Piston Damage

[YukonJack]

I have been a part of this forum for about 12 years now. In that time I have seen numerous postings and pictures of pistons that are destroyed. I myself wonder if the use of unleaded gas has anything to do with this. It has been said that unleaded gas won't hurt the valve seat because these engines came with hardened seats. But what about the pistons. I have never seen pistons get damaged like that without major abuse such as a valve coming loose or a high revving race engine. Anyone else have an opinion on this?  

[Don Coatney]

Interesting thought but difficult to answer. We no longer have a base line to judge by as all fuel today is unleaded. The engines I disassembled in the 60's were all filled with black goop as a result of leaded fuel and poor quality non detergent oil. However I do not recall any broken pistons with the top ring land wallowed out as we commonly see today in high mileage engines. Does not mean pistons were not broken then, just means I do not recall seeing them.

[TodFitch]

My opinion is that gas has little or nothing to do with it. An engine with lots of miles at low speeds/RPMs develops a ridge at the top of the cylinder. Now run it at some faster RPMs and the piston travels just a smidge farther and the top ring hits the ridge and bad things happen. That seems to explain why the damage usually broken top ring and broken top land on piston.

 

Now if you are doing a lot of "spark knock" you might have different damage to the piston and it might be related to the octane rating of the gas (more likely spark timing though). But that is not the same type of damage.

[Merle Coggins]

It may have to do with the fact that today's gas is more volitile making detonation a more likely candidate if your timing isn't just right.

 

Merle

[YukonJack]

My opinion is that gas has little or nothing to do with it. An engine with lots of miles at low speeds/RPMs develops a ridge at the top of the cylinder. Now run it at some faster RPMs and the piston travels just a smidge farther and the top ring hits the ridge and bad things happen. That seems to explain why the damage usually broken top ring and broken top land on piston.

 

Now if you are doing a lot of "spark knock" you might have different damage to the piston and it might be related to the octane rating of the gas (more likely spark timing though). But that is not the same type of damage.

I've never heard of the piston traveling farther as rpm increases.

[Don Coatney]

With excessive engine heat the piston will expand and travel further.

[greg g]

Merle, I'm gonna take a little exception to your suggestion about today's fuel.  The new stuff has fewer BTU's (at least the methanol stuff) supposeidly has a slower flame front, Whoomph instead of bang, and has less tendency to spark knock.  The average Compression ratio these days it nearly 10 to 1.  that was racing engine stuff growing up.  I have a tendency to think that piston damage evidenced by out high mileage engines has a lot to do with carbon build up from leaded gas, changing the cylinder compression ratio, or causing hot spots which caused pre ignition not related to incorrect ignition timing.  Also as the engines loosened up, piston travel might increase due to slackened tolerances allowing rings to make contact with the cylinder ridges.  But unless we are looking in there when the break, guess we may never know.

[Dave72dt]

I wonder how often in their past life someone has  done an overhaul by simply cutting the ridge out and re ringing the existing pistons.  Was piston land wear checked, was cylinder  taper checked, was the ridge even cut out?  Add to that was the wrist pin clearance checked as well as the rod brgs.  Some of those things add up to extra piston travel, plus ridges mean taper which means the rings move in and out as well as up and down which means additional wear on the lands.  New rings in old pistons means they don't move as much in the lands so any cylinder ridge can now be hit with the edge of a  ring.  Just a theory of mine, no facts to base it on.  Any of it sound logical?

[TodFitch]

I've never heard of the piston traveling farther as rpm increases.

I am guessing and no expert, but the higher RPM will put higher forces on the bearings. You are running a hydrostatic/plain bearing arrangement with a film of oil keeping the moving metal apart and the thickness of the oil film will likely change a little based on the load. Probably way less than 0.001". Same thing on the wrist pin bushing. Maybe only 0.0001" or even 0.00001" difference for everything combined but enough that the piston ring is tapping against the ridge at the top of the cylinder.

 

Don't know and I could be all wrong. It would not be the first time. But that is my guess.

[greg g]

a little bearing wear to make them undersized or maybe oval, ovaled out big end bearings, a little connecting rod stretch, worn wrist pin or wrist pin hole in the pistons, a little crankshaft whip, I could see a bang or two.

[YukonJack]

I am guessing and no expert, but the higher RPM will put higher forces on the bearings. You are running a hydrostatic/plain bearing arrangement with a film of oil keeping the moving metal apart and the thickness of the oil film will likely change a little based on the load. Probably way less than 0.001". Same thing on the wrist pin bushing. Maybe only 0.0001" or even 0.00001" difference for everything combined but enough that the piston ring is tapping against the ridge at the top of the cylinder.

 

Don't know and I could be all wrong. It would not be the first time. But that is my guess.

I agree that looseness in rod bearings could cause excessive travel in the pistons but would think the rods would be knocking pretty loud to cause that type of damage and the extended travel. Race engine builders used clay on top of pistons to check for piston/valve interference with high lift cams. That would serve no purpose if the pistons were to exceed their normal travel. Anyway, I was just curious. These damaged pistons seem like a pretty common occurance on these engines. I'm afraid to look inside mine.  

[Randy L Peterson]

Most of the engines I have tore apart.. Not just Mopars but some brand X.  The Pistons that had top rings broke and the ring lands wallered wide.. All had excessive Cylinder wall taper..    Lots of Miles and lack of Maintainence  and ate a lot of dust over the years..

[martybose]

I am guessing and no expert, but the higher RPM will put higher forces on the bearings. You are running a hydrostatic/plain bearing arrangement with a film of oil keeping the moving metal apart and the thickness of the oil film will likely change a little based on the load. Probably way less than 0.001". Same thing on the wrist pin bushing. Maybe only 0.0001" or even 0.00001" difference for everything combined but enough that the piston ring is tapping against the ridge at the top of the cylinder.

 

Don't know and I could be all wrong. It would not be the first time. But that is my guess.

It's purely anecdotal, but back in the 70's I worked on an 8500 RPM 440 wedge Mopar race engine that had a very carefully set zero deck height flat top pistons with steel rods and a small CC combustion chamber.  Any time we took it apart for updates or maintenance there would be an outline of the combustion chamber very lightly stamped into the top of the piston, indicating that the piston was coming higher out of the block than the head gasket thickness, which was 0.022".  We would just lightly sand the piston tops to eliminate the sharp edge and put it back together.

 

Marty

 

Marty

[Scruffy49]

How many of the broken pistons are just flat out OLD? Metal fatigue from so many heat cycles. Aluminum gets brittle as it ages...

[RobertKB]

How many of the broken pistons are just flat out OLD? Metal fatigue from so many heat cycles. Aluminum gets brittle as it ages...

 

This makes the most sense to me. Add in a little wear and..................voila!!!!!

[Dodgeb4ya]

Yep^^^^^