I've re-posted my response in this subject title as it where the data originates. It may be of some use.
Thanks all - here's something for the Bank Holiday reading and lockdown.
I have trawled through the links (which pistons are available for my Ariel) and come up with a piston comparison spreadsheet to try and understand the data provided by various owners. I also added some Hepolite data and details of a new standard 7.5 JP piston i've recently purchased.
The data is for all manufacturers and sizes. The average piston weight is 455 grms complete with gudgeon pin and rings. The JP weighs 478 gms and thats 6% heavier than the average piston weight. As David suggests there is scope for material removal from inside the skirt, not sure how much yet.
The surprising range of Hepolites as given is 400 gms to 550 gms. Maybe someones scales are suspect. Average weight of Hepolites is 448 gms.
There is not enough data to show a Hepolite weight increase for the range of oversize pistons. A guess but maybe add 10 gram per each oversize piston in steps of +0.010.
Also surprising is the gudgeon pin weight range 79 - 96 gms - a 17 gram variance when taking the total piston weight into consideration. The JP weighs 87 gms.
Piston expansion looks steady at 0.012 thou average at 250C.
JP data quotes a minimum recommended clearance of 0.004 thou. Most feedback suggests this should be 0.006 / 0.0065 thou. Which is what i am thinking for my alloy barrel.
A couple of observations on the JP piston.
Measurements across the bottom of the piston. The difference between the piston O/D versus 90 degrees across the gudgeon pin is 0.027 thou. on diameter. New Hepolite +0.040 and 0.060 by comparison is 0.010 thou.
The skirt O/D surface finish is faintly grooved. This is very fine and only just detectable with a finger nail. There is potential to remove these fine machining marks with wet / dry to provide a smoother surface.
Inside the skirt there is scope for material removal. My plan will be to remove the excess material flush and blend in with the internal machining. I'll weigh it afterwards as see what I get.
54 Square Four, 1934 RH500 twin port, 1936 RH500 single port, 1945 RH500 (1936 ) twin port, 1947 VB Buttermilk.