355 piston ring issues

I'm curious, how did you rule out valves (and guides). I would guess that rings would almost never fail on cars that don't cover many miles.

I recently considered buying a car with 20% leakdown on 1 cylinder and was mentally budgeting $20k for a major and for valves guides. If that particular car also needed rings, that would have definitely put me upside down on day 1.

 

Agree with roadracer. How did you rule out the valve quides. The thing that would cause a bad cylinder and or rings in that cylinder is badly leaking headers. Otherwise the 355 rings are pretty durable.

 

Talk to enginfixer. I just had a rebuild and Gary Sharpe thinks that rings are a problem--wearing cylinder walls and removing the coating of the cylinders

 

Yes they will if the headers are leaking and causing a wash of that cylinder. In that case the oil is weakened by the fuel that isnt burning. I had my heads done recently and while the heads were off, we checked for any wear lines in the cylinders. There were none and there was no sign of a ridge at the top of the stroke. The sleves were perfect in an engine with about 26,000 miles or 40,000 kms.

 

Perform a leak down with the heads off. We do it on every motor that is going together or coming apart.

 

Rebuilt 3 355's recently with the same problems. I have a 360 in the shop now with no wear, but do have another 355 that shows some wear. In each case, either I, or the mechanic that was taking the heads off found worn cylinders. In each case, the heads were being taken off to replace guides/valves. The common problem with each motor had been that sometime in their lives, the headers had been bad. A common cause? Not sure because I have had too few examples to say for sure, but it points in that direction. The highest mileage car I've had to do a full rebuild to because of the ring/liner problems had 40k, the lowest 17k.

 

I always thought it was not difficult to diagnose rings vs. intake vs. exhaust leak with leakdown test. When pressurized, the air will leak out of the cylinder via the faulty area. IE: If the rings are bad, the leak will be into the crankcase; if the exhaust valves/guides are bad, the leak will be heard in the exhaust; and if the intake valves/guides are bad the leak will be heard in the intake tract. This is the value of a leakdown test as opposed to just a compression check (the ability to know WHERE the fault lies, not just that there is a fault).

I think this is why the OP knows the car in question has bad rings.

John

 

Not really. If you have 80% leakage from bad guides which is not unusual you will not be able to tell if the rings are leaking. That situation is exactly why leak down testers that bolt to the block deck were invented.

 

I guess if you have bad initial leak down results, there could be more than one problem. In that situation, would expect air to leak from worst site, but might not notice second air leak site. Still, GOOD leakdown results are what we should be looking for when considering purchases, right?

John

 

Correct but when it is very bad the extent of required repairs cannot be known with certainty until the heads are removed.