I have just bought a new old stock piston AE make part/casting no 11455. This is a 85mm piston and is STD in size,Anyone have AE piston number part book so I can identify original fitment.Have checked against a M20 piston and pin size and pin centre to top of crown is the same so should fit into a suitable overbored barrel.I have old Hepolite and AE books but cant find this number in them any help would be appreciated.. Dave

Hi Dave...The pre war M21 used an 85mm piston with a 105 mm stroke...I don't know the dimensions of those though. Many other bikes used pistons of this size as well.
Basically, if the overall dimensions and pin size are suitable you will potentially be able to use it in your M20.
Addiional factors to consider are weight..If there is more than 1 1/2oz. difference between that piston and the standard type the crank should, ideally, be rebalanced.
Also, piston clearance at rebore will depend on whether it is a 'split' or 'solid' skirt type.
If there are no valve cut outs it is probably from a sidevalve...If you intend to do a lot of miles or if you have a 'mishap' you may also have to source a replacement...something to consider..Ian

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

I have 2 bibles with all the car, bike and truck lists from the 1970's I'll bring it home tomorrow and check it out

email (option): davmax@ntlworld.com

Cheers Ian / Dave for replys .. I have checked weight against a M20 piston .M20 piston weighs in at 470 gram with ring set but no pin this one weighs 497 gram so no problem with balance.It is a solid skirt piston with no valve cut outs on crown also has 1449 stamped inside on the boss for gudgeon pin.The skirt is approx 2mm shorter but everything else is same spec (apart from no top grove) as M20 so will I get a +060 barrel I have bored to suit.Would be handy to know original fitment as then I could maybe source oversize pistons or it could be helpful to other forum users.Ian you could be right with this being a earlier HM21 type piston I have access to welworthy, hepolite & ae books which list all the early pistons but will have to have a thorough check.Ian do you know the maximum bore size for a linered barrel ? I know BSA used pistons with valve cut outs for sidevalve models as the 1936 W6 I have has cut outs for valves and being a sidevalve this is the original piston and I was the first person to lift head of from factory. Dave