I have a M21 on standard bore and it never showed any signs of burning oil but the oil always gets black within 100 miles. Piston and bore still within spec. So honed it and put in new rings (NOS Hepolite) and it is still the same. Valves and guide were also checked and are in spec. Bike runs nice, no visible sign of oil burning other than normal usage. So it would appear I have blow by on the piston. Now the question is this normal for an M20/M21? I have used modern rings in a number of English bikes (not BSA)to reduce oil use but never seen one that has blow by but no oil burning. So lets pretend it is the 1940's and you have a new M20. Did they have blow by? I am not sure modern rings will help in this situation as the bike does not burn oil?????
Just got back from a 150 mile ride today and rode the bike very hard. It never missed a beat and ran perfect. Oil was clear when I left and is now black! I realize all oil eventually gets black but should not do it this fast?
My Velocette has probably 300 miles since last oil change and it still looks clean.
If you are using modern oils everything is perfectly normal. Modern multigrade oils have detergent additives and will turn black in an M20 as you say. If it bothers you change to what they should run on a straight SAE 50 or40) ie Castrol GP50. See instruction manual.
I have used both 40W and 50W. No difference. Have never used multi grade oil. So it appears for M20/21 this is normal? No other old bikes I have: Triumph, Norton, Velocette, AJS, Matchless and none of get the oil black this fast.
I had the same years ago: Oil black within a few miles.
It turned out that the balancing holes on the inside of the flywheels were full of very hard sludge, hardly recognizable as so. It apparently dissolved with the modern oil I was using.
I machine a recess just below the oil ring, about 5mm wide and 0.5mm deep (3/16" x 0.040"). I drill extra holes in this recess to help drain oil. My intention here is to reduce oil consumption, but may also increase the time it takes for the oil to go black.
Modern rings will not work in air cooled grey cast iron bores.
To work properly you need soft iron rings fitted into a coarse honed bore.
No finer than 180 grit & I preffer 120.
Rings bed in with a 120 hone in about 3 minutes.
The big problem is you must use a block hone & not a bottle brush type if you want the rings to seal before they are old enough for a pension.
