['If you can think of any other reason why BSA heads are so expensive [Given that the design went on for decades] compared with war specific 16H ones, let me know...']
I think the answer to that is just supply and demand...There are a lot more surviving M20s that have done a lot more work so there are more that have lost/damaged/corroded heads...To illustrate that, on the first Normandy Tour of 144 bikes 58 were M20s and 7 were Norton 16Hs....
Personally, I don't think alloy heads are intrinsically less reliable IF they are correctly fitted and maintained and the mileages I've done with one fitted bear that out.....What they are is more prone to problems caused by incorrect fitting which is not the same thing...A cursory look at many alloy heads will indicate that many have been misused in the past...Having spent many years rebuilding engines etc. for other people I lost long ago the belief that everyone knows what they are doing...and I include myself in that for many years after I first acquired a bike....So, the alloy heads have suffered and the (IMO) false idea has arisen that they are unreliable by virtue of their design..Regarding torque wrenches they are a modern 'luxury' that wasn't avilable in the past but they aren't an absolute requirement to achieve reliability...I ran M20s for decades before I bought one...Ian
Jeremy I have no idea about the relevant costs of an M20 or 16H head, other than there are probably 10 times more M20's existing now than 16H's therefore more demand for spares??
Regarding Torque Wrenches. They were a crude spring pull device back then and not in general use in ordinary workshops. Hence torque wrench figures are not included in the manuals. They were tightened by experience and even different length spanners to adjust the pull.
Figures today are quoted either from experience or by comparisons from more modern vehicles with a similar fitting or charts giving figures for different threads and sizes etc.
The biggest culprit of blown composite head gaskets is after fitting a new one, failing to re check tighten the bolts as many times as required during the first warm up sessions until it is compressed to it's maximum. Even then, the cardboard in the modern gaskets will still often blow out. Ron
The numbers could be it, but try finding an ally head. They seemed to have been binned when replaced by iron ones. Sidecar outfits were owned by folks that generally couldn't run to a car, not keen mechanics
