Got a set of longer bolts from the spares company and fitted them tonight - perfect! Problem solved!
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F: Frame Petrol Tank Bolts
- Thread starter James B
- Start date
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When I first acquired my Comet I found that both of the front fuel tank mounts (F81) in the UFM had their threads (5/16” BSF) damaged and essentially stripped. So one of my first tasks was to repair them. Like a lot of others I went down the Helicoil path and this involves re-drilling the hole (so making it bigger!) using the supplied drill bit then tapping with the supplied tap it to take the Helicoil that restores the 5/16” BSF thread.
The retention arrangement is a shouldered anti-vibration rubber bush (FT80) that sits in the tank mounting hole and also holds the tank bracket away from the UFM. A shouldered bolt then passes thru the rubber bush with its shoulder abutting the UFM and when you tighten that bolt it is meant to tighten its shoulder up against the face of the UFM without compressing the bush. If in the process the rubber bush does get compressed it loses some if not all of its anti-vibration ability. Here is the problem – when you drill out the UFM to accept the Helicoil repair you actually ENLARGE the hole in the UFM. Later on you will find that now the shoulders of the bolt goes into the UFM a bit further and the FT80 rubber bush starts to compress. As a consequence the bike seems to vibrate more than before. This is exactly what the situation with my bike was. And I put up with it for years.
Simple fix is a F81 shouldered bolt with a longer shoulder section so that the FT80 rubber IS NOT compressed– but where to get one? Especially when you have a small workshop and no machining facilities.
The bolt into the UFM is 5/16” BSF; the shoulder on the shouldered bolt is 3/8”. Just so happens that SS tube 3/8” OD x 5”16” ID is available at most hobby shops – sold as K&S Metal part #87119 which is 12 inches long. So with my tank removed and a set of new FT80 rubbers from the Spares Co, I fitted a rubber onto the 3/8" tube then pressed that lot against the UFM making sure the tube ‘bottomed’ in the UFM mount hole and the rubber was up against the UFM. I discovered that the tube went into the UFM – about 1/8” – so I carefully marked the outside of the tube then cut it off and slipped the resultant ‘sleeve over my 5/16” BSF bolt resulting in a ‘new’ shouldered bolt that was a perfect fit.
Then I did the same on the other side of the UFM. The BSF bolts I used started out at 1 ½” long and I found that I need to shorten them to 1 and 3/8” to achieve the desired result.
There is now zero compression of the FT80 rubber bushes when the bolts are fully home. Anti-vibration properties fully restored!
The retention arrangement is a shouldered anti-vibration rubber bush (FT80) that sits in the tank mounting hole and also holds the tank bracket away from the UFM. A shouldered bolt then passes thru the rubber bush with its shoulder abutting the UFM and when you tighten that bolt it is meant to tighten its shoulder up against the face of the UFM without compressing the bush. If in the process the rubber bush does get compressed it loses some if not all of its anti-vibration ability. Here is the problem – when you drill out the UFM to accept the Helicoil repair you actually ENLARGE the hole in the UFM. Later on you will find that now the shoulders of the bolt goes into the UFM a bit further and the FT80 rubber bush starts to compress. As a consequence the bike seems to vibrate more than before. This is exactly what the situation with my bike was. And I put up with it for years.
Simple fix is a F81 shouldered bolt with a longer shoulder section so that the FT80 rubber IS NOT compressed– but where to get one? Especially when you have a small workshop and no machining facilities.
The bolt into the UFM is 5/16” BSF; the shoulder on the shouldered bolt is 3/8”. Just so happens that SS tube 3/8” OD x 5”16” ID is available at most hobby shops – sold as K&S Metal part #87119 which is 12 inches long. So with my tank removed and a set of new FT80 rubbers from the Spares Co, I fitted a rubber onto the 3/8" tube then pressed that lot against the UFM making sure the tube ‘bottomed’ in the UFM mount hole and the rubber was up against the UFM. I discovered that the tube went into the UFM – about 1/8” – so I carefully marked the outside of the tube then cut it off and slipped the resultant ‘sleeve over my 5/16” BSF bolt resulting in a ‘new’ shouldered bolt that was a perfect fit.
Then I did the same on the other side of the UFM. The BSF bolts I used started out at 1 ½” long and I found that I need to shorten them to 1 and 3/8” to achieve the desired result.
There is now zero compression of the FT80 rubber bushes when the bolts are fully home. Anti-vibration properties fully restored!
Original F81 on the left and my 'manufactured' F81 on the right. Only other point of note is the original uses a 3/8" washer while the 'manufactured' uses a 5/16" washer so ensuring that the 'clamping' load is taken on the 3/8" tube and NOT on the anti-vibration rubber FT80.
This was my solution, but I had the whole plot in pieces. I think if one was careful it could be done in situ. This was for some very badly damaged holes. Sorry the picture is a little fuzzy if you blow it up, but you get the gist.
My kingdom for a well equipped workshop!
Black & Decker workmate or similar works OK for this.
I made my own bolts, with 1/8” more thread, AND a tapping size diameter pilot 1/4” long. End of problem, since engaging the pilot ensures the thread will start.
Tom Gaynor
Tom Gaynor