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Re: Which pistons are available for my ariel

Posted: Sat May 22, 2021 8:06 am
by ian.gallpen
Has anyone had any experience with having the piston skirt coated with friction reducing compounds such as Molybdenum Disulfide or high speed teflon? Triumph TR and MG engine rebuilders I have spoken to over here (Australia) swear by it. Given that friction leads to heat which leads to the piston expanding that can cause the piston to nip up/seize, I was considering using such a coating on a JP piston for my VH. JP's seem prone to nipping up. Cheers.

Re: Which pistons are available for my ariel

Posted: Sat May 22, 2021 11:28 am
by JohnnyBeckett
hi i have not used teflon on my ariels pistons but i have used teflon on my racing engines and it works well and they rev over 10,000 rpm

Re: Which pistons are available for my ariel

Posted: Sun May 23, 2021 2:45 am
by ian.gallpen
Thanks Johnny. Unless I hear anything to the contrary over the next week, I might try the Molybdenum Disulfide coating on the JP piston and report back. In the scheme of things it is not a big outlay (about $35A). Just for interest, what racing engines are you running. Cheers.

Re: Which pistons are available for my ariel

Posted: Fri Jun 30, 2023 8:27 am
by david.anderson
JP are no worse than any other brand with regards to expansion and in fact are better than many. However JP previously advised me that their 7.5 VH piston was the worst complained about piston in their range. Last I heard JP were advising a .005” piston to bore clearance, but in my experience that will result in a nip up. Even .006” is tight for Australian conditions especially with the scanty finning on the earlier cast iron cylinders. So I run .0065” without a problem. Many of the coating manufacturers advise that a closer bore tolerance is possible with their coating so if you work on a clearance of .0065” with the coating manufacturers recommended reduction you should be ok. I had a 10.5 to 1 forged piston made by Special Pistons in Melbourne and they recommended .007” clearance in an iron cylinder, but .006” with their moly coating. That coated piston is however in an alloy cylinder at a lesser clearance with no problems.
David

Re: Which pistons are available for my ariel

Posted: Sat Jul 01, 2023 12:26 am
by Simon.Gardiner
David

Have JP now re-worked their VH pistons, in the light of the complaints about them?
See comments in this thread including from Nev who's a bit closer than a lot of us are!
viewtopic.php?t=13063&start=10

SG

Re: Which pistons are available for my ariel

Posted: Sat Jul 01, 2023 2:37 am
by nevhunter
JP is under new management with in my view a much better approach. Slipper type pistons have reduced contact with the bore (obviously just by looking at them) There's not a lot of oil circulating in Ariels singles and they don't like high volume pumps either so I prefer a solid racing type piston with realistic clearances. and not too much cam to let more heat out into the cylinder wall. IF the motor is sleeved it was always the practice to allow another half thou. IF you're going for absolute minimum running clearances you risk seizure. A bit of silicon in the alloy helps reduce thermal expansion rate. IF you run a nikasil bore on an alloy cylinder you can reduce the clearance by about .002". The top 2" of many Ariel con rods show Blue colour so it's getting pretty HOT in there. Nev