The old domni 500ss and 650ss offered the same size valves as Norton used in their 750s and 850s - 1-1/2 in and 1-5/16 ex. They just increased the bore and didn't bother enlarging the valves size to suit. See 500 and 850 pistons below.
They should have upped the valve diameters - there's room for them and its what the bigger motors need.
Good evening Together,
As we all know the standard 850 Commando was NORTON‘s last effort to compete in the motorbike market, besides the Wankel or the Cosworth (both were underdeveloped or they didn'thave the cash to get the production started). But some reason that we all also know (no cash), unlike BMW they stuck to their oldish design of the well- known Commando.
While BMW developed a completely new engine around their old boxer- concept, Norton stuck to the outdated design with cast iron cylinder, the multi- piece crankshaft, which wasn’t only very flexible but also very expensive to produce, but also to the small valves of the 750- models.
All this doesn’t say that the BMWs had something new in their engines. Most of the development was also well outdated (the long push rods which made the valve gear rather flexible and so not very well suited to high revolutions/power output). And yet, they were good enough for some very interesting race successes.
Today the fastest BMW- boxer- engine I know is fitted with their traditional long- stroke crank, the short conrods of the R65/R45 engines and 48/42mm titanium valves. This engine was last ridden in a sidecar race at Spa/Belgium and was the fastest engine of the whole sidecar race. Oh yes, it has some very special porting, big 42mm BING carburettors (I think) and a special camshaft (Dr. Schrick???). It was built by the most famous BMW engine builder Dieter Busch. Ralf Engelhardt (the owner) told me that he can easily rev it to 10.000RPM (“but don’t tell Dieter”). And yes it has 1000cm².
I would have expected that NORTON’s would have done a similar thing to their engines, with bigger valves (maybe 41/36mm) a suitable and reliable aluminium barrel and a one- piece crankshaft, steel- conrods, maybe bigger big- end bearings and also plain main bearings. Yes, this development would have done them for another 20/25 years, as it did with BMW (from the first R50/5 well up to the last R 100 R Classic. No, we all know that they were in financial trouble (likewise BMW at the time, by the way) and so a complete re- design was out of reach for Norton. They went to Cosworth instead and went into real trouble. Who wanted the Cosworth or the Wankel??? I didn’t.
I myself did a lot of design- work on the NORTON cyl.- head and found that (just geometrically) in the 850cc engines 43/36mm valves can rather easily be fitted. The problem is the porting. You cannot easily raise the port- roof because you break through into the rocker- box (valve- spring seats), or on the right handside into the oil return- bore. So, all you can do is go sideways equal values to the right and left of the valve centre- line. One more thing one can do, is fit bigger valves (41/36mm) and align the ports to these bigger valves as precisely as you can.
You will find a problem with the rather big outer diameters of the valve guides of the 850- or the RH8 750- cylinder heads. These have outer diameters of 5/8” (approximately 16mm). As we all know you will have to alter the valve- angles with the Norton heads to clear the valves during overlap. So, the outer diameter would end up to be at least 18mm. But there are solutions. If somebody is interested, I could send you a drawing how I deal with this problem.
There is even a way to deal with the break through into the rocker- box, if you raise the roof of the inlet port. I haven't done this yet and I won't it's too much work for an old guy like me.
All in all, there is still a problem with fresh gasses getting lost during the valve overlap, finding their way directly into the exhaust- port. But, look at Paul Dunstall’s solution or again a raised inlet port roof.
I’m now 71years old and racing is far away for me, but I still have the aim to give other motorcyclists with much more modern bikes a ride for their money. I want at least the same performance as a BMW R 90 S. That’s why my Commando has 41/36mm valves and flowed ports (made by myself), a long rod- conversion (which is another story) etc.
I do know that I can never catch up with BMWs on German Autobahns because with these bikes you can ride full throttle the whole day without any trouble which I wouldn’t try with any classic British bike. These were simply not made for this kind of torture. But especially a Commando can still keep up with most motorcycles on twisty roads, because of their sheer torque.
That’s currently my way of thinking about NORTONs today.
Best Regards
Klaus Monning