F: Frame Petrol Tank Bolts

greg brillus

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VOC Member
A couple of observations, The bolts should be done up tight, or else they will vibrate loose .......... the shoulder or step on the bolt is very narrow, sometimes these seem to have carved their way part way down the threaded hole. This makes tightening the bolts a problem as it compresses the rubber mount too much. One easy way to fix this is to simply replace the correct part F 81 with the slightly longer FT 84 footrest hanger pivot bolt, and shim as required using slightly thicker washers under the head. Sometimes on some tanks the forward mounting lugs are too close together, and bending them apart is none too easy........You can linish down part of the larger section of the rubber mounts FT 80 so the smaller diameter part of the rubber passes through the tank mount more so. It is not desirable to overload these mounts, and it is worth simply modifying the rubber mounts to allow this............. Equally as important is the rear mount, if this is overloaded you will end up with cracks and fuel leaks from this area. A correctly mounted tank is one that can be grabbed from either side and the tank moved about so as to compress the rubbers, but also that does not mechanically touch on anything very rigid, such as some parts of the upper frame member, or many instances where the tank undersides are resting on the top of a crash bar. This will cause bad vibrations through the bike and can easily wear through the welded seam along the length of the outer base of the tank.............. Again resulting in a fuel leak.
 

oexing

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VOC Member
Common practice is to have a rubber tank mounting bush with a length of thin walled tube pressed into that rubber . Not so on Vincents obviously, but no drama to copy this. Then you´d have any 5/16 plain bolt at this place and you could fit just the optimum length of say 8 by 10 mm stainless tube into the rubber and nip it all up - done. A lot easier to start the bolt into the thread on the bike.

Vic
 

Vincent Brake

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VOC Member
What i do;
Indeed a longer bolt will do.
The helicoil will but up to the cylinder part of bolt and so will ruin it. No good. Than a bush is needed to overcome.
And bit bigger hole in the rubber.

Tap thread to the end,
First do cloth in so you can blast clean with air.
We dont want swarf in this precious bearing.
Machine up a new bolt, thread not reaching the stem by at least 3 mm


If thread apears gone totally.
Use a long 3 x D Time-Sert unf5/16
From Würth.
 

vibrac

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VOC Member
While we are on tank fixings I have some hex alloy bar about 6mm size (or 1/4) I tap each end and use it to space the under tank tabs one hex set screw in a few turns on one side and the hex spacer can be maneuvered easily into position for the other side to add the second hex set screw tighten both. Easier than threading a long bolt across the tabs, fiddling with nuts and washers and it saves an ounce or two. You could do it with round bar but hex is easier to hold
 

kettlrj

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VOC Member
I have machined my own front tank bolts with a little extra length on the plain diameter, an extra 1/8" of thread length and a 1/8" long plain lead in (diameter of thread root). These feed into the UFM thread very easily having been centralised by the lead-in diameter. I am running with an Indian made tank after my ex Argentinian tank gave up and started to leak around the rear mounting. The Indian tank came with the rear mounting pad incorrectly made and quite weak. I cut this out and fabricated a new one out of 4mm plate which gives greater strength to the back of the tank. This allows me to mount the rear of the tank on two anti-vibration mounts (the type with a central rubber element and a stud at either end). This lifts up the rear of the tank and means that the cross tube cannot be fitted. This is not a problem with the stronger rear mounting pad. I think that the original mounting using the two rubber pads is too stiff and is the cause of leaks in this area. The A/V mounts provide more flexibility and do not stress the tank so much. Time will tell if I am right.
Regards Richard.
 

danno

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Non-VOC Member
one of the threaded holes for the tank bolts in the headstock has decided to surrender its threads
James

Had the same problem years ago with the RH tank bolt. Someone from here kindly
machined up a longer bolt and posted it for a small fee. Did the job fine.
 
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