More on JP pistons

[nevhunter]

One of the problems you can have (apart from seizurees if the clearance is not enough) with Ariels is that there is not a lot of oil circulating to cool the piston so the heat has to get out of the piston through the cylinder wall. This requires that there be contact of the piston with the cylinder. Therefore "slipper" skirt pistons and large amounts of "cam" on the piston are not a good idea. Nor is the "W" slot piston appropriate.
It's not true that a thin piston expands less. A solid bar will expand as much as a pipe over the same temp change if it is the same material. It is true that it has more "give" when it's ready to nip up, but that's a marginal thing .
Upping the oil flow doesn't help a lot as the design of the oil scavenge pick up point is inadequate, the oil builds up in the sump and the engine runs hotter and does not deliver as much power because of the oil drag.. Anyone who has raced these engines knows this. A scraper designed carefully has been used by some to get the oil back out of the crankcase. Nev

[john.whiting]

I have heard allegations that JP pistons develop lumps and bumps after manufacture.I have also heard that heating them for a period[24 hrs was mentioned] causes things to settle down.I dont know.Engine reconditioners are very wary of them,and allways give plenty of clearance. I recently rebuilt a V4 air cooled Wisconsin engine,that had JP pistons in it ,and they had given good service,and were completely worn out,with broken top rings.This engine was in a Vermeer stump grinder operated by tattoed toothless yokels,it would have had a hiding.However,I fitted genuine USA made pistons and sleeves,as you dont want these kind of people coming back with a seized motor.As a matter of interest,I have started rebuilding a JAP 500 using a Harley sporty piston and a Harley conrod,making my own 1 1/4 crankpin,using Harley rollers and cage.I might invest $12 in an Enfield crankpin,and check it out .Cheaper than Alpha.Regards John.

[Graham.Goral]

Many thanks for all the replies... The Speed Twin pistons are perhaps the better way forward then as far as choice goes? Please would some kind soul give me the vital statistics of the 12410 piston so I've something to compare to? I've looked in the Heplex PDF catalog in the library on this website but it's a bit of a grainy scan and I can't make out the sizes.

Thanks again

[nevhunter]

John theree are lower dome Harley 1,000cc sportster pistons than the "normal" ones ( around 83 models. The high top ones have domes that are far too high for any normal use. As you are probably aware,( but maybe some others are not) you have to go into oversizes to get to the Ariel size. Nev