Identify Head

I focus on Commandos so forgive the question. I have a head in good condition that came as a part of a parts lot I bought. It has the rocker feed on the top. What is it for, or what other info do I need to provide? Just looking at it, I thought 750. It has not valves or rockers and the exhaust seats have some surface rust, but otherwise, it is in better condition than many I buy and use. I have a bunch more pictures, but I doubt that they will help with identifying it.

Looks to me an Atlas, etc. 750.

I used this site to idenitfy mine which were numbered 25319. No numbers on yours?


It does look like Atlas or N15CS to me too. Here are a couple pics of mine.

A quick and dirty way to tell if it's a 750 or anything else, is if the bolt holes are 3/8" or 5/16". They were downsized to 5/16" to make room for the larger bore of the 750. All others are 3/8". The head you sent pics of has the casting fins between the rocker spindles, so is either the SS head or a 750 head. SS heads in good shape are relatively valuable.

Mine are 5/16" and have those fins. Maybe the same head?

There is no casting number on top. It must be 750 because it's 79mm across the combustion chamber. It has holes for 5/16" head bolts. From what I read then it's an Atlas but should have BIRCO. I think it has been skimmed.

Oh that's the spigoted head version, it is mentioned on the site I linked to. I had one with a butchered exhaust thread and the undamaged non-spigoted one I ended up using. I had to have the spigots on my barrels milled off. You can see the area where the spigot fits in comes all the way around the chamber edge, unlike the later one. I'd have traded you for that one.

Here's my 23166 which has gone to another member here.

You can use those rings to transform your spigoted head ...

I would say you have a good enough eye to recognize, if it were a 650 head, that the bolt pattern-spacing would look small compared to a 750 Atlas/Commando.

t ingermanson said:

A quick and dirty way to tell if it's a 750 or anything else, is if the bolt holes are 3/8" or 5/16". They were downsized to 5/16" to make room for the larger bore of the 750. All others are 3/8". The head you sent pics of has the casting fins between the rocker spindles, so is either the SS head or a 750 head. SS heads in good shape are relatively valuable.

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Not always. Obviously, 750 Commando heads use 3/8 inch outer head bolts. Also many Atlas, N15, and P11's were modified to use Commando barrels or just drilled and tapped for bigger 3/8 inch bolts for better sealing, There is plenty of room for 3/8 inch bolts, however on some castings a thin socket is needed to clear the fins.

"Quick and dirty" means "there's a lot going on with many exceptions, but chances are, your problem will be solved with the following". Caveat Emptor, your mileage may vary, etc.

Greg's very familiar with Commando heads, so we can assume that he'd recognize one of those.

In this case (and most), the 5/16" vs 3/8" bolt holes is a reasonable "if/then" to identify a pre-Commando Norton head. Virtually noone is reducing the holes to 5/16", so that would have been (was) an easy one, no? Sometimes the flow chart will need to be longer than other times in the search for the telltale, but we'll get there.

With 50+ year old used motorcycle parts, anything could have happened, so possibilities of twin plugs, bathtub combustion chambers, 38mm inlets, or clamp-on exhaust spigots are as relevant and potentially confusing as any, if we're opening up the possibilities to any Norton head with modifications. Even in those instances, the 5/16" vs 3/8" if/then would still be an ok first step for narrowing a Pre-Commando head, particularly without a photo.

Sure seems to match mine by comparing pics, number, and description on the atlanticgreen site. Mine was the original off my '66 N15CS as far as I could tell, had the spigoted barrels to match.

Not claiming expertise but sure looks like mine which is N15CS (Atlas) spigoted head, by number and hole size.

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marshg246 said:

There is no casting number on top. It must be 750 because it's 79mm across the combustion chamber. It has holes for 5/16" head bolts. From what I read then it's an Atlas but should have BIRCO. I think it has been skimmed.

Click to expand...

Yes, obviously. It's a 750 head made before Oct. 1965 (s/n 114870). It's usable on a non-spigotted barrel if a flame ring is fitted.

- Knut