There are a few Metric/Imperial anomalies on the M20....
Spark plugs have a 14mm thread..That could be a throwback to the early days of Daimler (metric) vehicles etc.
The 20mm A/F nut size can be found on at least two parts, the crank shaft nut and the fitting that the valve lifter shaft runs in...20mm is .787 and doesn't conform to a normal Imperial size. .781" is 25/32 A/F, the nearest Imperial spanner size..
The nearest Whitworth size is 7/16" W which is .820 across the flats, so it appears the size is in fact 20mm...
Also, the two drive side main bearings are metric as is the gearbox main bearing...
On a more basic level there are the B.A threads...This is an Imperial thread but based on the metric system....
This leads you to wonder...Why did BSA use any metric dimensions or components at a time when the Imperial system was firmly established as representing 'normal practice'?....Ian
email (option): ian@wright52.plus.com