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M5 bolts in M6 lowrider mounts

Old 06-15-18, 11:00 AM
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M5 bolts in M6 lowrider mounts

I've recently bought a 1994 Trek T100 tandem, that comes with lowrider fork mounts that are not the standard m5 threads but m6 threads. The mounts are through the fork, not brazed onto it.
An m5 bolt fits through the m6 threaded hole without engaging the threads, so I was thinking, if I could just use a regular m5 bolt, with a nut and washer on the inside of the fork leg instead of the intended m6 bolt?
Doing that would enable me to use my Tubus Tara front rack, as the holes in the plate on the horizontal stays are not big enough for an m6 bolt, but would the clamping forces of the bolt and nut compromise the forks strength in any way?

I know I could probably drill the hole in the Tara to accept an m6 bolt, but if I can avoid making any modifications to the rack I would prefer that.
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Old 06-15-18, 11:09 AM
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....somebody once tole there are helicoil inserts that can adapt M6 to M5....not sure if that would work in this case. don't know if you would need to drill out the fork first.

try NAPA?

Last edited by saddlesores; 06-15-18 at 11:16 AM.
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Old 06-15-18, 11:34 AM
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5 is a 0.8 tpmm, 6 is 1 tpmm , so thread pitch is not going to match

a thru the fork tube mount may even be unthreaded,
so uses a longer bolt all the way through, with a Nut on it.
you can use a hoopless lowrider rack like the Duo,

those have a bracing tube on the inside and the outside of the fork ..

Or you can use a rack with a connecting Hoop. like your Tara...





...

Last edited by fietsbob; 06-15-18 at 11:37 AM.
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Old 06-15-18, 12:13 PM
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Originally Posted by masterchief
I've recently bought..
Your solution of 5mm bolts should work. If possible, secure all rack bolts with nylon-insert nuts or threadlocker (Loctite blue, or even cyanoacrylate gel in a pinch).
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Old 06-15-18, 01:15 PM
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I would drill out the holes in the rack to fit M6 bolts.

That is what I did, but a different bike than yours. My expedition bike has M6 rack bolts, I had to drill out the front rack to fit. I do not recall if I had to drill out my Tubus Logo to fit on the rear or not, but I use that rack on a bike that takes M5 and also use that rack on my expedition bike that takes M6.

ADDENDUM ADDED LATER:

And after drilling, touch up the paint where you drilled it to reduce rust. On black, I use black finger nail polish to touch up the wear spots on my black racks. I do not know what the best paint to touch up a silver Tubus is.

Last edited by Tourist in MSN; 06-15-18 at 03:30 PM.
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Old 06-15-18, 01:31 PM
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Thanks for the answers. My primary concern is really just if the clamping forces of the bolt + nut will damage the fork blade, since it's not the same forces as when threading a bolt into the threaded hole in one side of the fork.
It's totally doable, but I dont want to risk ruining the fork blades.
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Old 06-15-18, 02:02 PM
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My Mooney's fork was made with a much larger insert for an English bolt (the standard English generator mount) because Peter Mooney was unwilling to drill into the forkblades. (This was 1984. LowRiders were still relatively new and my fork failure was still in both our memories, Peter being both a clubmate ant present at the club dinner where it was announced I was in a coma.) He flat out refused to make a LowRider mount when I ordered the bike in '98. When I ordered the new fork, he said he had considered the generator mount because no drilling would be required.

So I have to drill every rack I put on that bike. Not an issue. (If I didn't have a drill, one of those tapered hand tools to enlarge holes would do the job just fine. LowRider racks die at the fender eye, not the upper fork point. Even a very "dirty" botched hole would serve.)

Edit: for compressing the fork blade, what matters is the sleeve the M6 bolt threads into. Can it take the pressure? You can probably see the sleeve end and mic it. Tell us that that is and we will know more. If the tube is sized to actually take compression you are good to go, It if is little more than a set of threads, this approach scares me.

Ben

Last edited by 79pmooney; 06-15-18 at 02:06 PM.
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Old 06-15-18, 03:26 PM
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Id be worried about the threads of the fork being damaged by the M5 bolt. As you ride, I imagine the rack will settle under load to the lowest point, and those bolt threads would apply preassure to the threads in the fork. Personally, Id just drill the rack. If you decide to use the M5 bolt, maybe use one where the part going through the fork isnt threaded. That should help reduce the chance of the fork threading being damaged.
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Old 06-15-18, 04:09 PM
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Originally Posted by 3speed
Id be worried about the threads of the fork being damaged by the M5 bolt. As you ride, I imagine the rack will settle under load to the lowest point, and those bolt threads would apply preassure to the threads in the fork. Personally, Id just drill the rack. If you decide to use the M5 bolt, maybe use one where the part going through the fork isnt threaded. That should help reduce the chance of the fork threading being damaged.
If the bolt is tight, it shouldn't be settling anywhere. Mounts like that get all their strength from the bolt tension causing the rack and fork to have too much friction to move, not because the rack is sitting the bolt.


OP,

Seems like the main place you're going to do damage is on the inside of the fork which is going to have a washer compressing the paint where it normally would not. I would be more a fan of opening up the rack holes than messing up the fork paint, but that's aesthetic.

Have you tried an M5 bolt, and will it fit through the inner diameter of the M6 threads? The inner diameter of M6 is very closer to 5mm.
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Old 06-15-18, 04:20 PM
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Just drill out the hole in the Tubus rack to accommodate the M6 bolt.
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Old 06-16-18, 11:22 AM
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what I have found a tube runs thru the fork
clamping on it compresses the tube on end, with little effect, not the fork blade

working blind I really don't see what the issue is...



try NAPA?
My home town..




..
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Old 06-16-18, 01:05 PM
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Originally Posted by 3speed
Id be worried about the threads of the fork being damaged by the M5 bolt. As you ride, I imagine the rack will settle under load to the lowest point, and those bolt threads would apply preassure to the threads in the fork. Personally, Id just drill the rack. If you decide to use the M5 bolt, maybe use one where the part going through the fork isnt threaded. That should help reduce the chance of the fork threading being damaged.
Had not thought of this, thank you. That will add to the cons of using an m5 bolt and nut.

Originally Posted by Kontact
OP,

Seems like the main place you're going to do damage is on the inside of the fork which is going to have a washer compressing the paint where it normally would not. I would be more a fan of opening up the rack holes than messing up the fork paint, but that's aesthetic.

Have you tried an M5 bolt, and will it fit through the inner diameter of the M6 threads? The inner diameter of M6 is very closer to 5mm.
Yes, I have, and the m5 bolt fits. But considering the comments in this thread, and my fear of damaging the fork, I will try drilling the holes in the rack.

I have a Specialized Pizza Rack as well, and its triangle bracket mounting hardware things are drilled for m6 bolts, so I could use that instead, I just prefer the feel of the Tara to the Pizza Rack when loaded.
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Old 06-16-18, 09:19 PM
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Originally Posted by masterchief
Had not thought of this, thank you. That will add to the cons of using an m5 bolt and nut.



Yes, I have, and the m5 bolt fits. But considering the comments in this thread, and my fear of damaging the fork, I will try drilling the holes in the rack.

I have a Specialized Pizza Rack as well, and its triangle bracket mounting hardware things are drilled for m6 bolts, so I could use that instead, I just prefer the feel of the Tara to the Pizza Rack when loaded.
You are only talking about a mm difference. It should have very little affect on your rack's strength.
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Old 06-17-18, 01:41 PM
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a tapered reamer (@ the auto parts store) or a *chainsaw file , will be adequate ,
you really are just taking out 0.5mm radially to have a 1mm diameter increase..

They * are not tapered, like a round 'rat tail' file is..
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