Hi. I have three Dommie 99 cylinder heads. They are all marked 22707. One of them is 22707H, one is 22707Y and the third has no letter after the number, it’s just plain old ‘22707’ (there’s no sign of a letter having been removed). All three heads have the same carb manifold fixings - vertically spaced screws in two pairs.
I decided to use the ‘plain’ 22707 head as it’s in the best condition. The underside of the head is stamped ‘S.S’ - does that mean it’s a 99SS head? If so, what’s the difference between that and a standard 99 head - it looks identical to the other heads. Or perhaps it’s from an early pre-downdraft 650SS, if such a thing existed?
The ports for the exhaust pipe fixing rings have the same thread pitch but different pitch angles (60 vs 55?) - the ‘Y’ and ‘H’ heads have a more steeply angled thread form than the ‘plain’ head. It’s odd that Norton would change such a basic thing for what seems like no good reason.
The threaded exhaust ports themselves are also a different bore diameter on the ‘Y’ head which means that the exhaust pipes for that head can’t be used on the ‘plain’ head or the ‘H’ head as the flange that fits into the bore is wider. The difference is only 0.7mm but it’s enough to stop the pipes going in on the ‘plain’ and ‘H’ heads.
It seems odd that Norton would go to such a large amount of trouble to cast different cylinder heads with only slightly different sizes in some areas for no apparent purpose or gain.
Can anyone shed some information as to what my ‘SS’ head is and why all the heads are different?
I imagine I’ll need to get some ‘one size fits all’ exhaust pipes and carefully grind a bit off the flange diameter where it fits into the head.
22707 is actually the skull casting mold for the picket fence side heads. The suffix does not change the basic head or even when they eventually stopped using the # on the molds.
It is used on both 88SS/136 heads and S650/136 heads.
See them here: http://atlanticgreen.com/nhth.htm
The factory exhaust port threads never changed. So one of your heads is a bodge to accomodate big flange pipes. Someone may have chased the port threads with a 60 degree UNS tap as they are much easier to find and possibly much cheaper. My UNS tap was only $160. The exhaust nuts may be aftermarket.
SS heads mainly had different springs to be compatible with the "SS cam.
Thanks David. That link to the Atlantic Green head info was very helpful. It looks like I can use my ‘plain’ 22707 head without any worries. I was worried that the ‘SS’ stamping might cause problems but if the only difference is stronger springs to cope with the SS cam, I can just use the standard springs with my QR cam. I’ve got the exhaust roses fitting nicely now so I just need to get some pipes. Thanks again for your help.