Hi
I am part way through a build on an m21 motor & am considering using a b33 piston

I know it takes the capacity to about 630cc or so but - can I use a std linered cylinder - I have a few of those here I could use
There appears to be enough on the liner to take it - just looking for notes from any one who has done this

Br
Jo’b

email (option): Jonnyob1atgooglemail.com

Yes John I did it and I expect Ian has. I picked Ian's brain at the time and he confirmed that the 2mm or so left of the liner was more that adequate and that he's known Gold Star barrels to be bored out to within 1mm of liner.

However for myself, I thought the engine felt lumpier than it did before, so I got a new liner pressed in and went back to a standard M21 piston. I'm not sure the effort and extra machining was worth the extra 39?cc

During the course of the lumpiness, you supplied me new cush drive parts which didn't improve things. Ron

email (option): ronpier@talk21.com

'However for myself, I thought the engine felt lumpier than it did before'....

A 633cc upgrade is in itself not a problem, I ran one to work and back for around eight years..However, particularly depending on the manufacturer of the piston the weight of the replacement is a factor...More than approx. 1 - 1 1/2 ozs difference between the original piston and the replacement means a crank rebalance is required to avoid the potential problem identified by Ron...

I have built two 720s for my own use. I rebalanced one but not the other and the difference between the two engines was very noticeable with the rebalanced engine running much more smoothly....Although the BSA balance factor is 58% I have always had them balanced to 60%, something I picked up from a long time British bike dealer and BSA enthusist in my area...There is a good arguement for rebalancing the cranks anyway as they were balanced statically and I imagine there was possibly a considerable degree of tolerance employed at the factory originally...The guy at Basset Down told me he would nearly always have to alter even standard cranks to varying degrees to get them 'spot on'..Ian

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

I bought the b33 piston last year - it’s nos & +40
The cylinder I have is at +30
I have an nos +40 m21 piston I can also fit but if I go the b33 route my +40 piston cannot be used in the same cylinder

So - its decision time - the bottom end is together & I really don’t fancy tearing it down again to check the flywheels - it was rebuilt in the U.K. by a chap in ringwood - so perhaps best to stick to standard

Job

email (option): Jonnyob1atgooglemail.com

Yes John. You have to use the correct B33 "long stroke" piston which is 85mm so at least 1.5mm to be machined from the liner. When Ainsley did mine. I think it was about 12 cuts with his boring bar then a final hone to size. He was moaning about standing by his boring bar for half a day :confounded: Ron

email (option): ronpier@talk21.com

You need to weigh your M21 and B33 pistons bearing in mind the figures I quoted...If it's an original BSA B33 piston they are the lightest of what's available...Did you buy it from me at Netley last year?....Ian

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

Hi Ian
Yes that’s the one
A new old stock + 40 bsa stamped piston
Weighed just now with pin & clips at 476gm

Br
John

email (option): Jonnyob1atgooglemail.com

Just weighed the +40 m21 piston I have here
That comes in at 401gm
So a difference of 75gm

Br
Jo’b

email (option): Jonnyob1atgooglemail.com

This is an interesting topic chaps, thanks.
I’m hoping to learn here, I’m not trying to be obtuse.
The cranks of these engines are effectively flywheels, as opposed to webbed crankshafts.
I understand that a heavier piston will require more stored energy than a lighter one, to be shoved back up the bore.
Am I correct to think you can only dynamically balance a bare crank assembly, there is no way of doing it with the conrod and piston assembled too?
Are you just meaning that the motor will benefit from having a more accurately balanced crank? Or do you mean that the crank should be balanced to suit the heavier piston?
I can’t see how this can be achieved, as you can’t add more material to the crank.
Hoping to gain a better understanding.

Tim.

Yes Tim the crank must be split and rebuilt without the rod and bearing.
This gives you an opportunity to see how bad the big end really is.

If the piston is heavy you can remove material on the big end side of the crank above the mainshaft.
Or add tungsten slugs below.
For a lighter piston it's the opposite.

It does make a difference.

Mark

email (option): pes.sales@btconnect.com

You can statically balance a single cylinder crank. This is how Me and Ainsley have done a couple of mine.
https://www.youtube.com/watch?v=0hoE_MuFpPI

I fitted a different higher comp piston to my WD/CO and the vibration got worse. I couldn't find any spec for the balance factor for these engines, so we took a guess and moved it be a few degrees. Built the engine and tried it and it still wasn't right, so went through the whole thing again and moved it a bit more which I'm happy with now. Ron

email (option): ronpier@talk21.com

So from what i take on this is
The nos bsa b33 piston I have is outside the recommended weight difference from an original equivalent +40’ m21 piston I have
This weight difference seems to me & what is being recommended/suggested is - for an engine tear down & stripping of a rebuilt crank - rebalance & engine rebuild
It’s all a bit too much for the time scale & my riding season ahead

In this instance I think I will stick to std crank & +40 m21 piston - I’ll have the cylinder bored, put in new guides & recut the seats for new valves

Thanks for the feedback - it’s an interesting learning experience
Br
Jo’b

email (option): Jonnyob1atgooglemail.com

Yes John well over 2 1/2 ounces. Just not worth the effort and expense to my mind. Ron

email (option): ronpier@talk21.com