I have started rebuilding the m23 engine that I have. Its a mixed bag of parts for sure. M20 flywheel and cases, b33 conrod, m21 piston, Matchless inlet valves on both, incorrect valve springs, unknown prewar bsa rocker arms and pins.

So far I have found new old stock valves and guides. I've located a set of valve springs. I ordered a piston from the M24 website which I received. The barrel has been honed and the big end renewed. I've trued the flywheels and balanced the assembly. So far so good!

I am wondering about the geometry of the m20 lifters being straight as opposed to on an angle? I am guessing I will loose a small amount of lift which equates to performance? Wondering if I would get more lift by using gold star cams or just longer duration of lift?


Adam

email (option): xeon25@hotmail.com

I'm not an expert on M23s but didn't they have a 3 bearing crank?...

If so then the crankcases are a different width to WM20 cases which have the followers roughly 1/2" further out from the crankcase centerline..or do you have the 36/37 M20 crankcases....?...

Also if you have a post 38 M20 crank you will have to change/alter the timing side mainshaft as it is longer than in earlier engines..

Regarding the cam followers the OHV followers are 'tilted' to point the pushrods towards the rocker arms..

You may lose a little lift but that wouldn't really matter, the question is whether the pushrods may jump out when the engine is revved hard...

Have you thought of using early B31 cases if its a 3 bearing engine?..

That would give you the tilted followers and would also save you the job of remachining the crankcases for the head bolts and pushrod tube studs...Ian

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

The cases where machined long ago by someone. The engine did run before I pulled it. I was concerned with the lifter angle. The lifters and rockers have ball sockets so I guess this allows for the off angle.

Adam

They are wd cases and crank.

Now I'm confused...I looked in the 1940 parts list and the M23 has a 3 bearing crank as far as I can see...

That SHOULD mean that the top end won't fit onto post 38 crank cases which most WD ones are, because the rocker arm ends will be approx. half inch inboard of the pushrods/cam followers in relation to the crank case centre line...(forgetting the fact the pushrods and followers will also be vertical instead of at an angle to the crankcase top face)

The same thing would apply if you sought to fit a (3 bearing) B31/33 top end onto post 38 4 bearing WD crankcases...

Does your engine have the outrigger plate on the timing gear to support the cam spindles or do the spindles fit directly into the back of the timing cover?..and does the bottom end have 3 or 4 main bearings?

Or have I got it wrong and the M23 has 4 main bearings as standard?...Ian

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

I have 4 bearings and out rigger plate. I will get some pictures. I've been confused by what I have as well. Everything seems to fit properly.

email (option): xeon25@hotmail.com

The push rods are leaning towards the head instead of being straight. I measured 5/16ths of lift on the lifters. By eye of course. I measured the travel on a fixed point on the rocker arms and got 5/16ths of movement, no negligible loss.

I've revved the engine pretty good when it was running and had no problems. I guess time will tell.

There is a pair of zb34 cases on ebay that I've been tempted to buy. I belive they would work as well?

Adam

email (option): xeon25@hotmail.com

ZB 34/32, B31, B33, M33 crank cases will all work with your top end and eliminate the 'inward lean' of the pushrods towards the engines centre line..

Also they have the followers mounted at an angle to the top of the crank case so will eliminate the 'vertical' cam follower problem as well along with the loss of lift...

You will, however, have to modify the timing side mainshaft of the M20 crank to fit it into the OHV crankcases listed above...

It's a simple mod involving some basic lathe work and the making of a machined spacer, though the crank will have to be stripped to do the work...I can give you all the dimensions for that...

Frankly, if I had been seeking to make up (or fake up ) an M23 this is the route I would have taken...

Early 'X' or 'Y' B31/33 crankcases or early 'Z'B31/33 cases (with the narrow joint flange) would do the job nicely and are probably the cheapest to buy...

Later 'wide flange' B Series crank cases are sought after for Gold Star 'fake ups' and routinely fetch four or five times the prices of the earlier types...Ian

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

Hi Ian,

Ok I made an offer for the zb 34 cases on Ebay here in the US and it was excepted. I just happened to have a shorter timing side fly wheel in my box of tricks. Its has the same part number as the wd m20 flywheel so I'm guessing its prewar?

Did you get my Emails?

Thanks for the help,
Adam

email (option): xeon25@hotmail.com

Hi Adam..Yes, I got you're mails and have replied...

What's the number on your flywheel?...Ian ...

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

It is 66-635. I just read the technical section on cranks. So I guess it's post 49.

Yes, these are the 'unified' forgings used for B33,M33,M20 and M21 when machined with the appropriate crank throw and paired with the right conrod....

Apart from that they are dimensionally the same as earlier flywheels ...They are also steel flywheels which is a big plus for OHV use...Ian

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

Ian your rigth i make my M23 with cranckcases drive side M20 and timing side XB31 and it runs well for years. I do think that the angle of the cam folowers is not good if you use a complete M20 case.