JP Piston

[charles.bulloch]

Hi, all. I have run 900 miles on a JP piston in my 38 VH engine and it still wants to nip up. I asked the engineer to bore to JP's recommendation which is 0.004", however apparently this group recommends 0.006" clearance. I will dismantle the machine shortly. If I have the bore honed, will I have to run it in all over again?

Charlie

[klaus gerhard]

Hi Charlie,
I have fitted a JP piston to my 1938 VH shortly. I was stongly advised to give it a clearance of 12/100 mm. The motor has done some 200 Kms so far and it has no tendency to nip up. I will participate at the Austrian rally the comming weekend and will be able to test the bike on the runs.
Hope this helps.
Regards KLaus

[charles.bulloch]

Thanks, Klaus, that is equivalent to 0.0047"

[nevhunter]

Charles... answer NO. A honed bore is better than a glazed one. Your ring gaps will increase a bit. Roughly 3 x what the hone removes on diameter.. A bit of two stroke oil in the fuel helps the run in. It doesn't need much. About 50 Ml to a tank. I personally thing those fits are a bit on the tight side. If the motor has been sleeved you usually add an extra thou for the loss of heat transfer.. Nev.

[david.anderson]

Charles
The information that JP gave me was .006” - .007” with the bigger clearance being for the 7.5 piston whereas .006” was satisfactory for the 9 to 1 piston in an iron cylinder. .004” was the clearance for an alloy cylinder. They advised me that their most complained about piston in their entire range was the 7.5 Ariel piston and did state that they were going to alter the grind to help the situation. That may have happened. However for Australian conditions I would never attempt to run less than .0065”, English conditions maybe .006”. I am running a 9 to 1 JP in A 57VH at .0065” with no problems. With the AOMCC club pistons I opted for .007” in my 49VH with no problems. A friend locked up his AOMCC club piston at .006”.
The run in period with the JP is about 500 miles so your piston is run in. It is just a matter of enough bore clearance.
David

[Ian.Taylor]

I have JP pistons in my 31 Norton 20 and my 36 ES2 with .004 clearance. Both are just fine with no nipping up.

[Les H]

I wonder what oil you might be using. Just an idea but maybe these modern oils do not allow enough friction to allow bedding in. If you think about it, say you used oil that was so good no wear took place at all. Then it would never bed in especially after only 900 miles...you need some wear to bed the piston in. Another thing is that too thick an oil might prevent enough volume being flung high enough under the piston to cool the piston compared to a thinner oil. Finally it might be worth trying a running-in oil specially designed to bed in old engines. Millers Oils make a good one

[nevhunter]

Whatever oil you use must stop the cam and followers scuffing. That's the most critically lubed part in engines without roller followers. Nev

[charles.bulloch]

Les and Nev, I am using Silkolene Chatsworth 40.

[robjameson]

I have a JP 9:1 piston in my VH.

When it was first built, it nipped up three times, all on long hills.

The piston was still usable, as was the bore, so I believe that dad polished the piston and bore to help run it in.

It’s absolutely fine now and has been for a long while. Worth noting that I had to change the slide for a 3 cutaway as I was too lean, which was causing additional heat which didn’t help, worth checking that too.

I don’t worry about all the JP negative press, works fine in my bike!