JP high compression Piston for Ariel failed again

[@goog2231]

I heard a while ago that the alloy used for these pistons is for water cooled engines so expands at the wrong rate for air cooled ones dont know if this is true or not.

[@4ager505]

After you pulled the cylinder, I noticed the 1st (top) and 2nd compression rings had their end gaps closely lined up, and located in the general area of the damage to the piston. I was taught to stagger the end gaps evenly around the periphery of the piston. Could your end gap locations be a clue to the nature of the problem? Also, I agree with others on here, looks like things have been running quite rich, which can cause problems.

[@thra5herxb12s]

Yes I always stagger piston rings. It's quite normal for rings to revolve as the engine runs, but this isn't how I fitted them though, and it's just coincidence that they are almost in line.. I explained why I set it to run rich, but I've never known a rich running engine to break a piston after a couple of hundred miles, and the last one went after 40 miles and wasn't running rich.

[@johnmurray7682]

A ring land under the top ring that's pinched or broken is a classic symptom of detonation. It's going to keep doing it while ever you let it detonate. And rich mixture won't always guarantee it won't detonate.

[@thra5herxb12s]

Yes the most comments suggest detonation, but I haven't been aware of any detonation or pinking, and it certainly wouldn't kill a new piston after 40 miles as in the previous failure, or would it. I have a compression plate under the cylinder already as can be seen in the video.

[@johnmurray7682]

@@thra5herxb12s check the timing with a strobe to make sure it's correct and stable over the rev range. The det most likely is caused by preignition. The baked-on oil under the crown after only 250 miles indicates it has been pretty hot. There was nothing wrong with the piston(s) but there is something seriously wrong with the tune. Lean it back to where it should be, get the timing right (slightly retarded from stock if the CR has been increased) and make sure there are no preignition sources eg. too-hot plug or overhanging head gasket edge. Check you have enough ring gap. Cast pistons like JPs mightn't be as tough as forged ones but it takes a lot of abuse to crack the lands and they should last a very long time in an application like this.

[@thra5herxb12s]

@@johnmurray7682 The timing is adjusted manually from a handlebar lever during riding, and is always ridden when it's at the sweet spot. There's no baked on oil under the piston, it's just the way it looks in the video, I have no head gasket, the cylinder is ground into the head, and the ring gap is a couple of thou bigger than the makers suggest, because that's the way they came out of the box. Like you say, it takes a lot of abuse to break a piston, and this has only had very gentle riding while it was being run in, as did the last one that failed in the same way at 40 miles. Any piston should be able to last long enough to adjust the settings during tuning.

[@steven-vn9ui]

There's running rich and running majorly too rich. I'm no expert but that amount of fuel seems unnecessary. I hope you manage to crack the problem. Lovely bike.

[@FLAPARON]

Use High octane fuel, or reduce your compression. You have Detonation (Pinking)

[@thra5herxb12s]

I did ask on many forums if a 9:1 compression ratio is too high for Super pump gas, and everyone said it would be fine at 98 octane in the UK. I dont seem to have any pinking, and others have ridden the bike at events and not mentioned it. It's only these pistons that I have a problem with. But I'm taking everything on board, because it's the first time I've tuned one of these engines for sprinting.

[@clivewilliams3661]

I presume the cylinder has been honed and that the piston and ring clearance is correct? i would be expecting a ring clearance of around 5thou per inch of bore. i would also check the ring gap at three position of the stroke to check for bore taper.

[@thra5herxb12s]

Yes all done by Godden Engineering and Conway Motors. It's only these pistons that break, so I just ordered a different make that will still give me the same compression ratio. I'm taking on board everyones suggestions, and it keeps coming back to the piston quality.

[@powerbikepete]

Have you tried john harding for a piston . He had one for my 37 triumph 3h

[@royferntorp]

The gaps are good. The rings are pretty much standard. I believe there is a suzuki piston that works.

[@thra5herxb12s]

Suzuki pistons are all flat topped. They wouldn't give a high compression with the hemispherical head on my old engine. It has to be a domed top piston.

[@royferntorp]

@@thra5herxb12s There is a 2 stroke piston that might work. 46mm bore

[@classicrider5274]

Sorry to see this again Dean. Have you considered the Ariel club piston for the rebuild? I think they did a fair amount of testing before they went on sale, with good results. Cheers Paul

[@thra5herxb12s]

They dont make one and have no plans to. If they did they would sell like hot cakes. I've just refitted my Gandini 7:1.

[@classicrider5274]

@@thra5herxb12s they were advertising a club arranged product at the end of last year for the VH in standard and oversizes.

[@thra5herxb12s]

@@classicrider5274 Yes I saw those advertised at very good prices as well, but I dont want a VH piston though, I can get those anywhere. It's an HS piston I need. Also a major snag at the moment, is there arent any singles spares available at the moment until a new volunteer steps forward to take on the task .👍

[@classicrider5274]

@@thra5herxb12s ok, got it👍

[@ANGRY_AMERICAN]

See if Wisco can do a forged piston with coated skirts and that amount of skirt marking is not good for only 250 miles is a balance or machining problem because cast can have different expansion depending on alloy then you have Hypernetic and forged with each require a different clearance and warm up time. I had a 250 2stroke that had less skirt scuffing after 3years of play/practice and racing. If you have cylinder bored again take it to a true speed shop and take the whole engine so the machinist can make sure the bore is perfectly centered because a crooked bore will destroy a motor fast. For the ring landing to break like that is poor materials a ridge at the top of cylinder or severe pre ignition. Anyone can build high HP engine and the difference between 250mil 250K Miles is in the machine work. I know of a synthetic additive that could also help with wear if you want to know ask

[@thra5herxb12s]

Thanks. Yes all maching has been done by my local engineers, who are Godden Engineering, started by Don Godden who designed and built the speedway engines, so they have a good idea when it comes to preparing sprint motors. They are also Conway Motors who are the Vincent specialists, so their attention to detail is very high and I have no doubt about the quality of their workmanship. It's only these pistons that I have a problem with.

[@misscrabstick]

Bad materials is my guess, the only time I have seen lands come off was on a Vauxhall Zafira that had been massively overheated and driven for a distance because the customer was an idiot.

[@thra5herxb12s]

I spent about 3 hours preparing this piston before I fitted it. It was very badly made with loose casting flashes on the inside and very sharp machining edges everywhere else. This is the third one that's gone in the same way. When the last one went, I refitted my 7:1 Gandini and rode over 200 miles to an event, and raced all weekend before riding 200 miles back again. I've just put the Gandini back in and it's still in perfect condition.

[@hiha2108]

Broken lands ar usually knocking damages.

[@PhotoBilly]

looks like tight ring gap to me...

[@nkelly.9]

JP pistons are not renowned for their quality