Just a quick enquiry anyone used a inner valve spring on their BSA M20,s I have a spare barrel which is fitted with inner valve springs no idea the model type of springs fitted maybe A10 or C15 ?.I personally dont think there would be a advantage....
I used C15 inners in one of my 720cc M20s...However, that was with touring cams which could produce some valve bounce when the bike was thrashed, as standard springs are pretty light...
With standard cams I have always run standard springs...This is the set up I have in the 720 M20 that has done the most miles (it's been running in that format for over 10 years) and it doesn't seem to need anything stronger...It would be worth measuring your original springs to see how they look against the specs. in the standards book..They do settle and weaken with extended use...Ian
