Hi Müs,
You are asking a slightly tricky question: I'm not sure there is a figure for the "official Douglas clearance" for the big ends, but perhaps someone here knows?
I assume the bearings are the usual split-bronze setup.
I ride my bikes pretty sedately, run in a fresh engine, and keep plenty of oil in the crankcase, and so I use rather small clearance on a bronze big end. I never measure it - just ream it out until it is (just) free on the crank pin. I'd be surprised if the clearance was more than 0.001 or 0.002" (say 0.025 - 0.05mm). I have never seized a big end.
I have a friend who now uses 0.007" (0.18mm) on his veteran BSA to avoid seizing. A much bigger pin than the Douglas, but to me this seems like very large clearance.
If the CW is to be ridden hard maybe 0.003" or even 0.004" might be a good figure to aim for???
Good phosphor bronze for the bearing, and plenty of good (straight 50?) oil, should keep you out of trouble.
Any other opinions welcome.
Cheers
Leon
