Running in a new piston on a 1956 ajs 16

[itma]

use the gears a lot, where you might not normally change down for inst., and don't let it slog under load, ie on long hills, or long bursts of speed.
it'll be ok.

[alanjennings]

One of our section members {Warwicks] seized his newly fitted piston on his 500 on the way to Stanford Hall on Sunday-I seem to hear about a lot of seizures of late!! Piston rebore clearances seems to be a bit of "a black art!" I did my V----t recently-sorry , Itma for mentioning this-I've now done 600 miles so far with no problems, I think 3 thou clearance was allowed-I hope for another 70,000 miles before repeating this-if I last that long!!

[itma]

600 miles on a V******t?

not all in the same year surely?

siezed piston?
I always ask [ahem] what oil was in it then ?

mmm???
mmm???

what? speak up............

[SPRIDDLER]

Jez.
The clearance question crops up regularly, so firstly click on 'Search Forums' and enter "Piston clearance". There's quite a lot to plough through.
For my part I've worked on the basis of 1.5 thou per inch of bore diameter. Since 2006 the GPM piston in my 1954 350 has run for about 25k miles, over the Alps and Pyrenees twice with full camping gear and never given any trouble.
I've asked JSL a couple of times for boring clearances for their pistons as last time (2006) I took a new piston from JSL to the long-established and highly competent engineering firm who were doing the rebore they asked me for the boring/clearance data and said it should have been on a leaflet in the box. They pointed out that clearances differ for many reasons - e.g. whether the engine is air or water cooled, material Coefficient of Expansion so forth.
I asked JSL who said they don't supply that data and that "any borer worth his salt" would know. Maybe that's so, but it left me to research it myself when I'd rather have been riding. (Thorntons, for example, provide the correct data for a whole range of applications on a leaflet in the box with the piston).
Although I haven't bought GPM pistons since then I undertook to resolve this frequent topic on behalf of yet another posting on the Help Forum by sending the following email to Jampot Spares on 23rd June 2009 and again on July 3rd. ....

[[Hi, Please would you clarify the clearances for the GPM pistons.
It is a subject which crops up quite frequently and it seems that the pistons are supplied without this data - leading to some confusion for the bike's owner or the engineering shop.
In essence it would seem that a rule of thumb is not ideal since I understand that the correct clearance is dependant upon the coefficient of expansion of the material from the piston is made.
Thanks
Neville ]]

I never received a reply to the initial enquiry or the resent email and left it at that as my request for the info was on behalf of those who buy from JSL.
Perhaps, a year later, the info is now available...

[Eric]

Spriddler I knew of your endeavours to obtain the clearance information from JSL that's why I prompted JEZ to ask them in the hope that the more who ask may get an answer. Club or no Club JSL customers are customers and the clearance information should be given to them by JSL not guessed at by the borer who no matter how much salt he may have on his chips will never be able to guess the correct clearance, only the piston manufacturer will know and the Club should take on the responsibility of passing this information to its customers. There does as you and others have said seem to be a plethora of pistons for singles seizing up, most seem to be supplied by JSL or put it another way the Club, something must be done, if JSL cannot supply the information then stop selling the pistons.

[legg]

quote:
if JSL cannot supply the information then stop selling the pistons.

Sounds fair enough to me.

[iandusud]

I don't think that the reborer should know what clearance to use as a lot depends upon the materiel and construction of the piston. AMC wire wound pistons are a case in point, requiring very little clearance. When I replaced the piston in my Velocette Venom I did a lot of research and spoke to Nick Payton who probably knows more about building Velocette engines than anyone. He told me that with an Omega forged piston 6.5 thou clearance was a minimum and even then they are know to seize, so he no longer fits them. With a Hepolite, which is what i fitted, he recomended 4.5 thou, which proved to be fine. For the record that's just under 1.5 thou per inch of bore, which is a good rule of thumb. However I agree that whoever supplies the piston should be able to give the spec for reboring.

Ian

[TommoT]

And it would be very simple for JSL to provide this information. JSL should demand this information from their supplier. No piston data, no business! Any serioius producer of pistons know material composition and expansion co-efficient. I too have had a few seizures with GPM pistons, and my machine shop has simply stopped reboring for these pistons.

[Don Madden]

There is more than the material that affects clearance. Good pistons, as the originals, are cam ground, meaning the bottoms are oval in section, & tapered with the top smaller, to accomodate expansion. Some have lengthwise slots for the same purpose or lateral ones to limit heat transfer to the skirt. Lacking info from the maker, the best way to be sure is to measure piston & barrel at room temperature & at 350F to determine how much each will grow. Alloy barrels grow at about the same rate, so need little clearance when cold, iron expands less. Cheers, Don.

[itma]

All the same its about time this vexed question had some informed and definative input to be put on file?

meanwhile sales of pistons stay bouyant