Engine Number ID

[Brooking 850]

Trying to identify this model 750 1971 Commando. Gap between head fin and barrel fin is very narrow (planed head or Combat) and bike is all together and new owner hasn't had the head steady off to see if there is a "C" stamp as yet.
No stamping above RHS tappet cover. No small drain plug in the crankcase, only the larger plug/drain.
There is a blanking plug in the crank case on the LHS where the camshaft timer/breather is normally mounted.

20M3S 151285
420R
Regards Mike

 

[Time Warp]

There was no Combat anything with the 20M3S engine.
They have an RH1 cylinder head and of all the 1971 heads I have there is no stamp on the top face. (Unlike from 1972 onward)

151### would be fairly late in the year 1971 which was the last year for it (20M3S) before the change to the 1972 engine cases with rear breather and altered oil pick up.

 

[Time Warp]

If the engine has the timing side port (for the breather hose) plugged, it better have a JC type sump reed valve breather or if a breather at the plate location on the T/S rear, a modified chamber wall (added holes) to not be a oil leaker.

 

[L.A.B.]

Trying to identify this model 750 1971 Commando.

Do you mean is it a Roadster, Fastback, Hi-Rider, as you already know it's (late) '71?

420R

Where is that (stamp?)?

 

[Brooking 850]

Hi Les, that 420 R is directly under the 20M3S
Its a Roadster , steel tank and steel side covers , twin leading shoe front brake , short clock holders (unpainted no boots)
Older style tail light (not the square box type)
No other breather Time Warp
Engine No stamping on the flat of the crank case
Current owner may well come on the forum, if so Ill encourage him to post pics or I can for him
Regards Mike

 

[L.A.B.]

that 420 R is directly under the 20M3S

Ok, I've no idea what that is.

Its a Roadster , steel tank and steel side covers , twin leading shoe front brake , short clock holders (unpainted no boots)
Older style tail light (not the square box type)

Engine No stamping on the flat of the crank case

The steel tank and side covers are almost certainly from a later model.

No other breather

There should be a breather somewhere.

Current owner may well come on the forum, if so Ill encourage him to post pics or I can for him

 

[gortnipper]

This is PL's bike then?

He got the NOCNZ site to work, was being blocked by his AV software.

 

[Brooking 850]

Yes thanks Dave
Standard breather only out of the timing chest

 

[gortnipper]

From http://www.gregmarsh.com/MC/Norton/CommandoID.aspx it looks like the bike would be a very late 750 MkII or MkIII. All models were MK II except that the Fastback was MK III with new style fork yokes - non adjustable type. So, looking at this may be able to help you identify the specific model.

There are the adjustable Atlas-style yokes used on early Fastbacks, the R, and S-Type.

These are not interchangeable with the next, non-adjustable pair with the stem fixed in the top yoke. Later versions of the bottom yoke had two 5/16" holes to mount the front fender of the SS and also for the option steering damper kit.

 

[Time Warp]

No other breather Time Warp
Engine No stamping on the flat of the crank case

That would be the normal stamp location for a just before mid 1971 and onward 20M3S engine, my March 1971 Fastback has the earlier location in the forward of that location concave curve.
That description sounds like any of the 20M3S 750's from that last build year, the exception as far as rarity might be if it left the factory as an SS model originally and had been modified since.
It will have the one year only steel power socket on the right hand side of the air box front panel.

It would need some form of breather if that hose is not in use.