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[Leon Novello]

One way to lower compression and lubricate the upper cylinder.
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[Creaky45]

Ric, I presume you mean the bottom 2 pics of my latest failure, the 500 original piston has just failed for no other reason other than it is past its use by date. (no fault of tuning or timing) whereas the top 2 pics happened 2 years ago 5000km after rebuilding the motor with Hitchcocks bearings etc. etc including big bore kit with alloy barrel and piston. (By a qualified mechanic, not me). As far as we could tell, timing and tuning was ok then also. Anyone want to buy a 535 alloy barrel, only done 5000km in as new condition? I will throw in the holy piston too.

[Alan R]

Hi CREAKY--- can you remember what Compression Ratio ( CR ) you were using , what grade of fuel and plug plus had the Cyl. Head been re-worked to match that pent-shaped piston ??.......Assuming that's the condition of the piston as removed then those two photos tell a very distinct story....The underside view suggests that a piece of piston was PUNCHED through by a physical force ie}--- it's not melted.. Yes, there are signs of a localised overheating with the greater part being near what I presume to be the exhaust valve area.....Conversely, on the crown view we see the opposite valve recess is clean ie}--- has been running relatively cooler... Also, given the overall clean condition and lack of any yellow oil stains I think this "Event" occurred not long after the engine was re-built.?? Given the prevailing circumstances within that combustion chamber at the time, I think that piston did very well to last as long as it did--- a cast one of the same shape would have given way much sooner and in a more spectacular way... IMHO that is.

[Revband]

I was planning to keep out of this thread!!!, BUT that first piston is typical of a badly tuned motor, yes it did overheat, to say melted is not quite correct, the alloy reaches a temperature where it weakens and the blast of the burning mixture blows the hole in it that is why they often look as if something hit the crown.

The root cause is often people tune an engine by just fitting oversize higher compression pistons and don't consider that other things need changing to suit, in a higher compression engine the spark plug will normally need to be changed to a more suitable type, the fuel mixture will also need to be adjusted to suit, it could be either or both these things which have caused the damage, but looking at it the hot spot has been towards the inlet valve which indicates the mixture as the culprit, had the hole been under the spark plug that would have been the main culprit, tuning these engines is not really to be done lightly by the home tuner unless they really know what is required.

[Alan R]

Hi REVBAND-----my thoughts as well although as the L/H side is darker ( Hotter ) therefore that's the exhaust valve side..??

[Gwilly]

Just going by the arrow marked on the piston, I'd say burnt towards the inlet valve Alan.

[Revband]

Yes it's the inlet you can see the Recess for the valve head is bigger.

[Creaky45]

Alan and revband, You blokes seem to know your stuff. I bought this bike with a sidecar a few years ago and knew nothing about Enfields. I joined a group and one bloke was a competent Enfield mechanic with a good record. He rebuilt the motor so it would have more power for the sidecar, hence the big bore kit. The compression ratio would have been high but I always used 95 fuel as recommended. I understand timing well so I know that was spot on. My main jet for memory was a 125 using a standard 28mm Mikcarb and, no I had done nothing to the head except I had the valve guides replaced and the seats done by an motorcycle engineering mob. In hindsight I don't think high compression was a good idea slugging along with a sidecar even though I had a 16 tooth gearbox sprocket. When it blew I did have a passenger but I wasn't particularly thrashing it. I could not afford to replace the forged piston so that is why I put the original one and the iron barrel back on. They had only done 5000km. Maybe I should have just not rebuilt the motor but we live and learn. The bottom pics are that piston after doing about 20000km. No-one including Hitchcocks seems to know why the piston blew. Just remembered I did fit a pod filter but that was at least 1000km before the big bang. Probably had something to do with it but my plug (a BR8ES) always looked good.

[Revband]

Creaky, I think TimNZ. Has already answered you question re the original piston, in short, bad design, I think your holed piston main culprit is running too lean, but never easy to give a definitive answer without actually seeing the motor. Perhaps you man is not the "expert" people think he is?.

[papasmurf]

Personally I would not tune anything that involved any Indian made components in the engine. (Having yesterday seen the destroyed components from the inside of a Chinese produced motorcycle engine, I would include Chinese made components as well.)
Having tuned engines myself back when dinosaurs ruled the earth. Every component needs checking closely and accurately measured and weighed. Orifices need to be polished and matched if more then one component is involved.
(As for aluminium pistons, someone jokingly said in a workshop I think this ally is made out of melted aluminium saucepans, and to be proven right when the tip of the turning tool removed the steel rivet use to attach the handle to the saucepan.)