hi all. im trying to get a piston + rings for my wm20 but im not sure of its size or if a standard one will fit, i do have a rough bore size which is slightly just over 82mm, does this make any sense?
regards barry

email (option): fullerbarry@hotmail.co.uk

hi barry,standard bore for an m20 is 82mm which equates to 3.2283 inches,to measure your cylinder accurately (to see if there is any noticeable wear) you could do with borrowing a set of internal micrometers, or a set of internal calipers and either a 75 to 100 micrometer or a 3-4 inch micrometer. there are other specialist instruments but you just want to get going i presume.if you have no experience of measuring accurately,perhaps you know an engineer who will do it for you.your cylinder needs to be 82.1mm or3.233inches as standard plus (0.1mm or0.004inches running clearance above standard.if it measures say (dont quote me on this)plus 0.03 mm or about 0.012 inches you may need a rebore.then you can decide if you need a new piston ,and which size to buy. providing your cylinder han no other damage ie score marks, grooves,pickups, and is within tollerance a light hone will assist in running in.henk who runs this site has pistons for sale (very reasonable too)look at previous posts for details .sorry to drone on but ive had a few dr holts'tonight. hope this helps, but if you are unsure ask at a local engine remanufactures cheers rick

email (option): richardholt@rocketmail.com

hi rick, thanks again, i was thinking i could just a piston/rings but yeh its best to get a proper messurement.
im still not sure how to go about restoring the bike
i think its best to put it in its army trim cos most of the parts are there but i need to get parts like the rest of the AA clutch and drive chains and magdyno, just little bits realy,
anyway thanks for you help and you will be hearing lots from me
best regards barry

email (option): fullerbarry@hotmail.co.uk

Hi barry

If we can help please ask - am local to you

Best regards

Jo'b

email (option): Jonnyob1@googlemail.com

Is there any data on other measurements that are crucial to our bikes?

Bearing sizes, end float, torque settings etc?

Is there any better book than the workshop manual which I have?

Thanks mates

email (option): britool51@hotmail.com

Hi robb

Try the technical section on the website page, lots of information there for you

Best regards

Jo'b

email (option): Jonnyob1@googlemail.com

Hi Barry if your piston and bore look fairly good see how much of a lip there is on the top of the bore. This will give a rough idea how much work that bore has done.
Next put the piston in the bore without the rings and put it in the middle of where the stroke of the piston runs.
Then use feeler gauges and see how much clearance there is between the bottom of the skirt of the piston and the bore.
Measure the piston at 90 deg of the gudgeon holes not at the bottom of the piston where the gudgeon holes are
If you can only push up to a 6 thou feeler gauge in between the piston and bore then the bore and piston are ok. Maybe even a few thou more would be ok if your bike is only a sunday runner or something like that.
Just give it a light hone and a new set of rings and circlips it will be fine.
If it does have a lip at the top of the bore you can remove this with a tool called a ridge remover or with elbow grease and rough sand paper if it's only a slight lip. Some people file the edge of the top ring so it doesn't hit the lip but I dont like that idea cause you get less of a ring then. Some new rings on other bikes have the top ring already edged.
If there is too much clearance between the piston and bore then you will need to measure them with micrometers etc and see which one is worn and workout then if you need to rebore.

If your piston looks ok dont throw it away if it's a hepolite or genuine BSA you could use it at a latter date if you ever reseeve etc. The Indian made pistons are rubbish so I'm told, I have never used them.
I hope this makes sense and maybe helps a bit. You can get a fair life out of a piston and bore if you aren't racing it which I'm assuming you're not LOL.
cheers Terry

hi thanks for the tip terry, im hope fully gona restore the bike but before i got the bike its been stored for over 25yrs thats when the last owner had it, there are some bits missing ie complete piston, im just looking around to se what parts i could get, which does not seem much at the mo or there out of my price range.

many thanks barry

email (option): fullerbarry@hotmail.co.uk

years ago you could get oversize piston rings to take up small bore wear i am talking car engines