valve action geometry

The early 77, 88  and 99 engines all shared the same cylinder heads. generally casting number T2225.  Then the inlet valves went up in size for the 22707  c/n versions. Roughly 1958 to 1961. The 500cc and 600cc Nomad bikes using a twin carb manifold version of that cylinder head.  As did tuned  Norton Twins ordered as Special Spec. This description later changing to Sports Special.  The first two versions of the term SS.

The first 650 and second phase 1961 88SS  used a modified 22707 cylinder head with the splayed exhausts and parallel carb mountings. The 650SS and 88SS bikes being classified as SS models ie Super Sports.  Some 88SS cylinder heads were produced with 26mm inlet ports

The early Bracebridge Atlas engines used a 23166 casting. This being carried over to the first AMC made 650 and 750 heads which shared the same casting but machined to suit. The same applied to the later 650/750 heads which shared a 25139 casting.

Finally the 650 Mercury, later 750 Hybrids (possibly after e/n 126***) and Commandos all shared a 06-0380 casting, again machined to suit the engine application. These heads all had the longer (0.10") valve stems and shorter pushrods. This change was apparently made to alter the rocker angle and minimise scrubbing of the valve stem ends. Perhaps this worked in terms of less wear of the valve guides but strong rumours then emerged of pocketed valves leading to rockers thumping the inside of the heads. Win Some....lose some!!!!!

Back in 1972 ish .....Norman White used to offered a Commando cylinder head mod where the position / angle of two of the valve guides was changes. This involved removal of the old guide, a small amount of Aluminium welding and then some drilling, reaming and seat cutting.

How exactly this worked I do not know.  If the rocker remains in the same position then there must be some change in the point of contact and possibly the angle of said rocker when this occurs. Was the pushrod length and valve stem length altered???

Is this mod mentioned in Norma's recent book?

After all this time ascertaining what happened with what part number all those year ago at AMC is becoming academic. BUT  I have always thought someone might want to measure the valve geometry on a twin head? But it will not be easy. The compression ratio-due to the head/barrel machining (not piston) is part of the issue, push-rod length, valve stem length, and of course rocker ratio. Get your dial gauge out!! The cam shaft is a factor but how and how much/when???

There is some books on the subject I suggest these are studied before you make with the spanners. I have done this job (measure rocker advantage/or not on a basic car engine. You only do it once!) If any one wants to try this please get in touch.

" The real question is  , when did the error arrive ? "

The early low lift cams with valve/push rods would have balanced valve end scrub.

Now install a SS hi lift cam and the rocker tip scrubs way beyond valve center. 

edited: A shorter push rod and longer valve repositions the valve tip scrub, due to  the valve adjuster  length is also being altered.

If the original model seven was perfect then there would have been equal sweep before and after the valve center. more lift by daytona&QR would alter the location of sweep by the adjuster. The SS would sweep even more beyond valve center due to the even higher lift.

 Also note the different cam grind base circle size requires rotating the valve adjuster. .040" is noted within norton cams even more base circle change in custom grinds.

By knowing the base circle and lobe lift would allow for perfect custom valve and pushrod length.

Very few would pursue this engineering effort or expense.

hello,

would a cylinder barrel base decompression gasket have any effect on valve movement and pushrod length.or cam in some way.

barry