I'm sure I have this somewhere but can't put my hand on it...Does anyone have the bore/piston skirt clearance figure for a 1936/7, '8 bolt' cylinder head, M20 and/or M21?....Ian

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

I have 2 D.W. MUNRO books on B.S.A. pre-war, post-war. Pre war book claims piston clearance bottom of skirt alum. piston .0035"/.0035" side valve engines. This is the spec. machinist and I used to check my 1937 M20 engine before re assembly My engine was within specs and is running very well do not know if this helps. My 1937 BSA manuals give no specs. And mine is 8 bolt head. Charlie

email (option): millcrocfarm@gmail.com

Thanks Charlie....0035" I get but the other figure should be different...e.g .0035" -.0055" ....Ian

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

I am sorry brain fart .0035-.0055 sorry about that, I do not type well. Charlie

email (option): millcrocfarm@gmail.com

Is this any different from later, "ordinairy" M20's ?

Piston to bore clearance is normally specified by the piston manufacturer, as changes in design will change how it expands.

email (option): pes.sales@btconnect.com

That would be the case generally...However, competition applications where the engine is not 'run in' using normal procedures can require considerably greater tolerances than standard to avoid seizures...The manufacturers recommendations are a guide but alterations for individual applications are often needed....

For example...In a B33 engine I built for sprinting the bore clearance needed was .0015" over the manufacturers tolerance to avoid problems... Pre unit Triumph motors I built for pre 65 scrambling smoked slightly right off the bench but could be raced without any running in...They had tolerances in the bore that were as close to 'worn out' as possible!....

I also found running in with the extra load of a large sidecar required a little more clearance in the bore to help in the early stages due to the extra loads...Ian

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

Charlie...Is your '37 M20 on the road?....Ian

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

Ian this is Charlies bike. He's going to send me pictures of it with some upgrades and a nice shiny fishtail. I quite like it like this anyway. Ron

email (option): ronpier@talk21.com

Very nice..I got offered a completely standard one of these many years back (with the optional footchange) for £800....Seeing Charlies bike makes be think I should have taken it up...Ian

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

HI Sorry took so long to respond yes bike is running quite well a friend has a WD and he took my 37 for a ride came back and felt it accelarates better than his. His is rebuilt. WE think because it has different cam profile. Anyway great bike to ride around 45-50mph paved roads near my farm. I like megga better also makes great sound. Fun to goof with the Harley guys! I am having it judged by the AMCA so I have to put on the original restored exhaust for judging then I put megga back on. Thanks again for all you guy's help in finishing this bike Charlie

email (option): millcrocfarm@gmail.com

Here is Charlies bike with the shiny exhaust fitted. Ron

email (option): ronpier@talk21.com