Tank mounting bodge/fix

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
The Vandal
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Tank mounting bodge/fix

Post by The Vandal »

At least one of the mount threads is gone on my tank. I was having trouble with all 4 mounts so bought some longer bolts in the hope that would fix it but one is still being problematic.

There's some thread still there as it'll engage but soon as it gets tight it skips on the thread.

I do find the stock way of mounting the tank pretty crappy, it's a nightmare getting the bolts lined up and the threaded bit on the tank is woefully inadequate in terms of thickness.

I'm considering cutting the heads off the new, longer bolts I bought and brazing them into the existing mounts and using nuts to fasten the tank. I've read the posts about welding up the holes and redrilling/tapping the holes, threaded inserts, cutting nuts down to slip on top of the straps etc but it seems like changing it to a male thread system will make life a lot easier in a few ways. No more fishing around to find the mount, I figure I should have enough thread to put two nuts on each and lock them together so no messing about with the lockwire anymore

Just wantoing to check I'm not being thick and missing something? Aside from the obvious, taking a brazing torch to a petrol tank!

I have a basic stick welder and basic skills with it too so could just pop a wee weld between the bolt and mount, would probably be less damaging to the paintwork than brazing, but I'm definitely more practiced with brazing than welding so think I'm more confident of my ability to make them solid if I braze it.
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jackstringer
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Re: Tank mounting bodge/fix

Post by jackstringer »

Try just running a tap down it.

I drilled and used a wire insert to fix mine. I did buy new nuts on the idea if trying to weld them in but didnt have to in the end.
The Vandal
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Re: Tank mounting bodge/fix

Post by The Vandal »

There's a tank on ebay that has nuts welded (or maybe brazed) on the bottom of the existing 'strap' on the tank but that seems like it'd be in the way of the rubber bits and/or make the tank sit higher and my bars already touch the tank at full lock.

Re wire inserts, it just seems like that strap is so thin that even from brand new there must have been very few threads present and it's just asking for trouble.
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jackstringer
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Re: Tank mounting bodge/fix

Post by jackstringer »

I had the idea to drill a big hole, sit the the nut in there and weld around it

Gravity does most of the work. The 32ltr tank on my R100GS PD only has a small catch to hold it down at the back.
The Vandal
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Re: Tank mounting bodge/fix

Post by The Vandal »

That's a good idea. Hmmm.

Do you think that there's anything to be gained by keeping it to a female thread on the tank? I guess if it were made the way I was thinking of doing it then those protruding male threads might be more prone to damage when the tank is not fitted to the bike? I can see how that would be/have been a concern for a manufacturer but it shouldn't be too problematic for me and would solve that problem of jabbing about trying to find the hole (sounds bad!).
Drunclej
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Re: Tank mounting bodge/fix

Post by Drunclej »

That's basically what I ended up doing with mine; don't leave the front two threaded ends too long, or (at least on mine) the tank can catch the handlebar clamp (1949), and chip the paintwork when you're fitting it on the bike. I think I just locktited the threaded studs in, and they've held up fine. My model 18 has had a few bodges over the decades, it would seem 🧐
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Duncan
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Re: Tank mounting bodge/fix

Post by Duncan »

As John says fixing the tank on with studs protruding can be difficult, you will also need to fix/hold the rubber spacers in place as these can't be slipped in easily, especially at the front after the tank is fitted.
The Vandal
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Re: Tank mounting bodge/fix

Post by The Vandal »

Aha!

That’s the kind of thing I was looking for!

Maybe nuts brazed to the tank at the front, cut down bolts brazed into the threads at the rear then.

My old man has some end mills that I could always use to recess the rubber blocks to accept the nuts if they protrude off the bottom of the tank.
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clive
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Re: Tank mounting bodge/fix

Post by clive »

I have had a number of fixes over the years on different models. First on a 49 G80 was to weld 4 nuts on the underside of the tank and countersink (badly) the tank rubbers so the tank sits down properly.
Worked fine but watch out when welding, I had a second tank and stuffed the one to be welded in the loft for the summer to clear the petrol fumes. A friend has recently set fire internally to his tank whilst welding and as result damaged all the paintwork from the heat.
Second tank is on a 48-52 bitza. I needed a quick fix on this and used mini exhaust rubbers.
s-l1600.jpg
These have a M8 bolt stuck on each end. The M8 bolt seems to very satisfactorily wind into a worn out thread but anyway it is held in place by the weight of the tank. The bolt at the other attaches it to the frame. Two draw backs, first being the rubber part is waisted so does not quite look like the original, second it makes it difficult to get the tank on if the front two are fitted as the top of the tank tends to be restricted by the handlebar bracket when fitting. I have had this on all 4 points for some 25 years and accept that the rare tank removal is a bit of a hassle. This would also hold if you weld the bolt into the tank.
The third tank was dealt with by a very competent engineering friend. I had asked him to look at a tank leak for me and he was appalled to find the get you home fix I had adopted some years ago when last using the tank. This consisted of some thin beer can around the hole acting as a shim to take up the worn thread. So he machined a top hat threaded on the inside and welded into the enlarged tank hole. The idea behind making it a top hat was to ensure the bolt could not be wound into the tank (again!) so causing a leak.
All good solutions depending on your skill and time.
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clive
if it ain't broke don't fix
The Vandal
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Re: Tank mounting bodge/fix

Post by The Vandal »

The non original look doesn't worry me too much. Given that I feel the original fixing method is somewhat inadequate I'm at peace with a non-original look.

After reading that you say an m8 thread bit into the worn 5/16 one I wondered if the existing holes could simply be tapped to m8 but the drill size for m8 is 6.8 or 7mm depending on thread pitch, 5/16" is almost 8mm so that sounds like it wouldn't work but then the drill size for a 5/16 thread is 17/64" which is 6.7mm so maybe...

The tophat idea is interesting, there's a few t nut style things available on ebay but none that are blind as far as I can see. I have access to a lathe and I'm not bad at using it so could make my own but this seems like a lot of extra effort for only a little extra gain over using the readily available though hole version.

I'll get the tank drained and off and this will give me more of an idea how to proceed I think. I reckon it's probably worth trying an m8 tap though this still leaves the thread very short.

I'd have to hand drill the holes larger to fit a tnut/tophat so my willingness to do that probably depends on whether or not I can slip a sacrificial bit of metal between the threaded strap and the rest of the tank as so that if I 'pop' through the strap I don't just go straight into the tank itself!

I found these 'D nuts' that look like they'd work well if I can drill out to the reqiured size. Nice and flat so minimal disruptance of height with the rubbers etc and lots of surface are to braze to the strap. Could do it with silver to keep the temp down and hopefully minimise damage to paintwork though my tank is far from mint anyway so this isn't too much of a concern to me.

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