piston failure

[mercurycrest]

That had to be around 650-700C to melt like that! What shade of blue is the rod? ;-) I've stuck two Omega/VOC pistons at .004 with milages you'd expect things to be settled in by & two up on a Comet, but they were nothing like what yours is. The second one was even useable again with a bit of cleaning up. 'Went to .006+ and end of problem.... knock wood. I'd expect the little end to have enough side play to make up for any sidways error there. Roy has a good idea about the rod being bent. That would certainly give sidethrust. Hey, at least it's better looking than Tony Wheatley's
Cheers, John

 

[nkt267]

Taking a good look at the conrod it is nicely polished all bar the top 2 inches which is dark straw to brown so it looks like the small end may have started to bind up.
The engine in my Comet used to be in a friend of mines Comet,my Comet was then a spares supply. We changed it over in 1970 as the bigend eye broke and cracked the back of the crankcase. Where my original cases are now is unknown..John

 

[davidd]

I have never run an Omega, but I know they provide good service. I am not convinced that it is the fault of the piston or your assembly. This leaves the big end and the cylinder as suspects.

David

 

[passenger0_0]

Can't say there's anything wrong with using Specialloids or Omega for that matter as long as they are either new or really good condition and been crack tested. I certainly would not go for ceramic coating as this reduces heat flow through the piston crown and raises temperatures in the chamber to potentially above critical levels. I found this out in a racing Triumph triple engine many years ago where the spark plug overheated and so much molten piston aluminium blew out the inlet trumpet that all slides jammed open. My recomendation would be to repair the damage, even fit a replacement Specialloid or Omega piston but restore the liner to head seal. By the way - have you checked that spark plug depth I mentioned earlier? Cheers David

 

[davidd]

David,

My experience with Vincents is exactly the opposite. Carleton Palmer was using Ian Hamilton's piston design, but after racing it for a while the area between the crown and the top ring groove showed distress at the exhaust valve. He lowered the rings and coated the pistons. The life of the pistons trippled and he was able to eventualy get seven addition horsepower out of the engine with no problems. The additional power came from other mods, but the bike got three national championships with excellent reliability. I will say that it has been remarked that the coating in general makes the combustion chamber cooler as the heat should be evacuated in the exhaust, thus allowing for a denser intake charge. I know nothing about Triumph combustion chambers, but the Vincent seems to do OK.

Getting back to the thread, If you work a Comet hard, you are entering an area where you need to do better than stock. The previous owner of my bike used a Specialoid that broke at the wrist pin, breaking the rod. I know that I am being unfair, but I see no reason to run a Specialoid. I think the Omega is fine, but I am not certain that it can do the best job for what NKT267 is doing, particularly if they have stuck in the past. I also think that there is something else going wrong with the engine to cause this failure. I would be curious about the thrust washers in the big end and the interference fit of the liner. The Omegas are only as good as the cylinder. I wonder if all that oil on the head joint has come up the inside of the liner.

Carlton Palmer also found that the wrist pin clearance is too tight for racing. He had Carrilo add a half thou and the heat discoloration and tightness went away. I do not know if this would be an issue on the street.

I do like you comment on the spark plugs. This is easily checked before assembly and your point is well taken.

David

 

[nkt267]

By the way - have you checked that spark plug depth I mentioned earlier? Cheers David

I have checked the sparkplug in the head and the end of the thread just protrudes inside the dome, for some reason I fitted an NGK B8ES instead of the usual B6ES but as this is a cooler plug I would not think it to be a contributing factor.
Interestingly I was watching 'How It's Made' on spakplugs and saw that the electrode is sealed in with some sort of loctite, I now find that all around the top screw on the plug is covered with a blue substance. I have cleaned the piston crown with a scotchbrite pad and will post some more pics of the piston, as far as I can see there is no damage to the crown at all apart from near the edge damage..John

 

[vince998]

Judging by the pictures, your mixture seems about right if a little on the rich side.
What i do notice is you had a leak at the liner head joint directly in front of the damage on the picture. (i think the oil was pumped up after the ring seats vaporised)
The damaged area on the piston is also nearest to the spark plug position so the first area to be hit by the flame front.
Maybe a localised leaning out of mixture caused by the leak (air being sucked in on the intake stroke) was the problem? (not enough to give signs of overheating, but enough for this specific area)
I´ve seen signs of detonation on many pistons, but normally at the far side of the flame front, and never to this extent (maybe once the edge of the crown started to melt, friction took over and did the rest. aluminium being a very good heat conductor means it doesn´t take long for localised heat to spread )