Ariel Owners Motorcycle Club

The Canadian who owned the +30 piston sent it to me, refusing payment of any kind, even for the postage. This makes him the first International Sponsor of my Cannonball entry. Anyway, with the piston in hand I confirmed its weight on my calibrated scale and was able to make accurate measurements of its dimensions and thus a more accurate estimate of the weight of a stock piston. This reduced the previous estimate by precisely 2 grams (0.07 oz.). Taking the average of the Australian and Canadian piston weights, the new "official" weight is 505+/-2 grams (17.81+/-0.07 oz.).

This is a very minor correction to the weight given in my previous post and it will make a trivial change to the final balance factor I determine. But, there's no point in having the accuracy and not using it, especially since it will save anyone else from having to go to this trouble in the future.

I recently bought a +60 JP piston for my 1954 600 VB. It was well made and at about £120 delivered not unreasonable in price. The piston weighed 710gm, with pin, less rings. The old +30 Hepolite piston weighed 580gms with pin less rings.The original std. piston probably weighed nearer 556gm. How this will affect vibration remains to be felt. Unfortunately it wasn't a runner when I got it. I'm expecting some vibration but hopefully with a low revving side valve nothing unpleasant.

Ian that's alot of difference, you will be better of to rebalance to your new piston. I have used 67% in my nh and vha.

I am presently trying to dismantle the BA Gearbox off my 1938 Square-4 I have taken off the front cover with kick start/gear change mechanisms etc. -- but I am struggling the get the nut off that holds on the ratchet and spring on the main box to allow me to remove the mid cover and allow me get to the gears--I have held the sprocket tight at the other end but the nut will not move ---Any Ideas--should I use a nut splitter and replace with a new nut? ----or is there another way?

Regards Ralph

Hi Ralph, try page 10 of the Four stroke section of the forum, very near the top of that page, but for future reference, it's probably better to start a new topic than tag on the end of an unrelated one. As an alternative you could try searching posts by Pete.Silson

All the best, Roger

Put the clutch centre back on, complete with nut. Hold the clutch centre with your clutch tool made from old clutch plates and a steel bar and then try the nut. If it doesn't move then, warm it up with a blowlamp and try. These nuts are not usually too tight.

I have had it where that nut is almost welded onto the thread. What you don't want to do is ruin the thread on the end of the shaft. (K/S end). If in doubt I would drill carefully two holes opposite each other and split it off. The nut is relatively soft, and a small hex. You may have to remove the shaft to clean up the thread, or replace the sprocket. insert a dummy shaft as the other is withdrawn (even of wood) and you can remove and reinstall the mainshaft without dismantling the entire box. Nev

I have a piston from a Suzuki SP370 trail bike fitted to my VCH, it was a std piston but the cylinder was O/S already but had a machine shop bore to suit with 2thou clearance and if remember it is 83mm bore now. All this work was done in 1990 and I have just put on my 3rd rear tyre and 2nd front and it has not been touched ,now there is something to chew the fat over.

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.

Update on JP piston std 500cc 7.5 an excess material removal. Now weighs in at 463 gms showing a 3% reduction