Compression / Piston problem.

I just finished up with a Head/Cam swap. The car has a stock shortblock that has been running just fine for ~90000 miles. I buttoned everything back up and the motor is still running great (with a PCMforless tune). No squeaking or other misc. noises, so I think the bearings are fine. When I was cleaning / prepping the block I checked the piston/deck height just for ****s and giggles. Get this: at top dead center, the #6 piston is ~60000 thousandths further down in the bore than all the rest of the pistons. WTF!!?? Who can I shoot for this? I'm running 11.3 : 1 compression on all the other cylinders. What kind of power am I likely to be losing because of the #6 cylinder? Could this difference in deck height be the result of a shorter rod? A bad crank?

Obviously, this thing has been running fine for 90K, but I've substantially raised the operating range of the motor (shifting ~6800-7000 rpm). What kind of problems might I run into with the #6 piston the way it is?

I assume you mean .006 (6 thousandths)?

For a factory block with no squaring and decking, that's not too bad. Check the crank to make sure it is straight just to be sure.

The piston heights have to be corrected on a mill. So you can record the heights and take them to a machine shop to have them corrected if you want it dead nuts.

-Mindgame

At .006 it's only about 1.24 cc's and should be 11.1 CR, but at .060 down it's 12.4 cc's extra and about 9.74 CR.

If it's really down .060 ("60 thousandths"), your are down on power a few % in that cylinder, or maybe .5% overall. Also, if it's .060 you have LOTS of quench height which ain't good, but with gas good enough for 11.3 you should be ok.

The big question is, how in hell would a stock engine have one piston .060 lower than all the others? .006 could be within production tolerances, but not .060!

So what was it? .006? or .060?

Bent rod's do that!!!!!
Failure of someone to check the rod alignment.

If you have had a NOS backfire or piston to valve contact,there in lies your answer

The piston is definitely down 0.060, not 0.006. It was never a nitrous motor. I have only recently changed the rev limiter from stock.

Originally posted by Elysian
The piston is definitely down 0.060, not 0.006. It was never a nitrous motor. I have only recently changed the rev limiter from stock.

Does the "low" piston lookjust like the others? I wonder if it could have been replaced at some time. A .060 shorter compression height wouldn't make too much sense in a stock replacement.

If that rod was bent enough to shorten it .060 it's difficult to believe tha piston wouldn't have scuffed and you'd see noticably different wear patterns on that cylinder. Was compression good in that cylinder?

Curious...

When we checked compression on the cylinders, there was some variance, but every one of them was higher than stock by a significant amount. The piston was not visibly damaged and there were no unusual wear patterns in the cylinder bore. I bought the car with 47000 miles on it, so it is possible that whoever owned it before me replaced a piston, I suppose . . .

If you hadn't check piston heights you'd probably just run it. Because you haven't found anything else to indicate a problem, my feeling is to run it like it is.

You might compare the #6 plug to a couple of others after some hard runs. It may tell you if things aren't right.

Good luck.

I'm running it as is already. It's running a little rough, but I haven't had it dyno-tuned yet. In any event, the #6 cylinder was running just fine for ~90K miles.

Sometime this winter I'll probably be building a stroker anyway, I just wanted to make sure the motor wouild live until then . . .

Just curious, what did you do for heads and cam?
(flow, timing, etc.)

AFR 195cc LT4 . . . 2.02/1.6 valves . . . 281cfm
CC 233/239, 0.569/.577, 110lsa