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I have my heads off my engine and since i know a little more about engines than i did when i rebuilt my motor a few years ago, i decided to measure how far my pistons are in the hole so i could more closely calculate my compression ratio. I bought a new handy dandy deck bridge and dial indicator. So i measure on the bottom of the piston and come up with .020, put it on the top and i get .008. Big difference. I get to looking closer at the pistons and realize i can see the rings between the piston and bore. I can move the pistons in the bores. I don't believe this is normal right?
I grab a feeler gauge and can stick a .020 blade between the piston and bore. This is a feeler gauge with the blades about 1/2" wide. Holy crap, thats not right. If piston to wall is suppose to be .007, is that total or per side. I assume its total, bore is .007 larger than piston right? I guess that explains why the piston rocks so much when i was measure deck height right?
Engine runs great, i did a leakdown a while back and it was a little high, i think it was 20-30%.
Pistons i have are TRW L2323 in a 455.
Mental note, find a new machine shop.
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You need to measure your piston clearance with it upside down in the bore. You need the skirt clearance not the clearance at top where the rings are. If you had .020 skirt clearance you would hear them rattling a block away.
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Oh ok. That makes me feel better.
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Like stated, piston clearance is from a point on the skirts. The tops of the pistons should slop around a little. The design is when the top of the piston gets hot, it will swell to about the equal of the point on the skirt. So it is cut tapered, with the top smaller. If it were perfectly round, the top of the piston would get so hot that it would swell and lock up. A .020 feeler guage seems a little much, but don't worry.
Now on to deck checking. You will get rock when you check deck height. So measure, then rock it to the other side and measure again. Then take the average. (So your down side .020 and up side .008 would be .014.) Then do the same on the other side. Then average the two averages to be perfect.
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Or you could check it over the pin. BTW if you' really have 20-30% leak, your done.
