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Piston dimensions and bore clearance?

Recently I bought a spare WDM20 engine in pieces. It was cheap and local and I already had some of the missing parts. The cylinder has been bored out to 84mm and the unused piston supplied with it is of unknown origin and without rings. Hmmmmm. As I own an M21 could someone give me the dimensions of the M20 piston?
Mainly gudgeon centre to crown, Overall hight and the cam.
Lastly is bore clearance measured at the crown or skirts?

Here's a pic of the piston.
Photobucket

email (option): spacemonkeym@gmail.com

Re: Piston dimensions and bore clearance?

Hi Matt..The piston looks a bit like a Norton one..but could be from just about anything. However, it should be OK to run it if it meets the dimensional criteria.
At 84mm (in an M20)the resulting capacity would be 520cc...Hardly a massive increase so it was probably sourced originally to go into a barrel already on +.060".
A BSA B50 has an 84mm bore as standard so a ringset from one of those might do the job if ring thickness is correct.
Another factor to take into account is the weight of the piston...A difference in weight of more than 1 1/2 oz. between that piston and an original one should necessitate a rebalancing of the crank assembly.
Gudgeon pin size is also a factor...the M20 pin is 3/4" diameter. However, as the M20 has a very thick small end bush there is some scope to accommodate a larger pin by reaming to suit.
I would recommend a bore clearance of .0045"-.005" at rebore.
Piston clearance should be measured at the bottom of the piston skirt on the thrust faces...ie.. at 90 degrees to the axis of the gudgeon pin, or put another way across the front and rear of the piston as fitted.
I can't recall the gudgeon pin to crown hieght of an M20 piston from memory...I'll measure one sometime today and let you know...Ian

Re: Piston dimensions and bore clearance?

Keith sent me a link to some dimensions for hepolite pistons (thanks Keith). The piston I have is good for overall hight and gudgeon pin hole. The compression hight might be a little short. Hepolite say 40.48mm crown to gudgeon centre, mine measures 39.825mm, 0.655mm short although the crown is flat not concave . piston weighs 345 grams.
The cylinder will need honing out as I can only get a .002" feeler between piston skirt and cylinder wall. How far out can these liners be safely bored to?

email (option): spacemonkeym@gmail.com

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