There are two types of tappet adjuster locknuts used on M20/21 and B31/33 models...Both are 5/16" cycle to suit the tappet stem thread...One type is a plain nut (to CEI pattern) but is thinner than a standard nut at .200" thick...The other is thicker at .312", the additional thickness being derived from the 'domed' form on the underside of the nut....Both types are hardened...

Does anyone know of a source for either of these 'special' nuts?....Ian

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

Hi Ian
I can put some aside for you
Br
Job

email (option): jonnyob1@googlemail.com

Thanks John...They can come with the other parts...Can you spare six?...Ian

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

I think so ian
I can confirm in the morning
I’ll have to open the Jiffy bag agsin 😃
Br
Job

email (option): jonnyob1@googlemail.com

Hi Ian
I have the required 6 lock nuts here - they are new old stock

Part reference for these for m20/21 is 15/6524
I have checked the 46 parts book & this number is shared between
B31/33
M20/21

No other reference is given

I await your confirmation
Br
Job

email (option): jonnyob1@googlemail.com

From a bsa list Standard nuts.

15-6524 0.525" hex 0.175" thick 5/16 CEI

email (option): pes.sales@btconnect.com