A10 TIMING BUSH

[unclemeat]

Hello Folks, Anyone got any thoughts about a brandnew steel cased timing bush that seems to be a rather "easier than should be" fit in the casing ? As time goes by and bushes are taken out and replaced, the tolerance grows, or rather the hole basically gets bigger as little scrapes of aluminium are removed to the point where a bush bought of the shelf is no longer the fit it should be. Of course the casing should be heated accordingly when taking a bush out and putting a new one in but does this really mean looking for alternative cases ? Are oversize (as in the outside diameter) bushes available ? Or will a small amount of locktite bearing fit suffice. Any advice or suggestions much appreciated.

"should have put this under TECHNICAL !!"

[fido]

I would contact SRM about this as I've no doubt they could offer a few different solutions. I would not trust loctite. The bush only needs to move a bit to cut off your oil supply.

[a10gf]

Hello, and welcome.

The danger with the steel cased bushes is if the inner part rotate, that will cut off oilsupply completely. Have read  stories about that. Some may work fine, others not, it's down to the manufacturing quality. Before starting any big work, maybe try measuring the dia of the bush you got, contact your dealer and ask them to check if they got something a little bigger, as the accuracy of any remanufactured parts can vary from batch to batch. Also check with a SRM or some other experienced techs if they recommend using the 2 part bushes or prefer the one piece stuff.

Regards
Erling

[unclemeat]

Thanks for the comments...looks like i'v got a bit of reading to do and some questions to ask before I proceed. One good thing with the steel case is that it adds rigidity and, if there was a seizure for some reason, hopfully the bronze would rotate within the steel case so as to avoid any damage to the anti-rotation lugs on the crankcase. With a solid bronze bush you wouldnt have this 'safety feature' - if you want to call it that. I dont think there is any reason to think that the inner bronze bush would rotate within the steel under normal conditions. These are held pretty tight within the steel (I think a high strength bearing fit loctite is used).
But back to my original question, i'v spoken to a number of people and this is what was said:

1. Have the steel outside diameter part of the bush cadmium plated just enough to restore the tolerance between the bush and casing (I would need to find out exactly what that tolerance was). Cad plating mmmmh !

2. Have a new bush machined up...it gets worse!

3. Just fit a bush, as you say with the biggest od i can find and maybe just a little high strength high temp bearing fit Loctite and that should do it. Of course making sure that no liquid blocked any oilways !!
Afterall, the whole bush cannot really move in any direction, its just that if you have a sloppy fit, oil pressure would reduce.

Anyway...wish me luck

[unclemeat]

Hello again Fido & Erling....just read that the fitted tolerance is 0,0016". Think I need to measure exactly what size hole I have. Unclemeat

[bsa- bill]

the two piece bush is pegged, if you look closely you should be able to spot the  peg that keeps the bush from moving in the steel backing.
I have just replaced mine with a solid bronze type, probably just down to personal choice though.

All the best  - Bill

[unclemeat]

Thanks for that Bill. 

Basically it looks like there are 3 types available then.

1. Solid Bronze bush
2. Steel case with pegged bronze inner
3. Steel case with bonded bronze inner

After speaking with a number of dealers who specialise in BSA twins, timing bushes (either type) are available in oversize outside diameters when specifically required. You may have to pay a little more as they are a special but they are available.

If we are only talking about 0 to 0,001" play ie. the hole in the crankcase is 1 thou bigger than the outside diameter of the bush, then the general consensus seems to be that an appropriate Loctite (641, 668 or even 648) would do fine - just a little now...don't go blocking those oil ways !!!

So after all this talking, phone calls and E-mails, that's what I'll do. I'll buy just a normal bush off the shelf, probably a 30 under size, Loctite it in, have the crank ground to clean, then the bush line bored to match.

Thanks for the feedback folks

Lets hope it all works..Bye for now.

[dpaddock]

If it's only a matter of fit, copper plate the bushing. (I'd think it's impossible to find someone who cad plates anymore.) Any plating shop can do copper for you.

[63a10r]

im doin a ground up a10 bobber ,which brings me to a veritable slew of questions.
a:the plating idea seems to be a good one. never thought of that. as for cad platers, my brother mutt does a fine job, and since his gig is restorin vintage bikes, he specializes in all things motor-sickle. hes at the pigstye. cheap too.
b:ive decided to try the solid bush, as it made more sense to me at the time. having read this thread, i gotta ask, what are the drawbacks of the solid bush? ive asked questions, and the general consensus seems to be that the 2 piece factory thing is the way one should proceed. i still have my doubts, but what do i know.
more questions later.

[unclemeat]

I think at the end of the day, either are fine, both designs have worked for high milages, afterall, solid bronze bushes without steel casings have been used for years in all aspects of engineering. One other advantage I thought of for the steel case version is possibly to prevent too much thermal expansion occuring ?

Unclemeat