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[ChrisD]

Well, guys. I went with my recently rebuilt 1996 535cc Bullet (new roller bigend and new mains fitted ~1000km ago) on an extended 4-day road trip around the mountains of the Western Cape. And it went well, comfortably doing over 70mph at half throttle on a windless day and accelerating well (I have the 19T gearbox sprocket so that was just 4000rpm). Along the way, one of the fellow riders pointed out that the front wheel was tilted a few degrees compared to the back wheel which put a bit of a damper on my feeling of pleasure (now I know why it snakes when overloaded).

BUT almost home again there was an awful clacking crunching sound and I lost power. Got it home on the trailer and started stripping the engine. The first clue was the oil filter being overfull of aluminium and iron. I had two of MrH’s magnets inside the filter and they looked furry, like old tennis balls. Then there were flakes of aluminium and iron pieces in the timing chest and partly blocking up the rocker feed pipes. There were even a few pieces of aluminium lying on top of the piston….Where did they bits come from?


When I got the barrel off, I found pieces of a shattered gudgeon-pin circlip - ooops. Sods law would have it too that the pin had moved far enough out of the piston to not only ruin my lovely alloy barrel but also allow the piston to tilt so that one side was badly eroded. So now I know how bits of alloy got into the compression chamber.

All three of the piston lands have collapsed (oil ring most evident in photo) so that I’ll have to break the rings to remove, and there’s been massive blowby so everything is oiled outside and coked inside. Look at the faces of the rings, completely stuffed. The top of the piston has an evident crack - which fortuitously hadn’t completed otherwise the damage would have been much worse.
And I haven’t yet had the courage to finish the strip to see if my lovely new bearings are also stuffed.
That was a really expensive trip.
Cheers, ChrisD

[Gwilly]

Kin el chris, thats a real downer, so sorry mate..

Just guessing at this stage but i wonder if the crank was not quite centred to the barrel, or the rod is slightly bent.

Just that its usually some sort of sideways force to bust out a circlip or even the groove shoulder from the piston..

Nice photos, just wish the subject was more cheerful.. regards gwilly.

[ChrisD]

Gwilly - yes, I'd had my suspicions about the conrod and now am about to bite the bullet and get a new steel conrod from Mr H. I've had the same indian conrod since new (~15000km) and had no other issues with it but who knows - maybe the addition of proper English parts to a crap indian engine confused it! Maybe it was alll the extra power. On the timing side of the piston you can see where the circlip broke its way out of the groove and again squashed the lands. I hope the alloy barrel can be saved - it will only allow for a 20thou overbore and cannot be resleeved.



I've also heard it said that the angles of the crown of the forged piston are a place for detonation because they get extra hot (Ace Cafe says so) so I'll plan to file them off. - at least I now have one of those pistons to find out how much I can remove on the grinder. Actually I have another of the accralite forged pistons (a 500) – which kept seizing because the manufacturers said only 6thou clearance was needed in a cast iron bore. Current thinking now is 8thou.
Cheers, ChrisD

[jefrs]

Combustion and detonation are often (usually) misunderstood. The fuel mix burns because the compression pushes the molecules closer together reducing the flame path, the gasses expand and it is this expansion (not explosion per se)that propels the piston down the barrel. Detonation occurs where the combustion goes supersonic and material cannot be pushed away but gets shattered instead. This can occur at peripheries especially where the exhaust and induction cycles are unable to sufficiently cool the system, glowing carbonised combustion products don't help (the piston appears to be designed with a crevice to collect crud); the difference of cold gas to hot gas being what produces power. Mixture, compression ratio and RON/octane are used to produce an even flame path. Detonation is heard as 'pinking'. No obvious signs of that happening on the piston crown.

New piston time, don't be in a hurry to take the bench grinder to it.

The circlip failure appears to be a purely mechanical parting of the way.

[jefrs]

Deepest sympathies by the way.

I've got the remains of a stock D14/4 Bantam piston 10:1(?) on my desk that was destroyed by detonation donkey's years ago. She got re-bored and an even higher compression comp piston fitted and she never pinked again.

[Leon Novello]

I would be thinking about nylon plugs instead of circlips, they don`t shatter.

[Tim NZ]

Detonation will destroy piston rings and lands, and can erode away and 'blow' out the circlip, which is the point of greatest mass and heat and thus most prone to the initiation of detonation. All of which can happen in seconds in a Hot motor, even when idling down...


Other wise a displaced circlip is fitter error. How deep is the gauge in the barrel?


Those bits of blown piston have reached into every corner of the engine. You should pull it down and thoroughly clean and inspect the main bearings, and EVERY oil way.


4000rpm and half throttle: you probably need to look at raising the needle or fitting a larger needle jet, along with radiusing the edges of the new piston valve cut-away's.


IF your con-rod was bent you would have an angled wear pattern across the thrust face of the skirt which would have been evident withing an hour of the motor being run? Which would also have been evident on the previous piston, AND the timing side main bearings would probably have been dead within 10,000km?


If you were seizing the previous forged piston when running with +0.006" clearance, you had other issues associated with tuning/mixture/cooling.

[Presto]

If it was metal ‘spurs’ (sharp edges) on the piston that created this problem then would it not be a case of pre-ignition, not detonation. Pre-ignition is combustion of fresh fuel/air mixture by an incandescent ‘hot-spot’ in the combustion chamber area. In pre-ignition the fuel mixture is combusted by the excessive heat of this ‘hot spot’ before normal ignition, generated by the sparking plug, occurs. Incandescence in the combustion chamber may be caused by a heavy build-up of carbon deposit, a ‘spur’ of metal that glows with excessive heat, or a spark plug of the wrong heat range that also may glow with excessive heat. But either way, as TimNZ suggest, this engine probably was not correctly ‘tuned’ and a possible the root cause may be found to be incorrect carburation – in which case the problem may indeed have been detonation (not pre-ignition, and not due to the sharp edges on the piston!).

[ed.lazda]

When I first got my Bullet 350, it always seemed down a bit on power. I did all the usual things with carburation, timing, etc. Eventually, the guy in the shop I bought it from told me it had some piston slap (I hadn't noticed, but I wouldn't have known what it sounded like). So I stripped it down and found one of the gudgeon pin circlips was adrift and had scored the barrel. It must have been like that for 10,000 miles. A replacement barrel and piston later, and it was running like I always thought it should.

[Barry N]

Looks very much like there is "nibbling" caused by detonation around the outer edge of the piston crown (top picture) around the inlet valve pocket.