Ok so i decided to start the rebuild of my BSA M20, i took i apart long time ago and it's been waiting to that day will come.
so the crankshaft had big side play, what should fill that gap? do i need to had shims as many as it required? or do that play is from a bad bearing?

the rod have a small up and down play, how good is replace the roller with OS ones? will it last or should i get a new pin? why are those pins so expensive??

I saw that there are few option for the sprocket at the crank shaft.
what is the best size to get? if i will use 21 the engine will have the power to start up good?

what piston should i get, i would like to get some more HP form it, so has i read there is a lot of option for that.

many thanks, Pete.

Hi Pete...The first thing to do is to obtain the correct workshop manual if you are not familiar with the basic principles of how the M20 engine is assembled.....

The method of assembly is explained in the manual, along with advice on tolerances etc...

Options for tuning the engine are limited and the tuning of side valve engines is in itself an involved subject...

Simple enlargement from 500cc to 600cc (M21 spec.) is probably the simplest option as a first step and will give you better performance. However, that will require the fitment of the longer stroke M21 flywheels and the M21 piston in place of the M20 parts...

Any decisions you make about gearing will depend entirely on what you end up doing to the engine...
A standard M20 isn't a particularly good subject for pulling higher gearing as it produces insufficient power to 'pull' higher gearing under all conditions throughout its rev range...Ian

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

Hi Lan, i have download a manual, but i'm not sure that i will get any answer that on it.
i'm not sure why there was side play in the crankshaft, so is the use of shims are normal?

about the big end to rod clearance, i guess the cleaning with sand paper and installing a oversize roller won't be in any manual, my question is it going to last or its a temporary fix? i'm not planing to do it again in that engine...

thanks, Pete.

Hi Peter...It is impossible to adequately determine whether the end float you have in the crankcases is correct without knowing the condition of all the internal engine parts relating to that...

In principle, when the crankcases are assembled there will be some end float...This is taken up when the crankshaft shock absorber assembly is fitted and tightened...

In the M20 engine the crank assembly is pulled over to the drive side and locked against the outer drive side main bearing by the tightening of the shock absorber nut....

The spacer between the two drive side bearings should measure 1.000" - 1.005" long and the drive side oil flinger plate (fitted between the flywheel and the inner drive side bearing) should be flat and unworn...

When the shock absorber nut is fully tightened the conrod should lie centrally in relation to the joint between the crankcase halves...

Regarding the big end there should be no up and down play in the bearing or any damage/corrosion on either the crankpin or outer race...

It is possible to hone out the outer eye of the big end and clean up the pin to accept over size rollers...It should be noted however, this is a PRECISION COMPONENT...

The outer eye should be honed using the correct honing machine to produce a parallel bore and the pin can only be cleaned up by hand if it has very light wear...Any more and it should be reground...

You will need to source oversize rollers and rework the crankpin and outer eye to produce a good surface finish and the correct tolerance between all the components for it to work as it should....Ian

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

You need professional help.

Hello Amit (?) / Pete.

Where are you located?
Maybe I could help..

Noam.

email (option): noam10@gmail.com

Hi Lan i checked every thing that can cause that gap and i found out that the oil flinger(it was stack in the crankshaft and lost a bit from the thick) and the spacer are worn out a little-bit so i guess that is the case of the gap.
As yo said, Thanks!

i will buy a NOS oil flinger. Should it bee flat all the way? i have 2 that are bend in the big-end pin nut, i tough that it's done for it wont spin in the crankshaft pin.

for the big end clearances, i guess it will be batter to open it and to take a look.
i tried to open the big nut of the big-end pin with a impact driver with no success so i will be need to make the crank holder(with the two pins...)



Hi Noam i'm from Israel as you guessed ;)

Hi,

If you need help, give me a call.- 0508678697
I am not an expert as some of the others in the forum,
But I rebuilt few engines, so maybe can help..

Cheers,
Noam.

email (option): noam10@gmail.com

Send me an email and I will send you plenty of service sheets and a workshop manual. Cheers.

email (option): britool51@hotmail.com