tank mountings

Dave Walker · Post by **Dave Walker** » Fri Mar 19, 2004 10:49 pm

can anyone provide a drawing of some studs to replace the tank mounting bolts on a 56 ish twin when the threads in the tank are worn out. I believe this is a well known fix and I could try drawing it up myself but would likely as not make some crass error or other. Can anybody spare me from reinventing the wheel?.

bjork · Post by **bjork** » Fri Mar 19, 2004 11:34 pm

Hi David, I'm not sure if the tank fixing on your bike is like mine, but I did successfully shorten helicoil type thread inserts on my G80 and fit them when faced with the same problem. I was worried that there would not be enough depth for the tap to work properly, but there was about 3/16" or 1/4" between the thread plate and the tank base, and then the tank base was 'dimpled' inwards another 1/8" or so. This was just enough to allow the use of the recoil/helicoil system. Hope this is of some help, I'm only familiar with this type of tank fixing and the 60's rubber bung jobbie.

Eric · Post by **Eric** » Sat Mar 20, 2004 12:00 am

You

Eric · Post by **Eric** » Sat Mar 20, 2004 12:00 am

You can usually get round the problem by re-tapping using any thread with a pitch coarser than the one already used. [:p] The new thread will have a deeper form than the old one. [:)] The only problem with the petrol tank is by the time you get to 4 tpi acme there is nothing left. [:(!] My tank has been re-tapped with Whitworth [;)] so I just use a Whitworth bolt instead of the cycle thread. [^] Sorry without checking I cant remember diameter [:(] I guess 5/16" then you can use everything else just as intended. [:)]

Biscuit · Post by **Biscuit** » Sat Mar 20, 2004 10:47 am

Lawrence I can't quite grasp the logic of the coarser thread. If the thread is worn/stripped, the core diameter just isn't there to take advantage of the coarser thread. One method I used in the past when my tank was on and off like a fiddlers elbow (as teenagers did) is as follows. Cut a piece of brass/mild steel to the same dimensions as the bracket on the base of the the tank, drill a 5/16" hole in the centre and over the hole braze/silver solder a 5/16" x 26 nut, if the facilities are available turn the nut down to around .4" diameter. Carefully open out the worn/stripped thread to accept the nut and with the nut uppermost through the enlarged hole drill and tap the tank bracket to accept 4 - 2BA C/S screws, screw the plate and nut to the the bracket. Caution while opening out the hole and drilling and tapping the 2 BA threads introduce a piece of thin steel between the bracket and the tank to avoid damaging the base of the tank. Also ensure that the screws and the nut do not contact the tank base.

Eric · Post by **Eric** » Sat Mar 20, 2004 1:48 pm

Alan you are right I posted my reply without engaging gear first. [V] This is because my tank apparently has a Whitworth thread and when I did once have it all sandblasted down there was no evidence of metal being added by welding. I wonder if by using a tap something like a self tapping thread form that does not remove metal but moves it around you can get away with it [?] You could always try there is nothing to loose, then you can try the helicoil method if that fails or your method as another way. I guess you could always go quite mad and open up the existing hole and make a tapped bush with a very thin flanged head to fit and bond it in with one of the very modern adhesives we have available today. I remember once many years ago using a locktite product to bond a collar onto a 5/16 dia spindle. This was a production item originally intended by design to be copper brazed, but as a company we did not having proper brazing facilities and not wanting to contract it out we hit upon this idea. The resulting bond proved to be more than adequate for the job. The other way of course is plug and weld, drill and re-tap, but this may not be acceptable because of potential paint damage.

Biscuit · Post by **Biscuit** » Sat Mar 20, 2004 8:13 pm

Sorry Eric and Lawrence, I attributed the posting to the wrong man. Your suggestion of the stud and collar is a better engineering solution, the collar does'nt have to be thin, as the requisite amount can be removed from the bolt sleeve. Thinking back, my nut was actually soft soldered to the brass plate, could'nt afford silver solder on 15 bob a week!. At the time I considered another solution, slotting out the damaged hole, sliding in a set bolt with a reduced thickness head, and then running a nut down the thread to clamp the bolt to the bracket. This idea was discarded due to the difficulty of filing out the slot. The idea put forward was on the assumption that David only had access to hand tools, as I had at the time.

clive · Post by **clive** » Sun Mar 21, 2004 7:42 pm

Sod engineering solutions what you may want is something to get you home when you have lost one of the tank mounts. I have found that if you can buy a mini exhaust mounting rubber you can get away with it. For a get you home bodge you may even be able to "cut" the thread in just using the thread on the bolt which is mounted on the "cotton reel" rubber and is mounted on aflat washer which neatly allows you to turn it in with the assistance of a self-locking grip. of course it wrecks the original thread but the bolt usually only falls out because the thread is dodgy and loose anyway. The other end then just slots through the frame as it too has a bolt on it. I have actually used this mounting for preference on my bikes for years. The join between the rubber and bolt (which is mounted on a flat washer) tends to give up after a few years with the vibration but responds very well to a dose of superglue. of course you do not need to be an originality freak or you will get an anxiety attack.

Dave Walker · Post by **Dave Walker** » Thu Mar 25, 2004 10:45 pm

Thanks for the ideas - there's a few there I hadn't thought of. The fix I've heard of seems to consist of drilling out the tank hole to a wider diameter (5/16 perhaps ?) and retapping with suitable thread. Then having a stud turned up with that thread on one end and a suitable 1/4 thread on the rest. The stud can be tightened & bonded into the tank and then the standard rubbers used with a nut (ideally nylock) on the bottom holding the lot together.

The advantage being that on taking the tank on and off you are no longer working the thread in the 1/4 steel mounting plate on the tank but on the body of the stud which spreads the wear over a greater length as you tighten the nut over the years to take up the crush in slowly decaying rubbers.

So I guess it's a stud with 3/8 of 5/16 thread on the end of about 1&1/4 of 1/4 thread with about 1/16 of 5/16 diameter unthreaded in between. Perhaps it is that simple and I've just answered my own question. But is there anyone out there who's done it before and can confirm I haven't forgotten something crucial?

Like if the studs are too long does the tank foul on anything coming on & off? or do the existing mounting rubbers etc work Ok like this?

Biscuit · Post by **Biscuit** » Thu Mar 25, 2004 11:10 pm

HI David, The bolt is 5/16"x 26, so the retapped tank bracket will need to be 3/8" x 26, There should be a steel sleeve through the centre of the rubbers which compress them to the correct amount, the sleeve protudes through the frame bracket, hence the sleeve is locked against the tank bracket and the washer under the head of the bolt, no need for a nyloc nut or locking wire.