I"m just about to fit an A65 cam in an A50(1969)motor,Ive been told there is no problem.I see that the overall lift is about .040 more on the A65 and the deration of course is different,anything else to worry about.
While you may not care, the Lightning cam from most 650 twin carb bikes is a bit cammy. The motor will jump on the cam at about 3000 to 4000 RPM. The mild nature of the A50 will probably be lost with the cam transplant.
The Lightning/Spitfire/Hornet cam is hotter than the Triumph Bonneville cam, for instance, and the cammyness is readily apparent even on my low compression A65.
This is not a negative for the transplant, but only a heads up.
I dont think you will end up with any cam issues , as LD says , it will have a different personality , but you noted that with lift and duration . The only thing I would add is to replace valve springs , I dont know for sure if A-50 are a different spec , but I do know thta if they havent been changed recently , they are probably 30+ years old and tired of being kept under tension .i'm sure it is second nature to most , but changing out valve springs is cheap insurance , as long as you buy quality replacements(BTW , Hotrod springs arent necessarily a good thing in a street engine, factory spec replacements should be sufficient, the cam dictates the springs ) .
My thinking with the previous post was two things , first , I have seen people replace valves and guides and reuse the original springs because they arent broken and are still in spec? if they bothered to check spec.It maybe a non issue , I have never seen a broken or collapsed spring cause a catastrophic filure in a Britbike , but it never made sense to me . second ,I have probably seen more than my share of boy racers who install highperformance valve springs , thinking they are just plain better than original replacement even without a hot cam , and end up wasting their cam , and probably losing performance in the deal before the cam goes south .
As Rich says , there were some differences in valves , springs , collars and retainers in various years , I dont know if the A-50 and A-65 kept pace with these changes . It seems info is sketchy on the A-50 compared with the A-65.
I believe I was once told that BSA only made two different cam grinds for the unit twins. These break down roughly into the single carb and twin carb models. The hotter cam, which came in the Lightning and Spitfire as well as the Cyclone and Wasp, was 68-473. This cam works equally well in either 500 or 650 application. When you put the top end together on the 500, check very carefully for coil binding and for the valve collar/keeper hitting the top of the guides. The valve stems on the 500 are longer because the chamber is smaller so some parts are different while others are the same, between 500 and 650. I believe rocker arms and pushrods are the same, but not sure. Just make sure the parts don't run into each other. Many of the Daytona 500's failed back in 66-69 era due to swallowing valves. SRM once told me that the factory race engines actually ran the 68-473 cam, that they could never improve on that. Though I do have a couple of Sifton grinds that are pretty scary.
According to the 1966 parts book the A50 Royal Star used the 68-0103 while the all A65's and a A50 Wasp used the Lightning cam 68-0473. I know, parts books can be wrong but that ceratinly makes the most sense to do it that way for a grocery getter, commuter bike like the heavy for a 500 A50. Oh, I forgot, we're talking about a Brit bike where logic does not always prevail. In practice they may indeed have all come with the same cam (????).
Look at the specs on the Lighning cam. It is a step above any stock Triumph cam, which was the race bike of the 1960's. It would not surprise me that nothing else worked better all around.