Stripped tank mount thread..

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Stripped tank mount thread..

Postby Jdraper » Wed Jun 12, 2019 7:54 am

One of my tank mounting points at the front has stripped :( . Any ideas for a fix? There doesn't seem to be enough thickness of metal for a helicoil. Should I just tap out for a bigger bolt? The tank is cosmetically excellent, so I am loath to build up with weld, but...

Jeremy

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Re: Stripped tank mount thread..

Postby clive » Wed Jun 12, 2019 8:16 am

Depends on your facilities. As a quick fix I have found that the thread on the mini exhaust mount is not quite correct and seems to lock well into a damaged thread. Anyway the tank sits on it and the bolt on the other side of the rubber fits to the frame. I carry one around when going long distance as it's an easy save when the tank bolt and rubbers go. Makes taking the tank off a bit more difficult but so long as you don't have to do all four its manageable.
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clive
if it ain't broke don't fix

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Re: Stripped tank mount thread..

Postby billbeavis » Wed Jun 12, 2019 5:24 pm

Sometimes a new bolt will be enough to fix it if the strip is not too bad. However in the end I had tophat inserts made which screwed in and retained the original bolt size. It doesn't really matter what thread you have on the outside of an insert and the drilling and tapping can be done with care and no tank damage. For instance put a length of thin spring steel between tank mount and tank shell before drilling the insert hole and go slow increasing the drill size in small steps. Support the tank with an extra pair of hands or as I did, devise padding and straps to hold it on a workmate. I thought I posted this fix in tech topics. Bill

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Re: Stripped tank mount thread..

Postby Andy51 » Wed Jun 12, 2019 6:07 pm

I drilled and tapped the tank mounting holes for 1/2 in x 26 tpi, then made up bushes threaded 1/2 x 26 outside, 5/16 inside. Screwed them in place with strongest loctite, no problems since. Andy

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Re: Stripped tank mount thread..

Postby 56G80S » Wed Jun 12, 2019 8:41 pm

I think that it's not straightforward when one doesn't have access to comparatively basic equipment such as a pillar drill. Even if I had one, there's no power at the lock-up. I have the taps and dies that Andy mentioned and could possibly manage to drill the mounts with the battery drill but there's no way I could drill through a round bar and get the hole concentric with the outer.

Threads on mine are slack and using new bolts and studlock just about gets away with it. I'd love to make a better fix.

Johnny B

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Re: Stripped tank mount thread..

Postby Jdraper » Wed Jun 12, 2019 9:15 pm

Thanks for the replies which have given me food for thought. As a quick fix over summer, I will have a look at the mini exhaust bobbin or oversize bolt. I do like the idea of a 'top hat' insert, but this is beyond my skill with mole grips and a hand drill. Perhaps the spares scheme could commission a few dozen to test the market - I would buy a couple.

For a longer term/permanent solution I am coming round to the idea of a puddle or plug Mig weld to flood the bolt hole with metal and drill out and tap a new bolt hole. There is minimal space behind the hole, but enough to put a thin plate to hold the weld puddle. If the plate has to stay afterwards, at least there will be a longer thread for the bolt to engage with. I am not a welding guru, but I have seen enough examples of their work to suggest this may be a solution. All I need to do is find someone who I trust and make the tank safe for welding! I think this will be a winter job.

Anyone got any major technical objection to this approach?

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Re: Stripped tank mount thread..

Postby Groily » Thu Jun 13, 2019 6:15 am

On one of mine, I just brazed nuts on the front mounts, same thread as original tapped holes. (But, as with welding, caution!!) Tank height is scarcely affected, so no fitting hassles and easier than filling with weld probably.
For the rears, I did it with studs, as there's enough wiggle room to get them to go through the frame mounts.
But not really one of the best-designed features on the bikes imho!

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Re: Stripped tank mount thread..

Postby Dixter » Sat Jun 15, 2019 7:58 am

Jdraper wrote:For a longer term/permanent solution I am coming round to the idea of a puddle or plug Mig weld to flood the bolt hole with metal and drill out and tap a new bolt hole. There is minimal space behind the hole, but enough to put a thin plate to hold the weld puddle. If the plate has to stay afterwards, at least there will be a longer thread for the bolt to engage with. I am not a welding guru, but I have seen enough examples of their work to suggest this may be a solution. All I need to do is find someone who I trust and make the tank safe for welding! I think this will be a winter job.

Anyone got any major technical objection to this approach?

Jeremy


Hi Jeremy, in essence, yours is how I’ve repaired a few. However, to retain the air space below the stripped hole (below... for welding, the tank is up side down) I insert a thin sheet, or two, of copper, below the damaged thread, and using a TIG torch wilh steel filler rod, fill the damaged hole with a series of circular passes creating a rosette weld.

The copper sheet provides a “floor” for the weld puddle and makes for a neat repair. When cooled, remove the copper strips, drill and tap.

NB: since the copper sheet insulates the floor of the gas tank from any high heat, the fuel compartment never gets hot enough to burn, create a fire, explosion, or damage the paint, and for added safety, I simply fill the tank with water.

Ciao, DC
Dick Casey
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Re: Stripped tank mount thread..

Postby 56G80S » Sun Jun 16, 2019 6:24 pm

Groily has reminded me that my friendly garage owner who has looked after my cars, and has just bought an original Rickman Metisse Trumpet, MIG weld nuts into a tank I had. Rounded off the nuts, drilled out the threads in the tank to an appropriate size. Unfortunately, these gave way and as the tank was a spare I sold it on to a Club member quite cheaply.

Perhaps, as Groily suggests, brazing would've been a better bet providing better capilliary (!) action and a bit of flexibility? IKBA.

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Re: Stripped tank mount thread..

Postby Yak » Fri Jun 21, 2019 12:02 am

My tank mount plates could not hold a bolt. very annoying. Eventually had them drilled out and welded "deep" nuts in them which were also rounded off. On my tank I still have a gap between the nut and bottom of tank. The weld proved too brittle so had them brazed in and after 20,000 miles or so I have deemed it a success. Must stress it was not I that did the welding/brazing. Still lose a bolt if I don't wire them up!
Agree with Groily, regarding the actual design of the mounting system.

Ray.

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