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Hole in piston

This happened after a couple of minutes at 70mph some years ago. I blamed preignition at the time, but I heard no pinking. It permanently knocked my confidence in the bike. But there is no sign of preignition burning the carbon off the piston. Vincent's 'Tuning for Speed' says the piston opposite the plug will look as of the mice have been at ot. But it's not like that.  I think the crown was too thin. Any views? The undamaged one has no sign of burning. The damaged one had a heavy dose of oil when it failed.

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Thats interesting, I think you are right , a thin top, perhaps cracked first then burnt thro the crack.What make pistons?, Look as if they have been worked on a lathe ,maybe skimmed off the top?.

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JP pistons. I found a hairline crack up from the pin in a previous one a couple of years ago. I don't believe it was pinking. Electronic ignition set by hand and tested by strobe, and never hear any knocking or pinking. I am familiar with the sound! It's an 88SS on +30, for which pistons are hard to find. I have a pair of NOS Heppolites low compression ones. I should fit them to be doubly safe.

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Never heard good stories about JP pistons, the one I have direct experience of had a missed operation, the clearance area at the ring area had been missed and was at the same diameter as the skirt. I witnessed the result, a quiet tickover on first start-up followed by seizure 20 yards later. And those appalling stiff rings that draw blood of removed by hand.

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I had almost exactly the same happen to my Atlas engine. I was trundling off to work down the M27 at a steady 65 mph when bang, followed by loads of white smoke. An engine strip revealed a similar looking piston top to Dave's. A radial crack with a chunk missing on the crack line.  The sides of this piston had just a little scoring, suggesting that a seizure had not been the main reason for the failure.

What surprised me was this engine (& piston) had run with no problems for 23,000 miles of mixed motoring. Not too far away, was an AE component supplier so I headed off to see them to ask an opinion. They had a demised piston chart but none of the pictures matched any of those shown with my example.  The salesman suggested that my piston had perhaps been cast with impurities in the metal mix which caused a weak spot. This is possible but why did the Big Bang not occur much sooner?  

My own theory is a combination of weather conditions, ignition timing and carburation were the cause.  That morning the weather was heavy, over-cast and humid conditions. The bike had twin carbs and magneto igintion; both well known for supplying varying fuel mixes and firing points. My money would now go on pre-ignition for causing the piston failure. I fitted a single carb and electronic igintion and never had the problem return.

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In this case I blame metal fatigue. Many thousands of stress cycles, all inside the strength of the material, but damage accumulates.

My Heppolite piston crown thickness: 6.5mm. The JP piston: 4.3mm.

Since stress is proportion to the square of the inverse of thickness, a 50% bigger thickness is less than half the stress, and maybe Heppolite could have 8 to 10 times the fatigue life.

After 5 years it's maybe a bit late to ask for money back?

Maybe I shouldn't be to hasty in entirely blaming JP.  The first piston problem was within a year of fitting a full flow oil filter. I think it is underlubricating. There is damage on the front and back faced of the problem piston. I assumed it followed the hole...but that makes no sense. There was oil everywhere as combustion pressure blew it all over the place.

It's time I removed the barrel and inspected again.

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I have had 3 similar failures over the years, 88, 99 and Atlas. With the 88 and 99, a piston cracked and then holed. The cylinder kept firing and the combustion flame torched the small end of the rod which opened out and let go. The cause on the 99 was an asymetric mag cam ring and over advanced ignition on one side. With son's 88, it was cheapest low octane unleaded, 2 up on a very hot day on the autobahn so the engine was cooked and both piston crowns collapsed then the rod let go. Pre-ignition probably. With the Atlas, no obvious cause. A quick rebore and all was once again well.   

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Dave.......there is a 99 left-hand piston for sale on eBay this week. It is 68mm + 30"in very good condition with a £30 price tag. The downside being it looks like the make is GPM.

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Thanks Phil. But I think I'll fit my NOS Heppolites. They are low comp. That should give comp ratio 8.5 instead of 9.45. Should be plenty, and safer.

I have a full flow filter. But that is on return side. If it constricted flow, the symptoms would surely be wet sumping when running? So many options!

 

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