Hi all...I want to put together a B33 engine. Is the crank for the B33 the same as the M20 crank or is it the same crank as the M21 crank? Is there any difference between the bottom ends of the B33 and the M series bikes?

Hi Bryce...The M20 and M21 cranks differ from the B33 crank.
Firstly, there are two con rod lengths for the B33. Early B33 engines share a con rod with the B31 which is 7 3/8" between centres...
Later B33s had the rod shortened by 1/2" so have a measurement of 6 7/8" between centres.
The M20 and M21 have a con rod that is longer than either of the OHV motors at 8 1/4" between centres.

There is also a difference in the timing side mainshafts...The side valve motors have a longer timing side shaft and the boss on the flywheel face is also shallower on that side...

Further, all OHV flywheels are steel, some M20 flywheels are cast iron (but most are steel) and most M21 flywheels are cast iron with the steel version being rarer...

This can be relevant if the flywheels are to be lightened (or revved) as the steel wheels are really required in that case...

This would be a long post if I went into the specific details of what has to be done to allow the use of the side valve flywheels in a B33 but both M20 and M21 wheels can be used with appropriate modifications. In either case one of the OHV con rods is needed, the early one for an engine fitted with M20 flywheels and a later short one if M21 flywheels are used...

I have built multiple OHV engines with the M20 flywheels but only two with the M21 flywheels...To use those requires a fair bit of work to get everything to fit, including relining the barrel and boring the crank case mouth out if the maximum capacity is required (720cc..90 bore by 112 stroke)...

Dimensionally in that case the engine is being taken more or less to the limit. There is also the less radical option of retaining the B33s 85mm bore to give a final capacity of 635cc with the M21 flywheels....

I did it to see what the result was like but actually consider the 94mm stroke x 90mm bore piston (600cc) engine with suitably modified flywheels and other tuning mods to be the best package by far for practical use..I use modified 350 heads on those as well but that's another story!...

The actual mods required to fit the flywheels aren't too complex...any good engine machine shop or toolroom should be able to carry out the work...The crank may also need to be rebalanced depending on the engine characteristics desired...

In fact, the exact specification of the flywheels and the rest of the engine really rather depends on what you want to do with it and deciding that is actually the place to start... Ian

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

Thanks for your reply Ian, much appreciated

That's very interesting + some new information, Ian.
I don't think there are that many people around these days who know all this.
Can it be added to the info pages, Henk ?

Thanks for the information Ian,
Thinking back when i fitted a set of M20 steel flywheels into my M33 cases i cannot remember there being any difference in the timing side shaft. Sure the M33 had the 4 bearing crank and it was just a simple swap of the rod and 3mm cutting from the bottom of the original M33 piston. Also a 3mm plate under the cylinder had to be fitted. Not being familiar with the later B33 cases i gather they are a 3 bearing crank and this is the difference in the timing side shaft length. Not a massive difference in CC but it made a nice engine and is still running nice and is fitted to my XM20.
Maybe i was just lucky with the parts i had at the time.

Tim W

email (option): t.j.walker@btinternet.com

email (option): ahum@quicknet.nl

Hi Tim..The M33 was created simply by fitting the standard B33 engine into the M Series cycle parts...It was not an M Series bottom end and consequently all M33s/B33s have a '3 bearing' bottom end....

As the WD and later post war M Series engines are wider on the timing side due to the introduction of the air gap behind the tappet chest in the late 30s redesign of the side valves, they have the longer timing side main shaft...That and the reduction of the timing side crank cheek boss enabled the fitting of the fourth bearing into these later side valve engines...

Pre redesign M Series engines have a 3 bearing crank (1936/37 engines with the 8 bolt cylinder head) among many other differences but share virtually no parts with the M/B33...

I have never tried to fit one of the cranks from the 3 bearing M20 engines to a postwar OHV model so can't comment on whether that is possible..In fact I've never had a crank to try though I did briefly own a complete bike years back...

Maybe you used one of these earlier cranks?....

If not, then I can't see how you didn't come up against the timing side shaft issue.......Ian

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

Hi Ian,

Just looked through my notes of the build of the M33 motor that I was to fit into my XM20.
The original flywheels were poor as i could not align them well enough due to damaged crank pin tapers, these flywheels has a casting number of 66-635. A set came up on eBay with the same casting number and they were advertised as M33, they were around 40 miles away and i picked them up. They looked good and visually the same as my damaged ones. I fitted my rod and pin and they trued up nice but never measured the pin to the mainshaft measurements. Only when i fitted the piston and cylinder did i find out they were M20 stroke being 6mm longer than the M33. Easy to sort out by turning 3mm off the bottom of the piston and a 3mm packing under the barrel.
So i was lucky for once, and as you say they must be early pre war M20 with the short timing shaft, maybe that is why they were advertised as M33/B33. Flywheels with casting number 66-635 could have been bored to any half stroke centres, the only other markings on the flywheels were a single letter stamping being a letter K on both flywheels. When i built the motor i had no idea about the different lengths of timing side shafts, so as i said a bit of luck and a nice powerful motor for a girder fork bike.
Thanks for explaining the early engines.

Tim W

email (option): t.j.walker@btinternet.com