Hallo,

M20 and M21 have the same Cylinder Barrel(66 - 88) but different Cylinder Heads
( M20=66-94 + M21=66-96 ).

I can't find any number or indication on my Cylinder Head.

IF there is a number on the Cylinder Head where can I find it ?

IFNOT ---> how can I see the difference between a M20 and a M21 Head ?

BR

Hi Jos...Where are you getting your numbers?...I have looked in the 47 parts list and the 49-54 parts list and niether gives those numbers for the cylinder head. Both lists also show the same part for both models.
Heads variants I am aware of are..1) 8 bolt type, early combustion chamber shape, cast iron 2) 10 bolt type early combustion chamber shape, cast iron 3)10 bolt type, late combustion chamber shape, cast iron 4)10 bolt type, late combustion chamber shape, aluminium.
In all these cases, as far as I am aware, both models share the same heads...Ian

email (option): ian@wright52.plus.com

Thank You Ian,

I got these numbers from the list " Replacement Parts 1940 Models " page 8; but I can't see on the picture if it's a 8 or 10 bolt type.

But indeed in the list " 1949 to 1955 " the number is 66/59 for both models.
Same problem knowing if it's a 8 or 10 bolt type.

The head on my bike ( a M20 'transformed' by one of the previous owners into a M21) is a 10 bolt iron cast type ( old or new shape combustion chamber unknown because I don't know how to recognise the difference ). Also I can't find any number on the head; outside nor inside.

One of these days ... I will try to measure the stroke ( 112 mm ?) before changing the valves and guides.

BR

Hi Jos...8 bolt heads were only fitted for the first two years, from 1938 onwards the engines all had 10 bolt heads along with many other differences.
An M21 of the 10 bolt type has a stroke of 112mm...Ian

email (option): ian@wright52.plus.com

AFAIK both of the iron heads would have been machined from the same inital casting so if a numbers is to be found it will be the same on both.
In most cases the number on a part is the number of the raw casting or forging prior to machining unless it has been stamped in and this was not done for most parts, cams & gears being the most notible exception.

email (option): wariron@tpg.com.au

I & T,

Txs for Your comments.

JB