I am building a 90x112mm WM20 engine with a custom 2" intake valve and guidance from Ian Wright. The valve is a one-off item from Manley Valve in the USA, a well-regarded manufacturer of NASCAR and drag race (sprint) components. Manley have not been forthcoming with help, to say the least.

My question is what is the best choice for the valve-to-guide clearance in this engine? I am using the BSA cast iron guide which necessitated a bronze sleeve to accommodate the Manley stem, which is .3405", vs the BSA valvestem of .3470.
I am at .0015" now and considering a final clearance of .0025. What do you chaps think?

My second query is regarding the crank balance factor. I was at a vintage race meet in Ohio this weekend helping 2 chaps running a ZB34 Gold Star. The engine builder suggested I get my engine dynamically balanced at a shop he uses. He said I need to specify the optimum RPM for my engine and the "balance guys" would do the rest. I am using a JE piston with modern rings and a pop-up crown, and the flywheels have been lightened along the lines of the ZB-BB Gold Stars. I plan to ride this bike much as you chaps do, but perhaps maintaining a bit higher cruising speed, say 50 to 60 mph. I'm inclined to say 4000 rpm for the balance figure. Is this a reasonable choice?

Thanks for any help!

Jeff in Rhode Island

email (option): jjbandoo@aol.com

Hello, my M20 have both engine and gearbox sprockets 19 teeth, rear tyre is 3.50". On this case engine makes 3547
rpm on top gear riding 90 km/h (55,9 mph)

Hi Jeff..Stem clearance is going to be a bit of a guess..The lower tolerance with the normal set up is .0035" and a reduction in this figure is not normally successful. However, with materials such as 'Colisbro' which is a low expansion guide bronze, much tighter tolerances are possible..I had a ZB32A head here with these guides fitted that I know could not have run with such a close tolerance with an iron guide.
I guess it is really down to what the guide sleeves are made from...Your .0025" clearance seems like a good place to start...
Standard BSA crank balance factor for the singles was 58%..I have mine dynamically balanced to 60% and have done for years with good results...Ian

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