THREADED HOLES-All the threaded  holes on all aluminum heavy twin heads are whitworth form. There are 4 different threads.
1. 3/8 BSF
Atlas & Commando-on the 3 head/barrel studs.
Atlas-head steady center stud
2. 5/16 BSF
Atlas & Commando- intake rocker cover stud
Atlas- 2 oil inlet on top of head
Commando-2 oil holes on side of head + 3 headsteady bolts.
3. 5/16 BSW
Atlas and Commando-intake port/manifold
4. 1/4 BSW
Atlas & Commando- rocker spindle cover plate screws (pre 66-short)
Atlas & Commando- exhaust rocker cover studs

Exhaust ports are 1-15/16x14tpi, The factory shop manual tech specs seem to be in error.

Click on pictures for full size view

The following pix is to show the small cooling fin profile for the following heads. Iron on the left and aluminum on the right.  

SMALL FINS=   NHTheadSMFN.jpg

The first Norton head was cast iron and had an integral intake manifold for a short prototype period of production. It was reported to overheat so only 6 heads were made (unverified).
No pix available yet.

Then this cast iron head, T2140 (1950), was the second Norton Heavy Twin head, and had a separate aluminum manifold. The T2140 marking is above the intake rocker box cover. Another revision of this iron head, marked T2212, seems to be visually identical to T2140. T2212  was on newer model 7's.  Small 1.3" intake valves on this 500 cc engine/head. It uses 1/2" reach plugs. The exhaust ports are each splayed only 20º from straight forward. Unlike aluminum heads, this central headsteady center stud mount is tapped for 3/8-26tpi. The head is quite massive with the iron material at a bit over 26 lb., more than double it's later aluminum counterpart below. It would have contributed to a smoother ride due to it's added vibration dampening of the tiny 500 cc engine. It is held onto the barrels by 3/8-26cei bolts. Notice the horizontal AND vertical finning in front of the spark plug. The vertical spacing for the intake manifold studs on these early heads is 1.50".
Norton p/n   D12/136 

This head is from a swing arm model 7 and featherbed model 88 that started about 1955. This aluminum casting  is also marked with # T2225  M above the intake rocker cover. It is cosmetically/visually  the same as the previous iron head. It is also held on with 3/8" bolts. The center head steady stud mount is now 3/8BSF. Small 1.3" intake valves and 3/4 reach plugs. It is much lighter than the iron head above at about 13 lbs. Again, both horizontal and vertical finning in front of the spark plug. The vertical manifold stud spacing is still 1.5" on this version of Norton Heavy Twin head. The C-C  head stud spacing for the holes forward from the spark plugs is 6-7/8".
Norton p/n  K12/136  ?

The following pix is to show the medium cooling fin configurations for the following heads.  

MEDIUM FINS=   NHTheadMDFN.jpg

This head is from a 1957 generator 99 #75050.
This aluminum casting  is also marked with mold #T2225M above the intake rocker cover. It is held on with 3/8" bolts. Medium 1.4" intake valves and 3/4 reach plugs. Again, both horizontal and vertical finning in front of the spark plug. The vertical manifold stud spacing is still 1.5" on this version of NHT head. The C-C  head stud spacing for the holes forward from the spark plugs is 6-7/8".
Norton p/n  R12/136 

The following pix is to show the large cooling fin configurations for the following heads.  

LARGE FINS=   NHTheadLGFN40.jpg

This norton head 88SS/136 is a late model standard 88  head (1961 88SS) and 99 and 99SS in it's final version through 1963, the last year of production. It's top core has a casting mold number of 22707, which is again located above the intake rocker box cover. The spacing of the intake manifold studs has increased to 1.625" allowing the slab/splayed manifold for dual carbs. It has slightly increased finning all over but especially in the exhaust area. The finning pattern in front of the spark plug is all horizontal now. This head has full coverage cooling fins, but still has the low level intake ports and 20 degree exhaust ports. This causes the side fins forward of the spark plug to be broader than if on the 40 degree head which can be seen below. It has the new "picket fence" pattern on the side of the head between the rocker shafts and now has bigger 1.4" intake valves.

It seems that both the intake ports and exhaust ports have changed at the same point in production. The intake has gone from 25mm small diameter, low level vertically aligned manifold mounting stud holes @ 1.5" or 1.625" spacing and changed to a raised semi-down draft 28.5mm port with horizontally aligned stud holes. The following pix demonstrated these two conditions.

INTAKE PORT ANGLE=   NHTheads017.jpg

OK now I show the difference between the prior 20 degree exhaust port heads shown on the left with orange markings on the longer fins and the 40 degree exhaust port heads with the shorter red stripes on the fins.  

EXHAUST PORT SPLAY ANGLE=   NHTheads016.jpg

All the following heads through the end of NHT production are only semi-downdraft intake AND 40 degree exhaust port splay.

Following is a down draught head from a 1961 Manxman, 62+ 88SS and all 650SS. It has the casting mold number 22707 ocasionally with a suffix of K or L on top core of the head. By 1963 the mold number has been eliminated completely. It additionally has the casting mold number of 23166 on the bottom core of the head. The 23166 mold fixture is used later on in the early atlas, but the 650SS head still uses the small head bolt pattern spacing. The 72mm barrel spigot relief is the edge of the combustion chamber.It uses the bigger 1.4" intake valves. It has 28.5mm ports, which makes it a quite potent head for these small size engines.This small bolt pattern head from a 88SS &650SS, has big fins which the pattern remains unchanged until the end of all NHT, along with down draft intake ports, and the exhaust ports splayed at 40 degrees.
The small bolt pattern head after the 650SS/136 is Norton PN 25358. Appearing in the 1966+ parts book, it the LATE 650SS head for spigotless barrels. The casting is unknow to me and I do not know what it looks like, but being this early before the commando style Mercury head seen below, it is very likely this early core but with a smaller than 72mm counterbore. This will be updated if more information becomes available.

Norton p/n  S650/136

*A few excerpts from Jim Bush on a late 650 mercury head.
I have a letter from NOC President John Hudson to the previous owner  from back in 1993. John stated that "the 06-0380 casting was the Commando head and would not fit".  Well I obviously have an exception as  the 06-0380 head I have is definitely machined for the 650 - dome size,  bolt pattern and top oil feed. The side casting ribs are almost all gone  (machined) as per any Commando. It has not been modified at all,
original casting all round, no welding. I surmise that this head is a  late 650 (Mercury) or a early 1970's replacement item, part of a small batch produced for ongoing stock/warranty work.
All 650's and 750's Dominator have top oil feed, and share same casting  25319# (with small exception 06-0380 noted above) All Commando's have side oil feed - casting #06-0380 The only reason I see they switch to side oil feed is the isolastic head steady mounting studs go where the top oil feed hole goes. Casting changes were not required to put oil feed on top or on the side.
These are the 650 Mercury head pix supplied by and with my thanks to Jim Bush.


These two pix supplied by Tom in Australia thanks

The BIRCO symbol and 23166 are both easily identified by the positives in the casting of this early ATLAS head. These heads have the late style re-angled degree exhaust ports at 80º included angle or 40º each from straight forward. It has carried over 28.5mm intake ports from the previous heads. The head bolts have been down sized for atlas only to 5/16-26cei.The spigot relief machining is 75mm. Oiling is still top feed.

The same head with the 25494 steel conversion  ring installed. This allows the use of this head on unspigotted flat topped barrels.

See the pronounced "picket fence" fin pattern between the rocker shafts. The rocker oil feed is different from the commando in that the oil comes in from the top of the head, the boss right above and between the rocker shafts.


This is the late atlas head 25319. It has the same combustion chamber and squish band as the 750 commando (no provision for a spigot).
They have 28.5mm ports. It is the core for my 68 Dunstall Atlas with a very nice porting job.

Below is the early 68-70 commando head casting number 06-0380. .These standard RH1 ports are 28.5mm or 1.12". The number and BIRCO are easily visible on the intake side as positives.

The space between the front top 1st and 2nd fins are full of aluminum. There  is still some finning pattern on the side of the head between the rockers.

The 71/72 head. This would be the 2nd series of commando head,. as a standard 71and 72  small port they have original  port size of 28.5mm. In 72 the head would be marked with a "C" in the center top of the head to indicate the combat conversion was done. The combat RH3 was cut .042" to raise the compression. It had a 32 mm porting job. It appears the factory learned more about the porting business as time went on. First hand inspection of several "combat" porting  jobs show how crudely (bad) they were done. Big.... yes.  By today's standards these heads have been ruined. The "BIRCO" on the right and the casting number faintly visible on the left side are negatives (depressions in the surface).

A 71 head with a "rotated away from exhaust" big  intake valve Dunstall conversion. It has 32 mm intake ports. The squish band being completely machined away. It was for use with a Dunstall 810 barrel kit and the R3 cam which was very similar to the norton combat cam. I suspect this combination was the claimed 75 hp engine.

This side shot shows the "finning" pattern is now almost all gone. The space between 1st and 2nd fins has come back.

This is a RH5 73 low compression 32 mm port head. NO birco or part number of any kind is remotely visible. The side profile is indistinguishable from the 71/72 head. The RH6 32 mm port head would be the semi high compression with .020 cut off the head. The 73 factory parts manual shows a RH1 available. Labeled as 30 mm for the carbs they will use, it is the same as early RH1 with 28.5mm ports. Some 750 heads received 850 sized guides and were marked with an S, however they seem to be fairly rare.

A 73&75 RH4 850 32 mm port head. The porting job out to 32 mm seems to have improved somewhat over the combat. The 850 head types are marked above the right hand exhaust rocker cover. They show a much bigger head gasket pattern compared to all the earlier head styles.

From the side, visually almost identical from the 71-73 750 head. The head bolts are farther out by about 1/8" but sometimes hard to tell unless the two heads are side by side. The RH4/RH10 marking above the right hand exhaust rocker cover is the give away!  . Many of these heads have 063807 casting # as a negative on the front bottom.
It has recently become apparent that the thin intake port wall under the spring perch results in a very high percentage of RH4 heads being cracked at one or both guides. A simple dye penatrant test reveals this unfortunate reality, which evades detection by a casual glance into the port. So far, the RH10 heads seem to be much less crack prone. In my opinion, the 30 mm 74/850 RH10 head is a rarer but much more desirable core for doing good porting work

Mike Wilson Memorial head RH4 porting. Bashed to smithereens when a valve head broke off and went on a rampage. The head combustion chamber has been completely welded up. A 1-5/8" radius combustion chamber will be machined back in. It will get the  42 mm rotated intake valve Dunstall treatment. The squish band will not be machined back in an attempt to keep the compression up there.  . A Lotus Elan combustion chamber is the basic effect desired.