AJS & Matchless Owners Club

1954 g80. The tank is held on by 4 special bolts, one of the front ones goes in, screws up, but will not tighten , stripped thread in lug on underside of tank ?.

Am I right in thinking its cycle thread ?.

What is the best way to repair it ?.

Thanks.

26 tpi cycle thread. You say a bolt will not tighten. First make sure you have all the correct washers, rubbers and metal tube insert for the thick washer. DO NOT TIGHTEN the bolts. Just nip them up until they just start to tighten on the rubbers. You will see a hole in the bolt hex head, this is for wire, to wire the bolts to the frame lug, so they do not loosen/fall out. The tank is then rubber mounted to stop vibration. You could helical the damaged thread. All correct parts are available from club spares.

I could never see why they shouldn't be tightened as the sleeve inside should stop the bolt from bottoming in the threaded hole. I did actually test this on my G80 and there was room to spare.

The rubber washers prevent any metal-to-metal contact as you say.

It's the same principle (although not as elegant) as the rigid framed Norton singles which used a shouldered bolt and didn't need lock wire.

Thanks, all correct rubbers and spacers present I think !. I bet cycle thread helicoils are hard to source, any tips on where to look ?.
Cheers. J

Try here: http://www.uni-thread.com Best place is at Stafford show. You usually get special offers.
Cheers Dave.

Careful with helicoiling, the plates are so thin that there's not really enough meat for the insert to grip. I've had only 50% success helicoiling these plates and prefer to have the hole welded, drilled and tapped with 100% success.

iansoady wrote:I could never see why they shouldn't be tightened as the sleeve inside should stop the bolt from bottoming in the threaded hole. I did actually test this on my G80 and there was room to spare.

The rubber washers prevent any metal-to-metal contact as you say.

It's the same principle (although not as elegant) as the rigid framed Norton singles which used a shouldered bolt and didn't need lock wire.

Hex head bolts have holes so they can be wired on, the earlier type were just a disc with a threaded stud welded on, plus a tab on the disc with a hole, so they could be wired on. Hand tight was good enough, but I guess some people thought the hole was for a tommy bar to tighten them. They were the ones that always had petrol pipe and joint leaks a more.The metal sleeve prevents wear on the rubber. Stopping vibration prevents parts from loosening. When copper petrol pipes were used, vibration fractured them. Tanks also split due to vibration. Vibration transferred through the frame caused damage and loosening of parts. Then onto the handlebars, where vibration is not only annoying, it causes a blood circulation problem called vibration white finger. Although the latter was not really discovered or considered years ago. Car engines and gearboxes are rubber mounted for the same reasons. Many more parts on modern bikes are found to be rubber mounted. I am sure there are many other reasons why designers did all possible to prevent vibration. Rubber mountings for tanks may not compress enough to cause metal to metal when new, but if there is any compression, they are not doing the job intended. Some AMC have no rear tank bolts just a rubber band.

Thanks thats great info, this british bike game is a steep learning curve for a youngster (ish) raised on jap bikes !. wash my mouth out with castrol R .

Am I right in thinking its 5/16 ?.

What is the safest way to weld the tank ?, fill with water ?.

Yes 5/16" X 1 1/4" most common. BUT, not all years and all models had the same bolts & rubbers. Parts lists need to be checked. Not sure about welding certainly cleaned but empty. What needs welding on your tank ? split seams caused by vibration, caused by over tightened tank bolts?

Hi, stripped threads in petrol tanks are a perennial problem - a bad design! I had 2 stripped threads on my '54 G80 tank, so I took it to a local engineering workshop and had the mounting holes drilled and tapped 1/2 in British Standard Cycle (BSC) (had to supply the tap). The engineer put the tank on his Bridgeport mill and drilled and tapped using the machine's chuck - cost £30 all in, similar price to a helicoil kit. I turned up threaded bushes, 1/2 in BSC outside and 5/16 BSC in the bore, about 1/2 in deep, and screwed them in with loctite. I now have properly threaded bolt holes with enough thread to prevent stripping in the future. Andy