Thanks for all the help and advice. I'm wandering around blindly in an area I have no familiarity with. Now, I have a question. If one were to fit an appropriate 90mm piston to an unlinered M20 bore, would you have a cheap and cheerful M21 boost? On the proviso that a suitable piston exists of course. Paper calculation would be 600cc or thereabouts. I think if I could get away without having to tear the bottom end down, I would be much more inclined to increase my WM20 to 600CC. I wonder if there is enough difference between a 600cc M20 and a 720CC M21 to make it worthwhile? Any thoughts?

I vaguely remember Trevor from Oz saying that a VW 1700 90mm piston had the correct measurement from Gudgeon to crown. I hope he can chime in as I believe he might ride something a bit quicker than stock.

email (option): cas.vanderwoude@gmail.com

Two things to consider with car pistons are weight and gudgeon pin size...A weight difference of more than 1 1/2 ounces between the original piston assembly and its replacement will lead to a requirement to rebalance the crank. I've looked at a few car pistons and without exception they are heavier than bike ones, often a lot heavier..
Also, many have pins that are larger than the 3/4" diameter BSA pin...That in itself can lead to further work making suitable modifications....Another consideration is that there aren't as many candidates as you would think (ignoring the points already made)...At, or before, the target size (90mm) most car engines transition from 4 to 6 cylinders, the latter reverting to smaller diameters for a given capacity...

A more fruitful source of pistons is motorcycle pistons...Many modern engines have large bores and short strokes, or in the case of V twins large bores and long strokes...Thus there are numerous pistons in a useable diameter range...Motorcycle pistons also use smaller gudgeon pins generally so that aspect is easier to accommodate and they are more often in the right weight range...Further, with modern combustion chamber designs many pistons are flat or nearly flat...

The essentials are the pin diameter, pin centreline to crown height, weight (compared to the original piston) and overall diameter...Take a look initially at the Honda XBR piston...I've heard that is a potential candidate though I haven't used one myself....Don't forget that the M20 has a different pin to crown height than the M21/short rod B33!!...Ian

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

All so very interesting and thanks for the lessons.

To put a cat among the pigeons, what about a long rod B34 piston?
its lighter but still has a beefy dome so it could be machined down. I'm sure Trevor or Ian or someone have already tried this...

email (option): cas.vanderwoude@gmail.com

Cas I think you need to confirm if this is for an M20 or M21 crank. I don't know, but I presume this has the same crown height as an M21 piston and therefor probably not suitable for an M20.......Surely there is not enough material in that dome to safely machine it flat anyway??? Ron

email (option): ronpier@talk21.com

As Ron notes, the long rod pistons only work with the M21 crank and there wouldn't be enough material in the crown to machine the piston flat, or even nearly flat anyway...The low compression short rod B33 piston would be the one if the crown heights work out...However, 85mm on an M20 crank will only give you 533cc so little real gain for the effort..Ian

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

My lack of knowledge on this topic is beginning to show, but I'd like to do some more digging on this. Where could I find out the pin to crown length for both the M20 and M21?

email (option): cas.vanderwoude@gmail.com

Michiel W Quotes the M21/B33 crown height in one of his posts in this thread. Perhaps he also has the M20 measurement written down? Ron

email (option): ronpier@talk21.com

I haven't measured the M20 crown height lately but the stroke difference between an M21 and an M20 is 18mm (.708")...So the difference in the crown height between the two pistons would be half of that figure (9mm/.354")..Using Michiels dimension for the M21 you should be able to work out the final figure for an M20 by adding the two figures together...

I'd get that figure confirmed absolutely at some point but it will give you a pretty accurate figure to work with...Ian

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

Yes, from the Hepolite catalogue:

M20 comp.: 1 19/32"
M21 comp.: 1 7/32"

They call it the 'compression height', distance from center of the pin to top of the piston.

Surprisingly the difference is 0.375" / 9.53 mm instead of the 9 mm as you indeed would expect.... So it looks the top of M20 piston gets 0.5 mm higher up in the bore than the M21 piston.

BR Michiel