Bad news after Leakdown test Suggestions

[bobm]

Just took car back to rebuilder for 500 mile checkup after complete rebuild.
The PO had changed to 2.2 PC's, the heads were redone and the cly's sent to machine shop. Mechanic and Machine shop said all look good and put on new rings. Yesterday after compression check , cylinders 1,2,5 had 130, #6 120 and 3 & 4 had 110. He did a Leakdown and found # 4 had 30%. He say's it is the rings. He wants to pull out and redo.
He is suggesting a different PC set?
The motor was initally rebuilt because of high leakdown.
Any ideas what should be done so that this does not happen again? Anything that should be done while open?
I am not a motor rebuilder so relying on his expertise. Would appreciate ideas from the guru's.
Thanks
Bob M

[304065]

Compression checks are of very limited diagnostic value.

OK, leakdown from #4, 30%, rings. That means he's at least using the leakdown tester correctly as it is supposed to tell you WHERE the leak is coming from. You can put a rubber glove over the tailpipe and over the #4 stack to see how much air is leaking past the valves, if any. If the valves aren't adjusted right or there is a piece of carbon under a valve seat, the leakdown will be significant. Often times a light tap on the valve adjusting screw ( and I mean LIGHT) with a plastic hammer will cause the valve to seat properly for the test.

So--- if there's bad leakdown on 4 even after verifying that it's NOT coming from the valves, then it's either the cylinder to head interface or the rings.

Perhaps he broke a ring when installing?

What were the precise dimensions of the cylinders and pistons, measured according to the factory workshop manual? He should be able to tell you this, if he didn't measure them, or didn't have the tools to measure them, then you could have worn-out parts.

Why is he suggesting a different piston and cylinder set? That doesn't make sense, there must be information we don't have.

My advice is, ask him to tell you EVERYTHING he knows about the piston and cylinder set that was installed, including the measurements-- if you don't get a good answer, you could have a broken ring, probably the top one. If you install brand-new rings into worn-out ring grooves they will break as soon as you hit the starter. The leakdown probably isn't lying to you if done correctly per the above.

Like my own engine years ago:



And when I tore it down I found this:



Good luck!

[savaden]

911 and other air cooled cylinders wear more at the top, creating not only a lip, but taper in the cylinder. Also the ring lands in the pistons wear. Simply honing the cylinders will only make the situation worse. Honing the cylinders will cause the clearances to be out of spec. For this reason it is standard procedure to either bore cylinders and replace pistons, or replace pistons and cylinders as a set. When your mechanic says you need another set of p/c's, he means a NEW set.

SV