Cylinder head dating

If it has the milled off vertical fins above the plug holes then it will be fine and the correct style, What the SS markings indicate I can't be sure, Perhaps polished ports or possibly stronger valve springs . Won't make any  real difference .

22707 can be either pre-1960 for the standard 77,88 & 99 head or post-1960 for the SS types. They all had the cross-hatching between the rocker spindle plates.

The same casting number was used for two completely different engine types. The 99SS is the exception to the rule. This used a 22707 early head casting for a post-1960 bike. Consequently it needed a special twin carb mounting block.

Check the inlet end and refer to the attached drawing. The carb mountings are the clue. Apart from the 99SS all the SS heads will need the wider splayed exhausts.

Check here:

http://atlanticgreen.com/nhth.htm

Research on the SS stampings so far,  has shown on barrels,  that the push rod tunnels were slightly enlarged to accomodate the SS cam and due to the extra lift and therefore exhaust rocker arm swing.The exhaust push rod swung  forward and would rub the barrel tunnel forward edge. The SS barrels are machined a bit on the front exhaust side in both push rod tunnels. It is quite possible the 61/62 99SS using the 88SS/136 head , without the down draft head, also needed the clearance machining in the head for the SS cam push rods as well the 61 88SS and was so marked SS. 88SS/136 are 20 degree exhaust port heads

The extra intake SS cam lift did not seem to cause a interferance problem in the intake tunnels.

. For 62  the S650/136 head was used on the 88SS and  650SSthese are 40 degree exhaust port heads shown on right.   20 degree port 88ss/136 head used 61 88SS and 61/62 99SS shown on left

In Tony's first post he seems quite clear on the diference between downdraught and non downdraught heads , so the above information (although of interest)  is probably over complicating his  issue. It would appear that he has a selection of pre -downdraught heads .Perhaps he could clarify this?. . With regard to exhaust pipes I have "Norton Pipes " that don't fit any known Norton! so he needs to be carefull. Even when they do "FIT"  the shape and lack of kick up often spoils the line of the bike. I believe that pre -downdraught the ex port angles are all the same, although the nice continually curved header is rarely seen now. Norton used to specify different pipes for a 99 and 88, due to the different cylinder height , not seen now.

Dave's excellent photo shows a number of key points that help to distinguish the standard head from the SS version. The left hand head is the Standard pre- 1961 77,88, 99 or possibly 99SS version. It can be identified by having vertical carb manifold inlet mountings at the back and extra finning at the front where the exhaust ports face near forward.

The right side head is the SS (Downdraft) version. This has different castings, at the back, for two horizontally mounted carbs. Also there is a big change at the front where both exhaust ports have been cut back at a much steeper splayed angle. 40* each compared to the standard head's 20*.  This is why the SS exhaust pipes will not fit on a standard pre-1961 cylinder head.

Also just about noticeable in the photo.... the SS head is slightly wider near the exhaust ports. The barrels of all SS barrels were also widened to match this and consequently have pointy corners each side at the front.  They also all had the widened pushrod tunnels mentioned above; though sometimes the maching of them was so poor the pushrods rubbed anyway. When the Commando arrived on the scene 6 years later their barrels reverted to the narrow style which is one way of telling Commando barrels from the Atlas version.

Proper 99SS heads were supposed to have the inlet porting machined and polished at the factory along with a polished crankshaft and conrods. Such components were generally stamped SS to ID them. The problem being that anyone can polish and stamp these parts and then claim them as genuine factory SS chunks.

Looking at the SS  99 barrel with the bust fin , I would think the head gasket could get a bit compromised by the  reduced  area in front of the tunnels. Think I will stick to the std cam and pushrods. My head gasket on the 99  has seeped a little oil for  25 years but has otherwise held up fine.  The std motor   (650 pistons)  is fast, reliable,, economical, and idles really slowly . Its a mistake to Hop up  these old warriors. I have  had some very exotic speedy machines but the 99 will always be my favourite go to bike. Well worth taking the time to get them properly timed and  carb set up. I have a light clutch, easy neutral selection,and no oil drips under the bike, its all possible with std stuff and fetling.

All 99 heads  will have the same port angle , The wider 40 degrees came with the downdraught head  as fitted to  the 650 SS, later 88ss  and mercury. A very few late 99SS  produced at the end of the run are said to have been fitted with the DD head  as stocks ran out. Never seen one .If you are near you can try my spare  new pipes, I will never use them,prefer Siamese for the sound ,extra midrange.light weight and ease of access to primary/rear chain.

Never heard of bronze rockers. Dunstall used to supply rockers that were much lightened and  polished to allow higher revs and reduce the chance of fatigue cracks. But all a bit pointless with the 99  which is now too long in the tooth for "hopping up" .Enjoy it for what it is,  A very competent classic bike.   If you want to go silly fast, pull wheelies and overdose on adrenaline I will sell you my VTR 1000.for peanuts!. If I carry on riding it my licence won't last.