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F: Frame Petrol Tank Bolts


James B

Website User
VOC Member
Hi all

Currently struggling with tank attachment - one of the threaded holes for the tank bolts in the headstock has decided to surrender its threads
I’m looking to repair it but first need to identify the thread. My current best bet is 5/16 BSF. Is this correct? Know thy beast, my normal go to, was unhelpful in this regard.
I’m planning to attempt a helicoil - unless there is a better way?

James
 

greg brillus

Well Known and Active Website User
VOC Member
Yes 5/16 BSF...... a helicoil will work fine, apart from one issue that can happen, and that is the replacement bolts are generally made of stainless steel. As stainless is about as forgiving as a brick verses glass, be sure to lube the bolt threads with some grease or antiseize which is best but messy. If you don't do this, it is very possible that when the bolt is removed it will extract the helicoil with it. Be sure to use grease on the drill and tap when you carry this out, as any swarf that is allowed to fall into the headstock will settle atop the lower bearing race, not ideal for the bearing assembly. Good luck with it all............ Greg.
 

davidd

Well Known and Active Website User
VOC Member
5/16" X 22 BSF FT84

Often the thread is longer and I think the Club offers a longer version of the bolt thread. It is in KTB.

David
 

davidd

Well Known and Active Website User
VOC Member
Did I give the wrong number? I just read it off the bolt head. It must be a series B trick!

David
 

vibrac

Well Known and Active Website User
VOC Member
A good tool for fitting those bolts is a socket with an extension and a short 5/16 socket ( mine is ground down) so you can get a good push I also grind a bit of a taper on one side of the large dia rubber it helps slip it in Without looking for the MO drawing isn't the rubber shown the wrong way round there?
Why the minutia? Time is short in the paddock and you get that sinking feeling to hear the words "the tank will have to come off":(
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
A good tool for fitting those bolts is a socket with an extension and a short 5/16 socket ( mine is ground down) so you can get a good push I also grind a bit of a taper on one side of the large dia rubber it helps slip it in Without looking for the MO drawing isn't the rubber shown the wrong way round there?
Why the minutia? Time is short in the paddock and you get that sinking feeling to hear the words "the tank will have to come off":(
Interesting that fellow sufferers (Vibrac) come up with similar solutions. I use a speed brace and socket to hold the bolt straight and keep a little pressure on until the bolts starts to screw in square. If you don't want to shorten your socket drop a 5/16" nut into the socket first so the bolt head doesn't disappear too far into the socket. Don't tighten them up too tight and risk stripping the thread. (Over the years it is most likely these bolts have been put in cross threaded at some time.) I only just nip them up so the rubber is not compressed or you end up with a rigid rather than flexible mount.
 

Albervin

Well Known and Active Website User
VOC Member
A good tool for fitting those bolts is a socket with an extension and a short 5/16 socket ( mine is ground down) so you can get a good push I also grind a bit of a taper on one side of the large dia rubber it helps slip it in Without looking for the MO drawing isn't the rubber shown the wrong way round there?
Why the minutia? Time is short in the paddock and you get that sinking feeling to hear the words "the tank will have to come off":(
Yes, the drawing is wrong and is a trap for new/young players. Silicone grease on rubbers and nickel/copper slip on the bolt is the answer plus a bit of "feel" before you screw it in.
 

greg brillus

Well Known and Active Website User
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

Well Known and Active Website User
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

Well Known and Active Website User
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

Well Known and Active Website User
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

Well Known and Active Website User
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.
 

vibrac

Well Known and Active Website User
VOC Member
How long and how many miles have you been without the cross tube?
 

danno

Well Known and Active Website User
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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