Thorn Cycles Forum
Technical => General Technical => Topic started by: Jonathan Hanson on December 16, 2015, 12:56:02 AM
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Everyone,
As I was installing an SKF bottom bracket on my (English-built) Thorn Nomad, I noticed for the first time an open, threaded hole in the bottom of the bottom bracket shell. Is this supposed to be plugged, and if so does anyone happen to know the proper size bolt?
Thanks . . .
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My Nomad has an eccentric insert like this: http://www.sjscycles.co.uk/eccentric-bottom-bracket-insert-fits-raven-mk2-sport-tour-etc-732mm-for-external-bearing-bottom-bracket-gold-prod16620/?geoc=us
The SKF business fits into the insert, then the insert goes into the frame. When you say the "shell", you're referring to the part of the frame where the insert fits, yes?
My frame has two threaded holes on the bottom, into which are fitted: http://www.sjscycles.co.uk/thorn-m8-extra-fine-cone-pointed-screw-for-eccentric-bottom-bracket-shell-prod12630/?geoc=us
which then clamp the eccentric insert.
Probably I am totally missing your question...
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ah, you probably have an derailleur Nomad, from long before my time!
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It is a water drain hole, simply one of those Thorn 'extras' not found on other manufacturers cycles.
Regards,
John
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Like this one perhaps.
PS its cleaner now!
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Thanks for the responses. Yes, I'm sure it is a drain hole; I just wonder if normally it should be plugged to keep grit from migrating into it, as in4's is. I just need to find out what size and thread.
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I am building up a new bike right now. Bottom bracket not yet in. I wonder if I should drilll a tiny little drain hole?
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I wonder if I should drilll a tiny little drain hole?
Hi mickeg!
Based on old road bikes that had perforated BB shells (ostensibly to reduce weight, rather than condensation), it used to be considered a Very Good Idea to make sure the (then loose-ball bearing) bottom brackets were shielded by a plastic sleeve to ensure the fenderless (without mudguard) front tire did not kick water into the works.
If I were to drill the BB shell, I would also tap it for a bolt to prevent moisture from being kicked into the shell from below while riding, then remove the bolt periodically to allow moisture to drain and condensation to dissipate. I have done this. An o-ring beneath the head keeps it snug and a snap-on socket screw thumb screw cap allows installation and removal without tools. McMaster-Carr usually has them, and sometimes Fastenal or Grainger for US customers. A custom Google search will locate them here:
https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=socket+screw+thumb+screw+cap
In the early 1980s, a small brass valve was available for retrofit to BB shells. You simply pressed it with your finger and any accumulated moisture would drain, then the valve would reseal after you let go of it.
In 1989, I also drilled and tapped a cyclocross-riding friend's BB for a small valve of the type used to release water from air lines and fuel sumps. It worked well.
Of course, these comments apply to non-eccentric BB shells, like the one on your new bike, mickeg. Post-1985 derailleur bikes often have a small bolt with tapped threads holding the cable guides in place beneath the BB shell. This can usually be removed for drainage and then replaced, as the cable tension holds the guide in place while the bolt is out. I've line-bored such bolts through the socket (usually they're 5mm x 0.8mm but occasionally are 6mm x 1.0) to use as a wire channel for lighting.
All the best,
Dan.
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Yes!
The bottom bracket drain valve still exists, made by ActionTec, the same company I remembered:
http://www.actiontec.us/attack.htm
More about them here: http://www.bikepro.com/products/bottom_brackets/bbaccess.shtml
USD$7.60 each
I have no interest in the company.
Best,
Dan.
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Thanks Dan, I think I will just rely on the bolt that holds the cable guide in.
If I recall correctly, I did drill a small hole in my Bridgestone but I store that bike outside so it probably is a pretty good idea on that bike. See attached.
Several years ago I tried to drill a tiny hole (maybe 5/64 inch or something like that) in the bolt that held the cable guide in for drainage, but the drill bit broke which of course meant that I could no longer use a screwdriver in that bolt. That soured me on that whole line of thinking.
Since I have frame savered most of my frames, I am not too concerned. I did not apply that to my Sherpa, but the previous owner told me that he did.
Someone at Thorn or SJS put a piece of tape inside the head tube to keep any water that got into that tube from going into my downtube or top tube on my S&S Nomad. I thought that was rather brilliant, so I did that on my new build. On the S&S Nomad, any water that gets into the downtube from that route would likely just accumulate there for a very long time, so that tape made a lot of sense on that bike.
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Okay, so can anyone tell me if the threaded hole drilled in the bottom bracket shell by Thorn is likely to be metric or SAE/British? That would help me find the proper bolt with which to plug it. Thanks!
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I can't say with certainty, but I would start with metric.
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Hi Jonathan!
I'll give it my best shot. Trolling through the archived Thorn brochures ( http://thorncyclesforum.co.uk/index.php?topic=6975.0 ), I see the Wayback Machine cannot at this time pull up the brochure for your model Nomad. However, it can access the 21March2006 brochure for the eXp:
( http://web.archive.org/web/20060321060820/http://www.sjscycles.com/thornwebsite/thornpdf/ThorneXpBroHiRes.pdf )
Page 1 shows a photo and lists an M8 x 0.75mm drain plug. I'd try that size first. Page 2 shows the same drain hole without paint.
If you don't have a thread gauge or calipers handy, you can try a rough "sizing" by removing a bolt from someplace else on the bike and gently seeing if it is even large enough to engage the threads. I don't think it is M5 x 0.8, and the hex head looks too small for M6x1.0, both standard sizes on bicycles. If the hold easily passes an M6 x 1.0 bolt without the threads engaging and about 1mm clearance on all sides, then I think you could feel confident it is likely the M8 x 0.75 Thorn mentions in the eXp brochure.
Hope this helps.
Best,
Dan.
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That does - thanks very much!
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A last note, Jonathon:
The M8 x 0.75 bolts are the same size diameter and thread pitch Thorn uses to secure the eccentrics on their Rohloff-equipped bikes. If you have no luck finding the drain bolt elsewhere (and if it is indeed M8 x 0.75mm), then SJS Cycles offers the ones intended for eccentrics here at only £2.99 each:
http://www.sjscycles.co.uk/thorn-m8-extra-fine-cone-pointed-screw-for-eccentric-bottom-bracket-shell-prod12630/?geoc=us
You would likely need to shorten it, but that could be done with a hacksaw and file or grinder.
Try the smaller bolts first to confirm before ordering. Dave Whittle of Thorn's Workshop would be the last word on this, but with the holidays nigh, it is likely some members of staff may be away on other pursuits.
Best,
Dan.
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Below are a couple of zoomed photos of the bolt in my MK1 Nomad, as per previous post. Unfortunately my micrometer and vernier gauge are stored away so i can't give you any measurements. That said the top of the bolt appears to read FS and A2 70.
Additionally, there is also a small washer used between the frame and the bolt on mine.
Hope that helps.
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Thank you again Dan. I'm in Arizona, but I have a good hardware store nearby that carries a full selection of metric bolts.
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You will have to be extraordinarily lucky to find a hardware store in USA that stocks a 8X0.75mm bolt.
I looked for one some time back thinking it might be useful to have a spare in the event that one of mine (for my eccentric) fell out.
I am not saying don't go to the hardware store, I am saying I wish you a lot of luck.
If it was me, I would probably just try to plug it with something like a little wad of aluminum foil or a 5/16 wood dowel or something like that. A wad of aluminum foil would probably allow leakage of any condensate out of it and could probably be removed easy enough with a long nose pliers. A dowel would probably have to be tapered to be threaded in part way and cut off but would be harder to remove than the wad of foil.
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Just thought of something...
The threads on male chainring bolts are M8 x 0.75. Again, assuming it will fit...(the threads on Ian's pictured bolt appear to have a much coarser pitch; the A2-70 indicates it is ASTM-302/18-8 stainless, so not marine grade/A4/316) this would extend the socket and allow continuous drainage while shielding the hole from water thrown up from the front wheel.
Best of both worlds.
A stainless one would fill the bill nicely. It would also allow a quick check on size to see if it matches the threads. A standard 5mm allen key will fit for easy installation,
Best,
Dan. (...who sends greetings from very wet and currently flooding Oregon)
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Some were M8x0.75 later ones went to M5x0.8mm, if the former you could use an inner chainring bolt and washer.
Hope this helps, Dave
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Thank you all! And thanks Ian for the photos. I'll report back when I've tried for fit.
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so Jonathan whats the story are you stripping the bike down giving it a full service ;)
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so Jonathan whats the story are you stripping the bike down giving it a full service
Sort of—I had a Shimano Hollowtech crankset on the bike originally, and I just didn't like it. The Q factor was too wide which resulted in a bad chain line. And . . . I didn't like the aesthetics. Meanwhile I'd been hearing great things about the SKF bottom bracket, and decided to switch to a Sugino crankset and the SKF (although I certainly gained some weight with the switch). I'm not sure why I didn't notice the open hole when I first built the bike, unless the bolt was there and subsequently walked its way out, which doesn't seem likely.
Here it is with the Shimano still installed:
(https://c1.staticflickr.com/1/767/23203195204_868cb13ecc_z.jpg)
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Lovely bike, Jonathan, beautifully attired -- a stunner to my eyes!
Those derailleur Nomads are really something. AndyBG has one, and I had the chance to see and really appreciate it up close.
Best,
Dan.