acotrel wrote:Is the cylinder base gasket the same on the 650SS, Atlas, and Commando 750 ? In other words is it possible to interchange the complete top end without machining ? I would really like to know this, it could give a lot of options. Also which crankshafts and rods are interchangeable ?'
IF you lay a 650 base gasket onto a 750 crankcase you will see that Norton moved the four rear cylinder studs straight backwards about 1/8 - 3-16 of an inch, also they moved the oil drainback hole to clear the back of the large bore and bored out the cases more for the bottom of the cylinder where it fits down into the crankcase. The Atlas did use the same crankcase and head casting, just machined differently and with a new cylinder barrel casting.
Sure, if someone had the tools and nothing to do they could weld up the rear base stud holes in the 650 cases and machine them into 750 cases.
The 650 was the first production Norton engine with the 89mm stroke and 1.75" crank pins, Norton had them ready and on the sales floor early in 1961. The first engine was serial #93601 and it's crankcases had humps added to the upper back below the cylinder base behind the magneto to clear the new big connecting rods as they swung around, it was tight in there! The bottom of the cylinder barrel also had to be notched to clear the rods in their larger orbit. The later 750 and 850 cranks and rods work in the 650.
The 650 was also the first bike to get the new downdraught cylinder head with the horizontal intake manifold holes and the exhaust ports angled back. For 1960 Norton had introduced a new cylinder head for the Model 99 that had different and larger fins, a bit larger intake ports and gave a point higher compression with any pistons, but it's exhaust ports still pointed more forwards than the 650 head they started to make about one year later. Norton had made the fins on the 99 cylinder larger to match the new 1960 99 cylinder head, but when the new 650 head with it's angled back exhaust came out they had to trim the front of the fins back so the 650 exhaust nuts would clear. So there was a lot going on during 1960-1962 with the Dominator twin.
Norton put the 650 head onto their Domiracer for 1961, and also in spring that year introduced the SS camshaft, tapered pushrods and valve springs that went onto all Nortons for 1963 and later. The 1961 88 and 99 still kept the 1960 style top end parts, but in 1962 the 88ss got the 650 head. The 99 was dropped at the end of 1962 along with the Bracebridge Street works and for some reason was never fitted with the 650 style head, although it is easy enough to swap the parts on, I am sure some have done this.
The 88 and 99 Nortons never got the larger crankpins and rods like the 650, but Norton did considerably strengthen their crankshafts from 1960 onwards by simply making the diameter of the sludge trap much smaller, thus making the wall of the crankpin much thicker, an easy and effective change on the production line. Later in 1961 a small handful of 500cc Domiracer engines were tried with the 650 crankpins, but after AMC shut down the Norton works and Doug Hele left for Triumph almost all development on the Dominators stopped except for the small detail improvements.
So the 650 that came out at the beginning of 1961 really set the pattern for all the 750 and 850 engines that followed, which did not really change much at all except for what was needed to accept the larger bores, tach drive and points ignition.
Doug Hele was the driving force behind all the Dominator twin development in the late 50s and early 60s, he really got things moving by testing the twins for durability and for production races and Daytona and letting the improvements trickle into the machines coming off the assembly line.