Author Topic: Thorn rear rack bolts  (Read 216 times)

wanderer23

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Thorn rear rack bolts
« on: December 28, 2018, 05:37:44 PM »
Hi all am a bit confused about the 5-6mm bolt difference that they mention on the thorn rear rack does it refer to diameter or length?
If it's diameter will I need to get the threads re-tapped?

Danneaux

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Re: Thorn rear rack bolts
« Reply #1 on: December 28, 2018, 10:10:09 PM »
The Thorn rear rack can be secured with bolts (machine screws, but "bolts" is the commonly used term) that are 5mm /or/ 6mm in diameter.

On a related thread, Tubus Cargo Evo and Logo Evo rear racks will also accept 5mm or 6mm diameter bolts with allen-keyed cylinder heads.

My Nomad has upper (seatstay) bosses that are natively M6 x 1.0mm in diameter. The lower (dropout) mounting bosses are threaded M5x0.8mm. I use Thorn's M5-to-M6 stainless adapter to provide a single M6 x 1.0mm there. The adapter is held in place by two M5 machine screws and spans both bosses. My Sherpa Mk2 used 5mm fittings throughout and I was able to fit the same Thorn rear rack to it with no problem.

See also:
http://thorncyclesforum.co.uk/index.php?topic=12784.msg95461#msg95461
...and...
http://thorncyclesforum.co.uk/index.php?topic=11473.msg83250#msg83250

Here is a useful tip: If you fit upper screws that are slightly long, if the shaft ever fractures, it provides a nub at the other end you can grasp with mole/vise-grips to remove without having to resort to drilling and a screw extractor ("Easy Out"). Similarly, if you run the machine screw with the heads on the inside and secure the rack or upper stays with nylock nuts on the outside, you also have an easy option for removing a machine screw if it eventually breaks -- just fit an allen key to the head that is on the inside (assuming you have enough clearance...it works a treat for securing lowrider racks on front lower dropout mounts).

Best,

Dan.

mickeg

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Re: Thorn rear rack bolts
« Reply #2 on: December 28, 2018, 10:19:42 PM »
The 5 and 6 refers to diameter, not length.  Some Thorn bikes take a 5mm bolt, some take a 6mm.  You are best off using the size that the bike was threaded for in my opinion, I have never heard of anyone converting a frame that was set up for 5mm to 6mm.

The Thorn Nomad Mk II frame has two 5mm bolt holes on each side, a stainless fitting with a 6mm threaded hole can be bolted to the frame with two 5mm bolts to allow 6mm rack bolts to be used, as shown in the photos.  (Second photo has a much cleaner bike, first photo was on a bike tour and airplane pressure changes I think cause the oil leakage.)

I have drilled out a couple racks to handle 6mm bolts if the rack was sized for 5mm.

I am in USA, we often refer to a 5mm bolt as M5, and 6mm as M6.  You might see it written that way on occasion.

Dan posted his response while I was typing the above so I am a bit redundant.

wanderer23

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Re: Thorn rear rack bolts
« Reply #3 on: December 28, 2018, 11:27:31 PM »
Thank you both for the info, I have a Sherpa and was just looking for axtra strength but I don't think there is space on the dropouts for the M6 attachment, I will not re-tap to m6 as I can see the special attachment spreads the load to gain the extra strength
Again thank you both for the help


mickeg

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Re: Thorn rear rack bolts
« Reply #4 on: December 29, 2018, 02:17:45 AM »
Generally the strongest racks are rated at up to 40 kg with 5mm (or as I usually say M5) bolts.  That is probably more than you would be putting on a Sherpa anyway, I have probably put over 30 kg on my Sherpa (I own both a Sherpa and Nomad Mk II) and had no problem with 5mm bolts.  If you are concerned about strength, you can get a high strength steel bolt instead of stainless.

I am not sure the exact weight rating on the Sherpa, I have seen different numbers, but generally I think the weight rating for that bike is in the upper 30kg range as long as you are not on extremely rough terrain.

A threadlocker is a good idea on rack bolts, I use the blue variety of Loctite which will prevent a bolt from vibrating out but the bolt can be removed later.

julk

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Re: Thorn rear rack bolts
« Reply #5 on: December 29, 2018, 04:14:18 PM »
I converted my rear lower rack mount holes in my expr frame from 5 to 6 mm, retapping in the process.
It was fairly straightforward, I used a length of thin stainless rod for direction when drilling and tapping to make sure I was keeping the holes in line. There is a lot of metal in those frame end dropouts.
You may save a little weight and money in the process!
Julian
« Last Edit: December 29, 2018, 04:17:32 PM by julk »