I bought a piston which I think is a B33 long rod piston with a plan to increase capacity on my M21. Have I bought the right one? (there are 2 more available)


https://www.ebay.com/itm/Nos-Genuine-BSA-Part-Piston-part-no-65-1263-Diameter-85-5-mm-1-mm-OS-B33-B34-M33/202708798605?hash=item2f3262cc8d:g:qLcAAOSwfuddBh8o

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

Cas you sent me a standard long conrod B33 piston some time ago, which I still haven't fitted in my WM20 yet (M21 crank fitted). Strange that he quotes the size as 85.5mm 1mm OS. The standard size is 85mm which makes it 1/2mm OS (0.020" 20 thou) Or is it in fact 86mm? The price is very cheap but you still have to provide pin and rings.

Initially I had some concerns as I think my barrel is the earlier type with a liner fitted (can't be sure till I pull it apart). I have a spare NOS liner on the shelf and have worked out that boring it to take the 85mm piston would leave a wall thickness of just about 2mm, which Ian says is acceptable as he's seen Goldie engines with a 1mm liner wall thickness. So your piston would require a further 1/4 mm cut (or 1/2mm if it's 1mm OS)from the wall, which still sounds acceptable??? All academic of course if you have a post war unlinered barrel, which as we know can safely go out to 90mm. The bigger pistons will give us at least another 35cc. Good luck Ron

email (option): ronpier@talk21.com

Yes, should be correct as the B33 long rod has the same 1 7/32 compression height as the M21 piston.

The size quoted on the piston is the required boring size of the barrel, so this piston should fit a 86.00 mm barrel.

The piston itself is of course smaller in size en non circular. The sexy Thai probably measured here 85.5 in the line of the pin or on the top crown/land which is also relieved in size. :blush:

BR Michiel

NB: the 86.00 is for the B33 barrel for which this piston is designed, boring size might be different for the sidevalve barrel

That is a long rod piston...

I've recently removed an original liner from a 'linered' barrel and replaced it with an oversize liner to fit a 90mm piston. So one way or another you won't have any problem fitting an 85mm piston dimensionally whatever the oversize, even if you need a new liner...

Original piston rings may be the problem as they are getting harder to find these days for B33s...The circlips are the same as a T140 Bonneville....Ian

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

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