G12 Primary Transmission

[MartinL]

Firstly thanks to all who helped me with my carb issues. The bike is now a second kick starter and goes well - sort off. Because my next issue is clutch slip.

I've today taken off the primary chaincase, and taken down the clutch. A number of matters on which I would appreciate some advice.

1. There are a couple of stipped threads on the inner case. These look like 1/4 inch BSF , is that right? and

2. anyone know of a good source for helicoils?

3. The drain plug in the outer looks a bit of a mess to me with a home made plug fitted. The thread looks like 1/2 inch whit - again is this correct?

4. The primary chain is very slack, and the gearbox way back in the adjustment slots. On top of this it is possible to pull the chain away from the clutch sprocket. Looks like its cream crackered even to a mechanical numpty like me so a new one is in order. The manual I have suggests that the chain can have a split link (the current one is continuous) is this correct? If not is fitting a continuous chain (I assume the link is fitted and peened over) a job within the scope this very average home mechanic?

5. The clutch itself, well I guess I will get some new springs for a start. Additionally the box of bits I got with the bike has a set of clutch plates that look much better that the ones I took off so they can go on. The manual refers to an updated conversion to a 5 plate clutch - is this a worthwhile mod?

Kind of a lot to ask, I have read the manual but it makes a Haynes book of lies look encyclopaedic by comparison so any help would be appreciated. Perhaps there is a more comprehensive book I could purchase?

Regards

Martin

[Biscuit]

Not to familiar with the aluminium chaincases, but I would suggest that the screws are 1/4" BSC, 1/4" BSF have the same TPI, but a different thread flank angle, which considering the age of the tapped holes won't make a great deal of difference. The drain plug should also be 26 TPI, not sure of the diameter, certainly not 1/2". AMC never used Whitworth threads. It is the same plug fitted as a drain for the oil tank. The problem with the chain could also be in part wear of the sprocket, a badly worn chain leads to rapid wear on the sprockets. Whilst modern bikes tend to go for the rivetted links, all the old Brit. road bikes I have worked on have spring links. When fitting these ensure the closed end of the clip faces the direction of travel. Be prepared for some fun when you fit the new primary, if the gear box is all the way back, you may find the secondary chain is to short. Helicoils - never used them, but there has been discussion on their use, a search of the site should locate it.Edited by - Biscuit on 06 May 2006 8:33:08 PM

[Merlin]

Sorry Alan pet subject going to,too many and there are two of them

[Biscuit]

Oh dear, I've sphericalled up again. I shall write 100 lines.

[Steve Martin]

Hello Martin, the chaincase screws are 1/4" x 26, Part 000484; for the Drain Plug, the list gives Part 012019, which on my '58 G3L is 3/8"x 28. [ Yes, I have checked it with a thread gauge, because it was much finer looking than I expected, and it doesn't appear to have been modified. Incidently, the oil tank drain plug is also 28TPI, which makes it very fortunate that I didn't try to force in the smart new 26TPI stainless screw that appeared reluctant to enter!] Both items are available from Jampot Spares, but if the drain plug hole threads are still reasonable, I'd be inclined to just use a larger plug screw; no-one will mind!
I have to confess I entrust helicoiling jobs to those that know. Adverts in the classic mags will help locate someone in your area. You could also check out www.Uni-Thread.com if you want to have a go yourself.
From your description of the primary chain a new one sounds like a very good idea. The Part Number is 022870, which is a 68x1/2" inc. a split link. I don't know the current view on fitting a continuous chain; my '59 G12 managed without problems with a split link, back in the days when I had fewer qualms about thrashing it! Again this is available from Jampot.
As to upgrading the clutch, it rather depends on your riding style. If you have a set of four decent plates already, try them out; it's easy enough to change later if you feel the need.
I agree that the manual for the late 50s-on machines is a little terse on some matters, but I haven't come across any more comprehensive published volumes. There is however, a very useful website of archive material, the number of which I rarely get right,[it's now on my 'favourites list'!] After a number of attempts, I believe it to be: http://217.157.196.176:8181/ajs/ ...
And, of course, there is always the access to the years of accumulated wisdom available on this site, so feel free!
Cheers, Steve.
Edited by - Steve on 06 May 2006 9:38:12 PM

[Biscuit]

Must be 1/8" gas (BSPT) Steve, I wasn't going to remove my plug to check!.

[Steve Martin]

Thanks Alan, I'd not knowingly come across this thread form before; where does the 1/8" come in on a screw 3/8" in diameter? On second thoughts, is it like the weird sizes given to petrol tap threads?
Cheers, Steve.

[Biscuit]

It's the nominal bore of the pipe on which the thread is cut. The thread is actually .383 OD. X 28, Whitworth form.

[itma]

An old dodge to check wear on a chain is to lay it flat, as it would be on the sprocket , and see how far it will bend sideways; a new chain will only have a slight curve, whereas a knackered one will go U shaped.
Look at the name on it, it may well be some far eastern tat; having said that it now seems that there are two grades of reynolds chain about,my local bearing factor from whom I bought chain by the foot now refuses to sell it me for use on a bike.I have to invent some kind of farm gadget to get the same stuff.
Bar far the better thread repair, if you have the material in the cases, is to tap the hole oversize, and screw in a threaded blank, pref LH, and then retap to original. trouble with helicoils I found is that in alloy chaincases there is not really enough depth of threads. If you are near Wilts, I can help with some sizes of helicoil.
properly fitted you should never have problems with a split link.Edited by - itma on 07 May 2006 1:48:55 PM

[MartinL]

Thanks for the help Alan and Steve. Further investigation has revealed a comprehensively bodged drain plug hole, which has been crudely retapped with a coarse thread, either UNC or Whit I assume, and a homemade plug made from a butchered bolt jammed in.

I propose to drill out and either retap at 1/2 inch UNF, make a plug which I'll drill and tap 5/16 UNF and then loctite it in to the case OR drill it out smooth, and turn up a plug (I have a mate with access to the local Tech College machine tools), make it and interference fit, heat the case and drop it in. It too will be drilled and tapped to pprovide a new drain plug. I think I'll only get one shot at this as there does not seem a great deal of metal forming the boss. Any thoughts on which is the best way to go?

I'll certainly buy a new chain and clutch springs and try them out.

The outer case is fixed to the inner with 1/4 UNF threaded cap screws in my case - I tried a few bolts till one fitted to find this out. Only one is stripped, and that one it is possible to get a nut behaind so thats the way I will go - there seems too little metal to drill it out bigger.

I suspect making the primary chancase oiltight will be the next challenge. It was splattered with some blue coloured silicone type stuff upon dissassembly. Should there be a gasket between the two cases? and any tips on whether or what sealant I should use?

Again many thanks for your tips chaps, when you're finding your way like me it really is tremendously helpful.

Regards

Martin