, 2004 | 06:35 PM

packrat56

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From: Willamette Valley, Oregon

This engine, although I do not know its history, has no ridge what so ever, is from the 70's, and seems to have std size pistons. I remember no cold engine knock, but that does not mean there wasn't one. I pulled 2 so far, one skirt is fine, one is broken on both sides of the outside skirt, as if the whole chunk might eventually break off.

How can I tell if it was heat? Or slap? You all agree, I ned to pull-em all????

Jun 13, 2004 | 07:14 PM

ga302p

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Packrat, you really need to pull this engine down; considering the other problems you have already mentioned in other posts. If you really want to know how much wear is on the holes. You are going to have to check three things: piston sidewall clearance, roundness of hole, and top to bottom taper of hole. You do all that with a taper checking gague. They measure down to 1/10,000 in. You can get by with being a little bit out of round, but if you have much top to bottom taper, you need to bore the block. It has been my experience that you can have a cylinder that mikes out fine at the top and is way too big at the bottom. That is what causes piston slap.

Jun 14, 2004 | 03:56 AM

scroob

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Ridge means nothing. It's an old wives tale. Cylinder bore wear is least at TDC. If you can even drag a fingernail on the ridge, you have a problem. Taper gets worse as the piston goes down. If you have a thousandth or two on the ridge, you probably have 4 or 5 times that at the bottom of the bore. If you picture a crank/rod/piston reciprocating assembly, and the piston moving up and down in it's bore, you can easily see why. Lateral forces on the piston are much greater at the bottom.