I was in the process of replacing rod bearings when I noticed a scratch in the rod journal. The scratch is in parallel with the rod and barely catches a finger nail. I pulled the heads and their was a score in the wall of the cylinder about 3/4" long from the top down. Sent the heads out to have them rebuilt and while their were at the shop thought about the score in the cylinder. Today I pulled the piston thinking I might have a broken ring and all of the rings look good. My question is should I replace the rings on this cylinder or just put it back togeather.
Thanks in advance, Terry

 

Are the rings recent? How old are the rings?
Hammer

 

It is never a good idea to put a piston with used rings back into a cylinder bore. The rings never line back up where they were and even though you would expect the rings to rotate in the piston groove during operation and find thier prior orientation that never seems to happen.
You are far enough into the engine that doing the complete job at this point would be a lot less time and money than trying to short cut it and have to go back and replace not only rings but gaskets etc. later.

 

The rings don't rotate? Are you serious? I have watched videos of several racing engines x-rayed while they ran that clearly show the rings rotating.

Thanks,

Garrity

 

Hammer, I am the third owner and their is 111k on the clock and have no history after 67k. I have found no evidence of any one going farther than the cams.

 

Garrity, I know you as a engine builder, what would you reccommend?

 

Hi Terrance.

I have to assume that you have an engine with original pistons rings. The one ring set that I see most worn out tends to be the oil control ring(s) believe it or not, but I always replace the set. I would replace them since they are widely available for you 928S4 for only about $20 a hole for OEM Goetz rings. Clean the pistons carefully with a soft plastic brush and place the new rings on them after checking the ring gap. Also coat the pistons with a nice spray of paintable moly spray, just avoid the ring grooves.

I would also use a nice hard stone to address the scratch on your rod jounal. It will only take about 5 minutes and give you piece of mind.

Not very expensive, just the right thing to do. Trust me you will sleep better!

Good luck,

Garrity

 

I'd send the crank out for inspection and polishing. Like $70.

Cylinder scratches bedevil everyone who rebuilds an engine. Gonna find them and they can't be honed out.

If there was a compression or leak-down test that'd give a clue as to whther a re-ring was really needed. Doing it now depends on your goals. Really does take a long time to get new ones seated.

Personally, I'd be tempted to put the rings back. Broken rings would show clearly on a compresion test.

 

I thought you had installed them recently. There's no way I'd use the old rings!
(unless you'd love to set a new World Blowby record!)
Hammer

 

The mathematics of the internal combustion engine can boggle the mind. If you have 110,000 miles on your car and it has averaged 50mph in its life, combined with turning an average of 2,000 rpm's, the rings have been up and down the cylinders 266 million times. Because it is a four stroke engine, the compression rings have been subjected to hellish flame and heat only 133 million times. Seems like enough. (Keep in mind this is a high average speed and a conservative rpm calculation. The slower the average speed and the higher the rpms only makes the numbers get worse.)

I can't remember the last time we did an engine and didn't change the rings. We consider the rings and bearings to be consumable parts, just like gaskets and seals. That being said, I'm sure that we are going to hear from a bunch of people that have reused their rings with "great" success. However, if we took a survey, we would probably find a group of people that routinely put on their dirty underwear after a shower, too.

greg brown

 

I want to thank everyone who responded. I have ordered ring sets for every hole and will remove the crank for polising and I guess while you are there, replace the mains.
My initial goal was just to refresh the rod bearings. I wasn't going to pull the heads but it is easier to do with the engine out, so when I did, found corrosion that had partially eaten into the ring seal of the head gasket on several cylinders, about 1/2 mile from blowing a head gasket.
I will feel better knowing that the motor went back together with everything being brought uo snuff.
Thanks again for your help, Terry

 

What is it with those head gaskets anyway ? Do they corrode?

 

Daniel,
The head was corroded and had to be puddle welded. The gasket also failed at the same locations but the aluminum rings were still intact. The corrosion was starting to run under the rings of the gasket. The only reason for failure that I can think of would be what was in the coolant.