E: Engine Exhaust valve not right

[oexing]

Aluminium may be allright for bearing material - BUT only if lubrication is safe. The tensile strength of alu alloys says little about bearing qualities. I did not accept these alu upper valve guides for a reason - and was confirmed with this thread. So I pressed in bronce guides into alu retainers. The bronce type is CuSn 8 , or more recently called EN CW 453K , excellent wear resistant and no risk of seizures due to lack of oil. And certainly all line reaming of lower and upper guides should apply piloted reamers, not an easy job.
Instead of hunting aviation alloys at ridiculous prices you can go for aluminium type 7075, max. tensile strength as elswhere but simple to get at.

Vic

 

[ClassicBiker]

Well a 2" diameter by 12" long piece of 7075 at $48 is higher than the 2024 by $4. But you are correct strength has nothing to do with bearing qualities. Bronze used in bearing is 80+% copper. I suspect that the 3.8 - 4.9 % copper in the 2024 had something to do with it being chosen.
Now actually making an aluminum retainer with a bronze insert is probably the best choice, but I suspect at the time the bikes were in production manufacturing and material cost were factors as well.
I like the idea of pressing in a bronze guide in to the aluminum retainer. What was the O.D. of the bronze and did you increase the O.D. of the portion of the retainer that extends and direct supports the valve stem?
Steven

 

[A_HRD]

2024 is heat tolerant. 7075 loses its strength at not much over 100C if I remember correctly.
Peter B

 

[davidd]

The 7075 pushrods wilted in the Flash. The 2024 pushrods worked fine.

David

 

[Robert Watson]

I have made upper guides that have been bored out almost all the way thru and cast Iron automotive valve guide material pressed in. They are available at the local engine rebuilders and have oil scrolls machined in them. The run on hard chromed valve stems and have been in there for thousand and thousands of miles.

 

[Nigel Spaxman]

Quite a few people in BC have found that the upper valve guide corrodes from condensation and from acid by products of combustion. Robert Watson told me about this problem. Perhaps in really hot dry climates this would not be a problem. The corrosion is in the form of a black alumium oxide paste. That stuff wears the valve stem. I fitted iron bushings into my upper valve guides to prevent this problem. I used some sintered bushings from McMaster Carr. Some other people have used bronze. In either case it is better than the original aluminum.

 

[stu spalding]

Quite a few people in BC have found that the upper valve guide corrodes from condensation and from acid by products of combustion. Robert Watson told me about this problem. Perhaps in really hot dry climates this would not be a problem. The corrosion is in the form of a black alumium oxide paste. That stuff wears the valve stem. I fitted iron bushings into my upper valve guides to prevent this problem. I used some sintered bushings from McMaster Carr. Some other people have used bronze. In either case it is better than the original aluminum.

Does anyone have anything against filling the valve caps with grease on assembly? I have been doing so for years and it seems to give a very steady "bed in". Cheers, Stu.

 

[Bill Thomas]

I like some of your thoughts, You were right about Mk2 amal slides, Had it on the Twin the other day.
I also oiled the seat stays on the Comet, Good.
But is your grease needed ?, I can't remember finding a worn top guide, Has anybody else had one lock up ?.
Cheers Bill.

 

[bmetcalf]

This tool https://www.homedepot.com/p/GearWrench-Valve-Spring-Compressor-912DD/205594537 can be used with the head in place. The end is hooked under a bar with suitable holes that straddles two head studs. To test it, I just reversed a head bracket to be the fulcrum point.

Here is the similar tool (KD #915) I have:

 

[oexing]

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.