Hi Vincent..Valve guides are (theoretically) taper ground on the O.D. .624"-.626" at one end tapering to .625"-.627" at the other end. (re. M20 standards book).
However, if the first tolerance was 'mid limit' and the second tolerance was 'bottom limit' then the guide wouldn't be tapered, being .625" along the whole length. Conversely, there could be a taper of up to up to.003" if the tolerances were anywhere towards the opposite end of the range.
On that basis a parallel guide of .625"-.626" should do the job OK.
Ref. the I.D. this should be sized after fitting to the barrel to produce the correct stem to guide clearance so would be determined by the measurement of the individual valve stem and the application of the correct tolerance.
The stem to guide clearance for both valves is .0035"-.005"...I usualy size them to the lower tolerance, so in that case stem size +.0035" would give the guide I.D. after fitting.
I'd have to measure some guides to give you an overall length..As a matter of interest the exhaust guide is 1/16" longer than the inlet. It is also counterbored at the upper end...I'll post these dimensions later after I have been to the workshop...Ian
email (option): ian@wright52.plus.com