Another Engine, another mystery. Another day in the life...

[zKars]

The last remaining mystery to me is the really obvious air leakage into the sump during the leak down test. Just how is that air getting down there?

So I see two clues. First the ring end gaps.

I popped #1 top ring off and stuffed into the bore, and measured the end gap.  0.020 ish. Hmmm, FSM suggests .010 to .015 is normal, with .040 max,. So .020 sounds ok, maybe a bit wide. 

With no obvious damage anywhere, about the only thing I see that is not "right" is a really random location of the three end gaps around the pistons. Some have the top ring gaps right next to each other, some are spaced roughly 1/3 of the way around. I likely moved them some while handling, I don't know how much weight to put on this. 

The walls were all well-oiled, how else does air get past the pistons? Usually I do this test with the motor buttoned up so I never get to hear whatever air escapes past the rings normally in a healthy engine? The leakdown tester was always saying the amount was well into the green/great/good region.  I don't think this means too much. The head gasket was great, no issues there. Oil was water-less, water was oil-less.

Finally I did a quick bore inspection. Only saw one scratch in #1 that can feel with a finger nail. Everybody else is nice and clean with clear hone marks.

 

 

[zKars]

So we have a block with 250,000 + km. stock 86 mm pistons,  evidence of engine work at some point, rings that don't look the age of the block, poor erratic compression ( that BTW does improve a bit with oil added) only one clearly bad cylinder valve wise, a timing chain with no tensioner, that must have slipped or was put back together without it, that resulted in in-consistent piston smoochin'',  

The other clues I noticed:

-oil pan gasket is not a stock gasket. 

-chain guides were very worn. Not down to the steel, but grooved and very thin.

So fellow detectives? What say you? Not that it matters, she needs a full round of rebuild love.

 

[Reptoid Overlords]

How did the cam look?,if they were running the valves too tight would it produce some of the results that you noted?

 

 

[zKars]

Cam looks fine. I did check the lash, some were tight but all were positive.

[240260280]

I agree with your theory...tensioner came apart and they just ignored the spring when returning the tensioner to its home.  Chain timing was off when they put the tensioner back or it skipped a  link or two due to the missing spring and caused the valve/piston contact.  The timing chain being off also contributed to the lower compression and the bent valves contributed to the variance in compression.

Edited October 12, 2017 by 240260280

[grannyknot]

I really thought there would be more damage to the top of the pistons.  The tensioner is definitely the key here, like you said the spring was misaligned  or pinched, the driver over reved it, the spring popped allowing lots of slop, the intake smacked the piston, the driver may have heard or felt something and backed off but the damage was already done.  Tough engines.

5 hours ago, zKars said:

No this is not a piston return spring. Must have been a hell of a ride!

Ping ping ping Ricochet Rabbit

[Zed Head]

I've wondered if a loose chain alone is enough to allow the valves to hit the pistons.  Doesn't seem like it should unless the camshaft can some how rotate faster than the crankshaft is pulling it, from inertia.  The inertia of the mass would have to overcome the valve springs and friction.  Maybe at high revs, on a downshift, or something similar.  With no tensioner on the loose side there would be a lot of slack to over-rotate on, if it's possible, even if the sprocket timing was right on the tight side.

[sweatybetty]

did you measure the pistons and bores? maybe someone bored-honed the cylinders without replacing the pistons, which would result in blow-by past the pistons. just an idea

[siteunseen]

5 hours ago, sweatybetty said:

maybe someone bored-honed the cylinders without replacing the pistons

That would be a problem!