The last point is an important one..As the M20 has very mild valve lift, any pocketing of the valve into the cylinder resulting from excessive valve seat wear/cutting can have a disasterous effect on gas flow...which at best is not brilliant.
Also the 'lip' between the seat face of the exhaust valve and the top of the valve must not be too thin if premature valve failure is to be avoided..It is this lip which helps edge of the valve to withstand the exhaust gas temperatures....Ian 
email (option): ian@wright52.plus.com
The inlet valve is in spec if it's head is 1.732" to 1.737", it's seat width is 0.125", it's stem diameter is 0.3475" to 0.3485" and it's stem to guide clearance is 0.003" to 0.005".
The exhaust valve is 1.607" to 1.612" and it's stem diameter is 0.3475" to 0.3485" the guide clearance is the same as for the inlet
The seat angles for both are 45 deg, these are the acceptable limits from BSA's own literature for overhauls.
Surprisingly the length is immaterial due to adjustment on the tappets. the onlt other thing it states is that the exhaust valve nust protrude no less than 1/32" above the gasket face of the barrel.
what i tried to find out is exactly what you posted here. When i take them apart i would like to see how worn they are. they are not very expensive to replace (valves and guides), but if it ain't broken, don't try to fix it
Thx
email (option): kschaank@hotmail.com
Also the 'lip' between the seat face of the exhaust valve and the top of the valve must not be too thin if premature valve failure is to be avoided..It is this lip which helps edge of the valve to withstand the exhaust gas temperatures....Ian
This is good information. So basically if the exhaust valve sits too low. it means either fitting in new valve seats or find a new cylinder.
email (option): kschaank@hotmail.com
Hi Koen..If the barrel fins etc. are good a new valve seat is the way to go. Hardened seats for use with lead free petrol aren't absolutely necessary for an M20 top run well but, if you have to restore the seat, an added bonus with the modern seats is that maintenance intervals (of the valve seat) are longer...Ian
email (option): ian@wright52.plus.com