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piston failure

nkt267

Well Known and Active Forum Website User
VOC Member
6 miles from home after a 100 mile round trip 2 up on the Comet this happened.
http://www.vincentownersclub.co.uk/photos/showphoto.php/photo/8023/title/piston-rim-failure/cat/807
The barrel shows corresponding marks on the timing side only, the drive side of the barrel is unmarked. Unfortunately I cannot get a decent photo of the barrel.
Does this look like detonation? I have never had a piston fail before, tell a lie I dropped the skirt off the piston on my B33 when I was 19.
Could this be due to the big end winding across to the timing side?
The plug I had in was slightly colder than the one I normally use, could this be a factor.
Any ideas on this subject welcome, even if it may be down to the nut that holds the handlebars:eek:..John
 

davidd

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VOC Member
John,

I have not seen this type of failure, although I am certain others on this forum have, but as I see it, thrust developed on the timing side of the piston and generated sufficient heat to vaporize the oil and then melt the piston. Possibly the centrality of the rod was not checked at assembly, or it became decentralized by reason of the thrust washer failing and wearing, etc.

I do not think the plug had a causal effect. It is unfortunate, but very impressive.

David
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
I have had similar mechanical malfeasance on my Comet.I was lucky enough to catch it just as it was nipping the oil ring in its groove.I doing 60 m.p.h. up a long grade,solo.I had done 3000 miles.Things I found:Slightly loose rear timing side head stud,evidence of blowing at the inner head joint and oil traces over the outer head joint,plug was a bit whitish.I was able to get home at 50 m.p.h. burning a bit of oil.What I think happened;The liner settled,upsetting the head joint.The possibility is that this allowed in enough air to upset the mixture,or perhaps with my rather unrestricted silencer the standard needle position was too lean.Yes,I had re-tightened the cylinder holding nuts.I am putting the needle up one step and going to a 210 main jet-bugger the expense!I have spent two hours making damn sure the head joint is o.k. and am putting in a brand new die-cast piston at .004" bottom skirt clearance.Previous piston was a 60 year old new Specialloid.Maybe it just didn't like anything other than Pool.I also proved the oil feed to the back of the piston.The con-rod was central and straight.
 

passenger0_0

Well Known and Active Forum Website User
VOC Member
Certainly looks like a severe detonation problem with all that metal sprayed off the piston - note the most severe damage occured in the zone above the top ring where there is trapped unburnt gas. Did the power drop off just prior to this problem occurring? Given it occurred on the same side as the spark plug suggests a local hot spot perhaps induced by running with an open throttle due to heavy load? Was the plug range too hot? Did you have an airleak in the inlet manifold or was it a really hot day? Over advanced ignition timing can also cause excessive cylinder pressures which would also cause this to occur. Many factors can influence the onset of detonation but all stem from running an engine too close to its thermodynamic limit. Good luck with the repair - David
 

mercurycrest

Well Known and Active Forum Website User
VOC Member
Any ideas on this subject welcome, even if it may be down to the nut that holds the handlebars:eek:..John
Well, John that pretty well sums it up.... btw, how long did you keep the throttle on from when it started to slow until it stopped altogether? When you get it fixed, make sure to specify the piston clearance to be .007, a Comet needs at the least .006 so I'm recommending a bit more for the mechanically challenged types. Sorry if this sounds trite, blame it on the vino.
Good Luck, John
 

nkt267

Well Known and Active Forum Website User
VOC Member
I was running fairly briskly 2 up, but this is not unusual for me and as I have done somewhere in the region of 5-6 thousand miles on the Comet would not have thought it a problem. The siezure was sudden with no sign of power drop off beforehand, and I stopped the engine immediately.
There is a possibility of an air leak at the head joint but as I have had an oil leak from the top end I cannot be sure.
When the liner was bored I specified 5-6thou clearance on the bore, so hopefully I was erring on the right side.
I was very careful with centering the conrod/flywheel assy, I ground out an old drive side bearing to give a sliding fit on the mainshaft to set up the shims.
I am concerned about the big end side plates though which is why I am going to fit a new bigend and conrod to my spare flywheels. When I look down at the big end I can see part of the bigend roller ring on the drive side of the con rod, but when I push the rod over I cannot see any on the timing side.
Either way the engine now needs a re-build I just do not want have a repeat of this problem in another 6000 miles..John
 

passenger0_0

Well Known and Active Forum Website User
VOC Member
Hi John, Sounds like you had the right clearance for the specialloid piston. As mentioned earlier, this problem did not originate as a piston seizure, rather it is a detonation issue so the big-end problem is something secondary. I've had this sort of problem before in other British bike engines with the most persistent cause being the spark plug not having sufficiently depth into the combustion chamber. In my race twin engine I had to counterbore both plug holes an additional 4 mm to get the plugs to the correct depth. The location of the failure in the piston is the main clue and I suggest you very carefully look in both the head and cylinder for any signs of local overheating or 'pepper marks' indication detonation damage. I note that both the head and liner around the raised lip are both blackened which indicates they may have been leaking here. Just out of interest there is plently of carbon on the piston so she obviously has done some miles before this problem occurred. Cheers David
 

davidd

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VOC Member
The only odd evidence I see is side thrust. I would expect detonation, but I am not smart enough to know if it were the cause or the result. A white hot piston will detonate any incoming mixture and the air leaks could have been there or be the result of the wild heating that was developing. You will have to check everything for straight and true, but it sounds like it was built correctly. If I were building a Comet, I would not use a Specialoid. I would opt for a CP or Arias and have the ceramic coating, not because of the failure, but because the single is subject to local heating issues in general when big loads are applied. I think the big end may tell you quite a bit about the side thrust. The piston generates 36% of the friction in an engine, so when things go wrong with it, it is always bad.

David
 

roy the mechanic

Well Known and Active Forum Website User
VOC Member
just a thought, is the rod straight, also is the bore 90 degrees to the base of the muff? it will be difficult to test with the debris ,but will be worth the effort on rebuild.
 
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