Which pistons are available for my ariel

[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.

[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

[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.

[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

[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

[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

[KenS]

I have a 1946 VG 500 single and in the process of its restoration and riding it for its second trip around my back yard, it heat seized. After an hour, I was able to turn it over aging with the kick start. I had originally assembled the engine in 1987 with the parts I had, and not checking cylinder or piston clearances as I assumed the clearances were correct, and I did not have access to an inside and outside micrometer at that time. Today, I do have these tools and after I disassembled the engine to see what was happening, I discovered very minor areas on and around the Gudgeon pin indicate signs of wear and zero markings in the cylinder barrel.
I have measured my diameters: BORE 3.2703"/83.066mm PISTON 3.2667"/82.974mm and calculated clearances and have a mere 0.0036", insufficient according to what I see on Dragonfly's Piston web page. When looking at a replacement piston and ring set, I noticed that my piston, which has convex top and a slit in the skirt, the replacement is domed.
Can someone tell me what piston I have (attached photo), and will the VH500 replacement (Piston, complete, 7.5:1, 81.8mm +.040) work in my application?

[KenS]

Inside my piston are molded lettering as follows:
D54
HEPLEX
11497 BA
413
PatNo 616320

[Simon.Gardiner]

11497 is the 5.6:1 'dished' (concave) piston (originally used for the trials bikes, I believe). At one time they were the only ones still commonly available.
There's an extract from the 1955 Hepolite catalogue - with Ariel piston info - in the online library (if you've not found it already!):
https://www.arielownersmcc.com/members_ ... /ariel.pdf

The 7.5:1 (10503 equivalent) will work fine as long as you can get the clearances sorted out. It looks like a JP, which some of us are a bit uncomfortable with - however I've seen it posted somewhere that they've now changed some of the piston specs to make them less prone to tightening up.

Hope that helps

SG

[Roger Gwynn]

The expansion rates of the material that JP have always used are almost identical to those quoted by Hepworth and Grandage, the only real criticism of the JP pistons is the extra weight caused by using a parallel bored pin, rather than a tapered one. My advice is to use an old pin if a good one is available.
A split skirt piston should use .002/.004 clearance and a solid skirt one .005/.007. So you will need to have the bore honed out to suit the new piston or clean up the one you have and put it back. That is an odd place to seize.