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A hole in on

ernie

VOC Assistant Secretary
VOC Member
Great French Rally, merci a tous la bas.

I have a momento of the trip, anyone got one of these, I would like to discuss diagnosis and future prevention.


I just got back from France where the Rapide holed a piston.

Herewith picture.

What is the story here? Never had that problem before. All seemed fine last time I checked the plugs etc. not a big deal to repair but I do not want it to happen again. Shall I lift the needles a notch?

Standard carbs by the way and standard settings, standard B Rapide. +40 pistons in a rebore 3000 miles ago. And I have thrashed it since with no problem. I was at half throttle climbing up a shallow incline at about 70 in top. Warmish day but nothing exceptional. Any ideas?
 

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Ducdude

Well Known and Active Forum Website User
VOC Member
I have seen this kind of damage before on pistons in engines were there was a good bit of detonation or knock caused by poor fuel with not enough octane in it, hi compression in the cylinders caused by carbon deposits build up, and incorrect timing...Any one or combination of any of the above can cause pre ignition in the cylinder and that can actually remove piston material from the piston crown and eventually blow a hole like you see here.. Often times there will be tell tail small dimples in the head of the piston where the material was removed. Sometimes if you look at the sparkplug under magnification you will see very small balls of silver material attached to it, this material used to be in the piston... If this is your issue the solution is simple, find out the cause of the pre ignition and adjust as needed..Or it could just be a carb out adjustment…or a bit of both
Cheers
Eric
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
Didn't Bill Easter have the same on his way to the North American Rally? Maybe Steve D. has the scoop on Bill's bike?
 

Ian Scott

Active Forum Website User
Non-VOC Member
This may be of little assistance as i'm sure the correct heat range spark plugs were fitted but I thought I'd mention it. I've seen several cases of holes in pistons (not bikes though), this was a long time ago (early 60's), where a batch of Rover 110's coming out of the factory had an odd Champion N8 plug in no 6 cylinder, the other 5 having the correct N4's. This oddity came to light mainly when a misfire developed and a hole was found in no 6 piston. I've also seen this happen on other cars with a decent compression ratio in their day of 8:1 and over with the wrong plugs fitted.
Any chance the compression ratio was increased during the rebore?
Best of luck,
Ian.
 

Ady00

Forum Website User
Non-VOC Member
Hi

Great French Rally, merci a tous la bas.

I have a momento of the trip, anyone got one of these, I would like to discuss diagnosis and future prevention.


I just got back from France where the Rapide holed a piston.

Herewith picture.

What is the story here? Never had that problem before. All seemed fine last time I checked the plugs etc. not a big deal to repair but I do not want it to happen again. Shall I lift the needles a notch?

Standard carbs by the way and standard settings, standard B Rapide. +40 pistons in a rebore 3000 miles ago. And I have thrashed it since with no problem. I was at half throttle climbing up a shallow incline at about 70 in top. Warmish day but nothing exceptional. Any ideas?

Ive got a piston just like that from the front pot and the 'main' culprit turned out to be the ADT which to my untrained eye looked ok when I had the mag rebuilt. I was running on new std carbs and standard settings. All the best with getting it sorted.....Ady
 

ernie

VOC Assistant Secretary
VOC Member
Ian thank you. Don't think it was the plug - mind you NGK BP7ES very recently fitted - 1000 mile - before that it was B7ES. Both plugs looked good before I left. C/R is still standard B Rapide at 6.5 or so!
 

Ian Scott

Active Forum Website User
Non-VOC Member
Hi ernie,
maybe you should mention which cylinder had the failure(or have I missed this?), not that I will be able to help but it might help others trying to determine the cause. If I hear of anything relevent, I'll post it.
Regards,
Ian.
 

Bazlerker

Well Known and Active Forum Website User
VOC Member
On my bike the front runs slightly cooler than the rear.Air leak? Wots the plug look like?..is the center electrode real white and scared looking?..Have you had the head off - what do the valves look like?
 

clevtrev

Well Known and Active Forum Website User
VOC Member
On my bike the front runs slightly cooler than the rear.Air leak? Wots the plug look like?..is the center electrode real white and scared looking?..Have you had the head off - what do the valves look like?
I think Ernie would be a magician to take a picture of his piston without taking the head off.
 
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