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1948 M33

I have a '48 M33 that has been retro fitted with an M20 engine. If I fit an M33 engine will I need to change the engine and gearbox plates?

email (option): roy.speake@yahoo.com

Re: 1948 M33

No You won't...though you will have to change the exhaust system, carburettor, head steady, some of the oil tank fittings (along with the addition of a rocker feed pipe) to provide a rocker feed for the OHV motor and you'll need to check that the petrol tank hasn't been changed to the 'flat bottomed' side valve one which won't clear the B33 head....
You may also need to change the engine sprocket to suit the B33s power characteristics depending on what is currently fitted...A 19T engine and gearbox sprocket would be the correct ones for a solo M33...

Saying all that it's well worth the effort...The B33 has about 40% more power with the attendant performance benefits, is at least as reliable and will do about 25-30 more miles per gallon....Ian

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

Re: 1948 M33

Ian that's fantastic, now all I need is an M33 engine as close to '48 as possible lol

email (option): roy.speake@yahoo.com

Re: 1948 M33

You can fit a B33 top end to an M20 bottom end with a bit of light engineering.
However buggering a good WM20 might not go down well with some here.
Fairly common to use M20 bottom ends on race bikes in the day.

email (option): bsansw1@tpg.com.au

Re: 1948 M33

Thanks Trevor that's interesting but like you say why mangle a good M20 engine. I was hoping to sell or even p ex for a complete M33 engine if there's one out there?

email (option): roy.speake@yahoo.com

Re: 1948 M33

Roy I think I'm right in saying, that what you need is a B33 engine. I think the bike was called an M33 when a B33 engine was fitted in an 'M' series frame. Ron

email (option): ronpier@talk21.com

Re: 1948 M33

If you are going for a OHV engine for 1948 M33 the number would start YM101 on. If just fitting a B33 engine the number will start YB101 on. For 1949 the letters change to ZM101 for the M33 or ZB101 for the B33. B33 engines are quite hard to get. It might be worth fitting a B31 engine short term, this will at least give you 4BHP more then an M20. You could then look for a B33 top end to fit or put in a T140 piston and get 400cc.

Re: 1948 M33

[You can fit a B33 top end to an M20 bottom end with a bit of light engineering...]

How did they account for the half inch misalignment between the cam followers and the ends of the rocker arms of the OHV engine caused by the wider timing side crankcase of the M Series motor?..Also, the cam followers themselves are upright, not tilted along the opposite axis, so the pushrods would be misaligned in two directions...I can't imagine it would be a good plan to tilt the pushrods to the required angle to achieve alignment...and anyway that would require new tappet heads and ball end pushrods I would have thought.... Also, how would the head bolts be arranged?....Ian


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

Re: 1948 M33

Thanks Ron I will do a bit more digging.

email (option): roy.speake@yahoo.com

Re: 1948 M33

Thanks Ian, I'll have a look at some contempory literature and see what crops up. I've always wondered what that lettering indicated, my '43 WM20 is the first BSA I've owned other than a yellow n black drop handlebar bicycle back in the sixties!

email (option): roy.speake@yahoo.com

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