Stu,
that black deposit on valve stems in alu upper guides is not related to corrosion effects but plain and simple wear due to bad bearing qualities of unlubed aluminium. Just try to polish alu with wet´n dry plus light oil , you will get
black fingers, not white as aluminium, or get Nevrdull and polish alu - black.
For that reason I did not accept alu as upper guide and had bronce bushes pressed in, one mm wall thickness is allright, no space problem to do it. There is a lip on the upper end so the bush will not drop.
For ease of mind I dremelled a window into the head to provide access of oil from the rocker to the space above the upper guide , can´t be wrong, but anyway that CuSn 8 bronce bush is great with a minimum of oil supply.
As to alu types for upper valve guides, 2024 is quite good, 7075 stronger, for high temps use 2518 is great - if you can get that, typically for pistons. It is not easy to get at info about strengths at higher temperatures. I´d say it is not a big factor for these guides, you only have max. 150kg loads on them purely from valve springs, no other effects I can think of. So even if 7075 will go down by 50 percent at maybe 200 degrees C in times it is not critical. Just think about cylinder head material, is that just as good ???
I´d be interested in getting more info on failed alu pushrods on Comets, some pictures ? You will have fitted steel end caps to these I guess ? Taking into account the shortness of Vincent pushrods I find it hard to see big problems in the choice of alu types for them. The 400 cc Horex from 1954 and a bunch of seventies BMWs had alu pushrods - and really long ones - with steel ball ends of course and I do not think they were made of aircraft types, in fact I made my own pushrods from lengths of alu tube you get anywhere, two mm wall thickness, 10 mm o.d. . All components in the valve gear of BMW flat twins or the 400 Horex single are a lot heavier than these from Vincents so I can only assume that any failures with alu rods here were possibly related to overly aggressive cam shapes or other defects in the assembly, or just too thin, hard to draw conclusions when you cannot inspect all installed components .
I will have alu pushrods anyway as I believe the heat growth of alu cylinder plus head does not go well with steel rods. With steel rods you can only set nil valve clearance cold and live with clatter when all items get hot. So alu will be a better idea in this respect.
Vic
No. 1 , 2 , 3 from left are alu rods, 2x Horex , third is BMW. No 4 is BMW stainless steel, three from right are titanium, no good, low expansion.
My experiences with hardchromed valves in cast iron guides and open valves in a 1935 ohv Guzzi, lots of scored stems, went for bronce types, no more problems.