Questions? Looking for parts? Parts for sale? or just for a chat,

The WD Motorcycle forum

WD Motorcycle forum
Start a New Topic 
piston / bore measurements


I have a Matchless G3L cylinder which needs a new piston. The measured size of the bore is
69,35 mm. It still needs to be re-bored, because it isn't quite round any more. No scratches.

Are the sizes on the pistons in inches (+ 020 )?
How much needs the clearance to be, and do you measure this when all parts are cold?

Of course I'm going to have this done by an expert but I think he needs to know
the clearance for these old bikes? I guess it's a lot more then the modern bikes.

+ 020 and +040 seem to be easy to find, but bigger sizes are very hard to find for Matchless?

Cheers Menno

Re: piston / bore measurements

The size of a standard G3L piston is 2.7182" (69.042mm) to 2.7192 (69.067mm)...

Oversizes are +.020" and +.040"....There are no pistons listed over those sizes so a +.060" if available, would be an after market piston such as Hepolite...

When reboring, the bore to piston clearance should be measured at the bottom of the piston skirt at 90 degrees to the gudgeon pin axis (the front and rear thrust faces of the piston...)

Clearance for std. and oversize pistons is .0034" (.086mm) to .0054" (.137mm). Measurements should be made with the components at room temperature...

Mail me off forum for additional information...Ian

email (option):

Re: piston / bore measurements

Hi Menno,

Assuming you are dutch :

try at Peter Classic Bikes by sending him an email ( he purchases Pistons from a big Italian factory)
try the Matchless club in Holland Onderdelen fonds.

good luck with the "experts" doing the job !


Re: piston / bore measurements

thank you both. pm send

Re: piston / bore measurements

You have 3 e mails...Ian

email (option):

Re: piston / bore measurements

Ian Wright
You have 3 e mails...Ian
Thank you for the info. Very helpfull!!

Nieuwe pagina 1