My motor is apart and thought it'd be wise to change the mains. I've had a look on Russell Motors and with my limited brain-power and lack of exploded diagram, I can be 100% sure what part numbers I need - can anyone help with numbers or even supply main bearings?

My piston, although not terminal, has seen better days and could do with replacing. There's a part number of 66-1024 on the inside of the piston with 178 7C marked opposite. On the crown (picture below) looks to be B followed by another part number (66-1020??) and O.S - any idea what size piston I have and also what size I'll need for a rebore?

email (option): lee@twowheelstested.co.uk

Hi Lee,

A 66-1024 is standard size ,so you may need another standard
If you hone, or +20 OS if barrel is clean and you re bore.

All the technical info and bearing No’s can be found at the technical section here in the site.

http://www.wdbsa.nl/Bearings.htm

http://www.wdbsa.nl/Drive%20side%20bearings.htm

Sorry, don't know about the "B".
May be the OS is 0 oversize?..

Good luck,

Noam.

email (option): noam10@gmail.com

Hi Lee..You need to measure your cylinder accurately to determine the degree of wear before opting for a rebore. If you don't have suitable bore gauges or bore micrometers it will have to go to an engine reconditioners for checking. It's worth looking at the valve stems, valve faces and valve guides while you have it apart as well. I'll mail you a diagram that shows the corect way to measure the bore....Ian

email (option): ian@wright52.plus.com

Lee, to my eyes that looks like 66-1026 stamped on the top, which corresponds to 1mm/.040 oversize. Best get those calipers out and measure the piston too.

ChrisJ

Hi Lee...A standard piston is 82mm dia...BSA pistons go to +.040" (1mm) oversize...aftermarket pistons go to +.060" (1.5mm) oversize. Go for an original BSA piston if size permits or a 'period' aftermarket NOS one if you can.(Hepolite etc.) Italian GPN pistons are very heavy and the Australian JS pistons tend to be similar.
Russells may have originals, if not try Bantam John...Ian

email (option): ian@wright52.plus.com

Thanks chaps. I guess the first step is to get it measured and see what we have.
Would Russell's be a good place to buy a piston?

email (option): lee@twowheelstested.co.uk

for a short time i worked for an engine builder making engine for stock car racing in the uk they would weigh the piston and then machine them to the lightest one for balance could the heavy italian pistons be lightened