Oh dear, getting confused again, maybe it's old age but I tried to fit a NGKB5ES spark plug into the 1958 M21cylinder head but noticed the plug I took out was shorter. According to the info I had, the alloy heads have shorter plugs - 1/2" and the iron have 3/4". So I wondered if I had bought the wrong one and after putting it in, very carefully moved the piston up and see if it was obstructed. Sure enough it was and on taking it out noticed that the gap had been pressed closed and a smear of carbon was on the top of the plug. I thought the alloy heads required the long reach plugs not the shorter ones. This being the case, is it possible that my M21, 1958 has an M20 or older M21 iron cylinder head on it? Is this possible? There is considerable rust on the fins which would flag up iron as a possibility but I thought by 1958 all these heads were alloy. I checked the other parts I have already bought, valves, springs and guides and they are universal to both M20 and M21 so I have no worries there. The other potential problem is when i come to doing the piston and rings. The M20 appears to have 4 (I believe one of the groves is not used), and the M21 only 2 rings. The stamp on the engine onto where the barrel sits states plainly M21 so should I expect a 2-ringed piston, or indeed if the barrel is an M20 could I be further confounded by finding 4 groves! I have always believed people when they said the M range are simplicity itself but I am now wondering if it's me who is simple. Please can one of you chaps who know these things explain what could be going on?
M20 and M21 barrels are the same part, as are the 3 piston rings.
M20 and M21 heads are the same part and changed from iron to aluminium alloy around 1950.
Short reach plug for iron long reach for alloy.
The M21 piston is shorter above the gudgeon pin than the M20 item.
thank you very much - that is a weight off my mind.