Hi Noam...The 'long rod' type B33 piston is the one you need. The 'long rod' refers to the B33 conrod used up to 1954 (the same length as a B31 rod).
In 1954 the B33s rod was shortened by 1/2" and the piston crown hieght increased by the same amount to maintain the overall hieght of the cylinder. This later piston is not suitable for the M21 application..
To build the larger engine you need to bore the barrel out further and fit a 90mm piston....that will give 712cc....Ian
email (option): ian@wright52.plus.com
Hi Ian,
Thanks allot for the info and swift reply!
If I would like to go for the 712cc, what would be the piston to use?..
And on the B33 PISTON- Nothing has to be done?
Just put it with its "arrow" domed head and nothing to balance?
What tolerance to use for piston skirt and ring gap?
By the way,
I heard about fitting Jeep pistons to the 500,
and also Fiat and Opel in order to get higher displacement.
Any clue?..
Thanks,
Noam.
email (option): noam10@gmail.com
Hi Noam...I don't know anything about Jeep, Fiat or Opel pistons (VW is another one I have heard of being fitted) One thing I do know about some of the car pistons I have looked at is they are very heavy....You can just fit the B33 piston straight in.(but you must have the flat topped one to do this)
Use the same ring gaps and clearance as for B33 applications. Also standard ignition timing. If you don't have one fit a solid copper head gasket cut to fit the larger bore diameter around the cylinder.
I get pistons made for the bigger engine...I have a couple but I think they are spoken for (I'm waiting for final confirmation on the engine rebuilds at the moment)...I'll let you know on that...Ian
email (option): ian@wright52.plus.com
Thanks again Ian.
As always, your answer and info is great! 
Ok then, please let me know regarding the 712cc piston when you know,
And as to the B33 long rod piston-
everything I saw so far was with a domed head and valve groves,
such as eBay item 200888879539.
http://www.ebay.co.uk/itm/200888879539?ru=http%3A%2F%2Fwww.ebay.co.uk%2Fsch%2Fi.html%3F_from%3DR40%26_sacat%3D0%26_nkw%3D200888879539%26_rdc%3D1
Is there another type I would need to use? with a flat head?.. 
(This one on eBay was flatter than few other ones I found which where really high )
Cheers!
Noam.
email (option): noam10@gmail.com
Hi Noam...There is a completely flat 'long rod' B33 piston which is the one to find for an 'easy fit'..I think the ones with the chamfered, higher top are either early B34 pistons or the higher compression version for the M33.
It MAY be possible to use one of these but you would have to machine out the head to provide clearance for it and if possible remove some material from the top of the piston as well. Judging by the weight of these high comp. pistons there is some surplus thickness in the piston crown....I haven't tried this or looked at it myself though to confirm this is possible.
If the inside of the head was machined back approx 3mm and the piston could be reduced to leave that much (3mm) protruding from the top of the barrel that would be advantageous as it improves the gas flow inside the head (between the valve and cylinder) by increasing the 'throat' between the top of the cylinder and the inside of the head, above the edge of the bore.
There is also no loss of compression with this mod...Squish clearances over the piston needs to be .030"-.040" after all machining work is completed...Ian
email (option): ian@wright52.plus.com