Piston clearances for a 650 G12

[Packers]

I am replacing the pistons in a G12 650 with 8:1's from the spares co.
From the Workshop Manual p4, if one subtracts the mean piston diameter at the top of the skirt of 2.8291" from the mean cylinder diameter of 2.8355" you get a clearance of 6.4 Thou which from my experience is excessive. The bores and pistons must have been matched on assembly to give an ideal clearance. The pistons I have removed from the engine have a clearance of 3.9 thou at the top of the skirt and 3.2 thou at the bottom of the skirt.
Question: What is the ideal clearance for the new 8:1 pistons.
Can anybody help ?

[Eric]

The modern pistons are known to seize far more readily then the originals if using the original clearances, so you need to get specific details of clearances from the piston suppliers.

If you already have such close fitting pistons why are you re-placing them?

[g80csp11]

The clearance that you have already sounds good to me . ideally 1 - 1.5 thou per inch of bore , so a max of 4.2 thou
and in my experence , the modern pistons are far more likely to sieze that original pistons. I know others will disagree but i would avoid JP pistons as they require high clearance . The new pistons shoudl reccomend a clearance if supplied for air cooled engines. ask Jampot spares the question if thats where you bought the pistons , they have a responsibility to supply this information

[SPRIDDLER]

Welcome, Paul

You really need to get that info from the piston manufacturer as it depends upon the constitution of the metal casting. This subject has cropped up many many times so I suggest you use the search facility on the Forum, ('Forum' 'Search Forums') then using 'Piston clearance' as your search words and you'll get many previous posts on this topic.
I allow 1.5 thousandths of an inch clearance for every one inch of bore. That is to say, 0.001" clearance per 1" of bore. Do not be misled with the clearance mentioned in some places of only 0.001" TOTAL clearance (NOT PER INCH) which is only relevant to the (virtually unobtainable) factory-supplied wire-wound pistons.

I emailed Jampot Spares Ltd on June 23rd 2009 as under but never received a reply.....

Hi Jim
Please would you clarify the clearances for the GPM pistons.
It is a subject which crops up quite frequently on the 'Help' Forum and it seems that the pistons are suppled 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

Make sure your chosen borer is familiar with boring air-cooled cylinders, and I've learned from ITMA that the boring should be done from the bottom, unworn, end of the barrel to ensure that the boring starts true.

[A Moderator]

Moved to here from Frequent Questions.