Carbon on Piston

[scottanf]

I have a 1978 SC I purchased in 2005 that the previous owner had completed a top end rebuild on 5000 miles ago. Last fall the clutch release bearing packed it in so we did an engine drop and got the engine out on a stand. While it was out we did a leak down on the cold engine and found that 5 cylinders were at or below 10% but #3 was 40% + with all of the leakage around the piston . I work for a company that owns a few jet turbines so I borrowed a bore scope and had a look at all the cylinders through the plug and exhaust (see pic’s). From the pictures you can see that #3 has a ton of carbon on the piston top and valves compared to #2. New lower studs, reworked heads including new exhaust valves and cams that have been reground confirmed the top end rebuild. The car smoked only at start up when it had been sitting around for awhile and did not show any signs of oil burning while running. I am suspecting that there is a problem with the rings on #3 from the rebuild. Also looking at the service records that came with the car it had been run for a awhile with a clogged cat. I’m in the process of ordering the cam tools to start taking 1,2,3 apart an wonder if anyone else has experienced similar issues with their 3.0. First 2 pictures are #2 rest are #3

[shbop]

Any idea what the two black spots are on the wall of pic one?

[scottanf]

I assume that a few fakes of carbon came off the plug or off the piston. Guess what I'm trying to figure out is why the engine did not smoke or show high oil consumption with the low compression on #3.

[gtihop]

looks like a scrape on the cyl wall up and down? could be a problem

[JeremyD]

Originally posted by gtihop
looks like a scrape on the cyl wall up and down? could be a problem

+1 - I'd be much more concerned about that - looks like a ring end gap that wasn;t properly measured

[cstreit]

+2 or a busted ring...

[scottanf]

I think you're more than likely right on the ring, either improperly installed or broken when rebuilt by the last owner 5k ago. Tools for the cam are on order so I should know in a week or so. It's a long way from California to Newfoundland:-)

[Walt Fricke]

Scott

For the taking apart stage you don't need any special cam tools. Just use a large enough socket and a half inch air impact wrench on the big nut on the cam.

The tools are for putting things back together and tightening them.

Walt Fricke

[scottanf]

The heads are off and #3 had a broken compresion ring but here is where things get a little strange. What I thought was carbon was in fact a different dome shape on the top of the piston from the other 5. The markings on #3 are 95L70++ and the other five are 93L7++. I think some fool has installed a 3.2L Piston as a single replacement on a 3.0 L engine. Can someone confim this for me based on the markings?

Thanks Scott

[Henry Schmidt]

It's easy to tell a 3.2 95 mm piston from a 3.0 95 mm piston.
The 3.2 piston has a 23 mm wrist pin.
The pin height is also higher in the 3.2 piston.
If the dome configuration is different you may be dealing with the difference between 8.5:1 78-78 US piston vs an 8.6:1 Europe piston.

The cylinder looks like alusil so the difference may also be Kolben Schmidt vs Mahle.

[scottanf]

All 6 Pistons are Mahle and have 22mm wrist pins. The wrist pin is set higher on #3 piston than the other 5. Could this be an issue of US vs: euro pistons.