My '58 M21 project is coming along nicely now and the engine with new mains, big end, rebore, valves and piston is now sitting in the frame. Feels like real progress at last.

Getting the engine in was a struggle and while I was sitting in the garden enjoying a celebratory cup of coffee with my helper, I saw a Robin go in the workshop and come out after a few minutes. When we went back in it had left a small deposit on top of the barrel gasket surface - I think I'll take that as a good luck omen!


Before I bolt the head on can someone please confirm the squish band clearance etc?

The top of the piston at TDC is 1.44mm below the top of the barrel gasket face and the (solid) copper gasket I have is 0.9mm thick. The head has had the lightest of skims to give a good gasket sealing surface. As this was a basket case I have no way of knowing if these measurements are within tolerance and will give the correct compression ratio etc.

Can anyone help please?

Gary.

email (option): gj.owen@hotmail.co.uk

I believe the piston crown should be flush with the cylinder deck at TDC.

If you have an aftermarket piston it might account for the clearance you're seeing.

The collective wisdom of this forum is that there should be 40 thou or 1 mm clearance between the head and piston, so with your discribed current set-up and even with the head surface lightly skimmed, the compression will be less than optimal.

email (option): teladelujo@msn.com

Gary

Its perfect, the piston won't hit the head, so leave it and get it running.

All these old bikes were built with plenty of clearance between piston crown and head - partly to allow plenty of clearance for con-rod stretch when revved and also to allow for muppets who leave off gaskets, get (OHV) valve timings wrong, etc. etc.

Squish band theory and minimal piston/head clearances need to be considered in highly tuned engines only, where the combustion chamber/piston is designed to make use of the squish.

I utterly disagree with that...

The squish area was introduced on production side valves to improve combustion following experiments by Ricardo and others...

Correct squish tolerances can easily be obtained in the M20 engine that allow the 'squish' to function correctly and to serve to reduce the flame path length and improve combustion...(as BSA and others intended)..

I am wholly against the 'near enough is good enough' philosophy that's in favour of ignoring settings that were put in place for perfectly sound reasons, based on developements by the factories and others...

In this instance there is no disputing the benefits of squish, it was regarded as a major jump forward in the development of side valve engines and was quickly adopted throughout the industry...

You will find it in all current side valve engines as well...

The only real difference between an older engine and a modern one is that the squish clearance cannot be made so close..(and it is a case of the closer the better)...

However, I have proven by experimentation with the M20 that the tolerance can be held within the range considered to be both functional and beneficial..That is between .030" and .040" set with the engine static.
The final figure will reduce further with the engine running due to crank flex etc. and to avoid piston to head contact .030" should be considered a safe minimum...

With the correct piston and base gasket fitted the piston should be flush with the gasket face and fitment of a solid copper 1mm thick head gasket will suffice to obtain the desired result...

It is good practice to check all dimensions on assembly however...

In this case it seems the piston is unusually far down the bore, so base gasket thickness should be looked at. A light skim of the cylinder gasket face and/or alterations to the head gasket thickness can also serve to bring things within tolerance...

If the piston is not standard in its dimensions it is worth considering that it may all need to be looked at again if the engine is rebored at a later date and another piston fitted...Ian

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

Ian Wright

I utterly disagree with that...

The squish area was introduced on production side valves to improve combustion following experiments by Ricardo and others...

Correct squish tolerances can easily be obtained in the M20 engine that allow the 'squish' to function correctly and to serve to reduce the flame path length and improve combustion...(as BSA and others intended)..

I am wholly against the 'near enough is good enough' philosophy that's in favour of ignoring settings that were put in place for perfectly sound reasons, based on developements by the factories and others...

In this instance there is no disputing the benefits of squish, it was regarded as a major jump forward in the development of side valve engines and was quickly adopted throughout the industry...

You will find it in all current side valve engines as well...

The only real difference between an older engine and a modern one is that the squish clearance cannot be made so close..(and it is a case of the closer the better)...

However, I have proven by experimentation with the M20 that the tolerance can be held within the range considered to be both functional and beneficial..That is between .030" and .040" set with the engine static.
The final figure will reduce further with the engine running due to crank flex etc. and to avoid piston to head contact .030" should be considered a safe minimum...

With the correct piston and base gasket fitted the piston should be flush with the gasket face and fitment of a solid copper 1mm thick head gasket will suffice to obtain the desired result...

It is good practice to check all dimensions on assembly however...

In this case it seems the piston is unusually far down the bore, so base gasket thickness should be looked at. A light skim of the cylinder gasket face and/or alterations to the head gasket thickness can also serve to bring things within tolerance...

If the piston is not standard in its dimensions it is worth considering that it may all need to be looked at again if the engine is rebored at a later date and another piston fitted...Ian

email (option): gj.owen@hotmail.co.uk

Don't worry I'm sure it will run just fine. Despite what some people say these engines where not state of the art supertuned racers even in the 30s. They where "cooking" engines designed initially for the working man to get to work on and maybe hitch to a chair to take the wife and kids for a spin at the weekend. They did that job admirably, as they did their army work later. As Nigel mentions they had to put up with idiots working on them who didn't know anything about how to do it. The fact they are still here today is prove of their ruggedness.

Gary - just run the damn thing, its not a rocket ship despite what people try to do with side-valves.

- Built to questionable tolerances, repaired and assembled by countless qualified and unqualified people, using whatever parts they can find over a very long time period - Be happy nothing hits anything as you turn it over.

Currently have a c11 engine on my bench where the NOS piston hits the flywheels at BDC - no 'reason' why it should be 5-5mm too long, but it is - silly me not checking it was the correct length before fitting.
Soon cured by chucking up in a lathe and machining a lump off - or better still gripping in a vise and sawing a lump off - as would have been done in times past.


Ian - i know all about Harry Ricardo's work on squish, read it in 1972 and still have "the high speed internal-combustion engine" on my shelf - its a seminal piece of work.
As i presume you have read it, you will know that most of the WW1 and post work he carried out (and patented) was largely to do with preventing 'knock' on the crude 40/60 octane fuels of the period.
Squish indeed seems to have been his idea, but a quick read through does not give any dimensions to the clearances he was using.

The 0.5-1.0mm clearance often quoted seems to have come from the two stroke work of Ernst Degner in the early 50's on small cylinder size two-strokes.

Interestingly as fuel has become more 'sophisticated' over last 20 or so years, there has a been a move away from large squish area's in both racing two-strokes and automotive engines.