Hi All,

I'm closing out this thread that covers determining the cause of low compression in cylinder #4 in my 2008 MCS. I bought the car not running with a known bad head gasket which I stupidly assumed was the only issue. Work done, re-assembled, and low compression on one pot. I assume it's rings, not exactly a shocker at 128k. After replacing so many internal components, getting the head machined and taking my time to clean as I went, I don't want to abandon the car or the engine and it looks pretty manageable based on this TIS article. It just requires me to re-do the work to get the head off. Ugh, but at least it's clean this time. I have a few questions about this process and wanted to throw it out there.

1.) If I replace rings on #4, does it make sense to do the other 3 as well? The time is manageable, but the costs depending on the next 2 questions is not trivial. Compression on the 4 is 1.) 110 2.) 110 3.) 110 4.) 30. So not high compression on the others, but very consistent.
2.) Mahle makes a ring kit for $20. OEM is $60. Anyone have experience with Mahle rings in an R56?
3.) do I need to replace the connecting rod bearings as TIS says? It's only $60 for one cylinder, NBD. But $180 for the rest.
4.) I put it in a new +.3mm head gasket during round 1. I put about 20 minutes of running time on it. Do I need to replace it? I assume that I don't a new exhaust gasket as I saw many re-use theirs in videos.
5.) Maybe it's not obvious but I don't intend to take the block out of the car so I can't have it machined. Should I do anything to the cylinder, like a hand honing? When i had the head off there did not appear to be any damage to the cylinder walls at all. I'll try to measure regardless but not sure it'll be super effective as I don't have a fancy micrometer.
6.) any reason to not think that this will solve my compression problem in #4? If it's a busted piston skirt, I'd of course replace. But would rings most likely be enough?

Not trying to cut corners here, just trying to not throw a ton of money at this. Thanks for any input.

 

My above response, in red, is for doing a minimal repair on the one cylinder. If only doing the one cylinder, I'd get it broken in then trade it in for something else. Personally, I wouldn't do just the one --- I'd pull the block and re-bore all four. That, or part it out, depending on condition of the rest of the car.

Finally, without re-reading your closed thread, are you sure the low compression is caused by rings, and not some other part? Definitive results from a leak-down test?

 

Thanks for the reply. My POS leak down tester broke so i don't know the number, but compression is minimal, and when I apply compressed air to the cylinder it comes out the oil filler. When I did the tear down the first time there were chunks of timing components but I didn't see anything that looked like it chipped off a cylinder. based on the compression and result of the compressed air, I don't know what else it could be but I'm all ears.

 

Closing the loop on this thread.

I have completed the work mentioned in this post. Replaced the piston on #4 with a good used one from ebay. I ended up replacing all the rings just for ***** and giggles I suppose. So I pulled out all of the pistons of course, and honed the cylinder walls. This was all a new experience to me, have his never been anywhere close to this deep in an engine before. I had not been aware that you hone the cylinder to get the rings to seat in the cylinder. I did NOT replace the bearings on the crank @ the connecting rod. They were not very worn and I'm rolling the dice. I understand that. I also re-used the head gasket and exhaust gasket after confirming they were in good shape.

Completed assembly today and it fired up. Lots of smoke but that cleared when I ran it pretty good. Lots of pep and seems to do what it's supposed to. I will test compression again when I've put 100-150 miles on it.

Thanks all to OBW and anyone else who stopped by.

edit: I guess when I posted this I didn't know it was busted ring lands. It was. Rings were still there but the oil control ring was missing a piece, which I found in the oil pickup. Cylinder walls were in good shape though so I just honed it like the rest.

 

I have that exact repair before on an old mazda protege, it worked out fine, even though it is not the 'right' way to do it...