Ohhhhhhhhhhhh boy, Tim; empathy and sympathy your way on the broken bolt and all that has followed from it. There's few things more bedeviling.
Austenitic stainless steel (as your bolts undoubtedly are/were) quickly work-hardens and then makes for a mess to drill for an EZ-out (screw extractor), just as you've found. Not that it will help you this time, but I try to leave my bolts long enough to protrude from the other side so if one fractures, I can grab the stub end with a pair of vise/mole-grips and remove it more easily from the other side with no drilling or extractors needed. Running your bolts from the inside-out and then securing with a nut on the outer face can help similarly in avoiding a full-on drill-extraction down the road. Nice to have a ready option if a bolt breaks while on tour.
As for your present problem, I agree with Fred; a sharp bit at slow speed with cutting oil in a center-punched hole will do wonders to get through the surface work-hardening.
Now to your questions, which I feel I can answer as a hobbyist framebuilder who has built a number of forks and frames successfully...
does anyone have successful experience of moving an unused boss on the fork and re-brazing with a DIY butane/propane blowtorch.
Oh, dearie me, as Phil Liggett might say.
I have to advise against this in oh-so-many ways, Tim. There's much that can go wrong and you really don't want that with a fork, which has less tolerance for failure than, say, a rear triangle (and the results of said failure can be tragic). Of course, the warranty will go as well.
SJS cycles sells replacement Thorn forks for remarkably low prices, and I'd advise going this route. By the time you figure your time/labor, the gas cannisters, the torch, flux, brazing rod, bosses, and a respray, you'd be hard pressed to come in much under the £99.99-£100 for most standard replacement Thorn forks, which are pre-powdercoated to match your frame.
Obligatory cautions on general principle out of the way and speaking academically...
If you wish to try brazing new bosses on *a* fork, you'll have best luck with either MAPP-gas (no longer available and the substitutes don't burn as hot) or oxy-propane an oxy-acetylene torch with a fine tip and a soft, carbonizing flame to remove the old boss, taking care to heat just the boss enough to whack it off cleanly with a light hammer, then hand-sanding with emery cloth to remove the remaining brass or silver. I've had good luck removing and reusing some bosses, but by the time they've been brazed on once, they can distort when heated again for removal. Nova Cycles (
http://www.cycle-frames.com/bicycle-frame-tubing/ ), Ceeway (
http://www.framebuilding.com/ ), or one of the nicest people in the industry, J. Gaerlan (
http://www.gaerlan.com/bikeparts/bike.html ) are excellent sources for new bits and I'd strongly suggest laying in a stock of fresh bosses and considering the old ones unsalvageable justincase.
These can be bought for £25. I have brazed before with the proper kit so I am asking if this is feasible.
While these will work for plumbing and the like, they just don't get hot enough to make a secure connection on a steel frame, and the tips are less than ideal. A MAPP-gas unit will do it, but not the garden-variety butane or propane torch. An air-injecting swirl-tip will do it sometimes if you're silver-brazing, but then you have to get things really clean and have really close tolerances, unlike with brass, where the added heat will work with the flux to burn off more in the way of impurities and also fills gaps in non-critical joins. As you know from brazing yourself, you have to get the parent metal (tube, fork blade) up to temp as well as the braze-on bit, to the point where the flux cleans the joint and the brazing material will melt on contact. The thinner unbutted sections of lightweight frame tubing and small bosses...okay, but on something like a fork blade, I think the heat-sink effect of the blade will be too great to get a good, effective joint without a hotter torch.
I realise I will probably have to get the framed resprayed, or at least the fork.
Yep.
has anyone brazed stops onto their fork to create a conduit for hub dynamo cable and the speedometer? Or has anyone brazed a plate onto a fork to take a speedo-sensor? Also what about stops for the cable supplying the rear light?
Yes, no, and...yes, I have, and made the little fixtures to do it. For the unbutted sections of Tange Prestige tubing, I used a spring clamp well out of the HAZ (heat-affected zone) with a spoke serving to hold the braze-on gently in place so the tube wouldn't dimple even under light heating and silver-- that stuff is *thin* at 0.7/0.4/0.7mm wall thickness, and heats so quickly you have to really watch it. I'd never use brass for braze-ons on such thin-walled tubing, and I'd keep cable stops in the butted sections.
I would suggest avoiding braze-ons for things that can change. For example, light wire guides are okay, but computer pickups change from model to model and computers don't last forever, so I'd pass on that or anything that is "faddish", proprietary, or subject to change or periodic redesign. Zip-ties ain't bad and can be configured to either look good or look nearly invisible, as with the light-wire and charging-system runs on my Nomad, shown here:
http://www.thorncycles.co.uk/forums/index.php?topic=4523.msg38847#msg38847 http://www.thorncycles.co.uk/forums/index.php?topic=4523.msg38853#msg38853http://www.thorncycles.co.uk/forums/index.php?topic=4523.msg39091#msg39091 My Danneaux's Sherpa gallery shows how I often glue wires inside the lip of the rear mudguard, the wire exiting from grommet-lined holes at two points in the 'guard:
http://www.thorncycles.co.uk/forums/index.php?topic=3896.msg17113#msg17113 Many of the same benefits of internal frame wiring without the disadvantages.
I am considering drilling a hole in the back of the fork and then taking the speedo cable and a parallel cable from the hub dynamo to the bar-bag
Please don't do this. If the fork already has brazing vent holes (which Thorn take great care to place within the crown shoulders to prevent water entry, so not a Thorn), you can take a length of dental floss and a vacuum cleaner and suck the floss through the
existing holes to use as a fish to pull wire through for a computer pickup or light wire, but you'll be limited on the wire gauge that can be accommodated. I have drilled through downtube lugs to internally wire frames, but would not do so on a TiG-welded frame that lacked lug reinforcement. I also would not run through a fork any wires larger than the vent holes could accommodate. Better to place the guides external to the fork blade.
There are other considerations as well. A Dutch friend of mine has a bike factory-equipped with full internal light wiring; the wire entry/exit points served as a means for water entry as well, which eventually resulted in a rusty BB interior. Also, wiring tends to eventually fatigue and break where it enters the frame and is much harder to replace when the time comes. Yes, the clean look is nice, but it does come with a few downsides. If you eventually go this route, I would suggest using Futaba R/C micro-grommets and a dab of clear or black RTV silicone at the entry points. The grommets will ease flexing and the silicone will prevent water entry.
I really hope this helps, Tim. If it were me, I'd have a further go at removing the broken bolt, then repairing the munged threads with thread-repair compound or heli-coil inserts or both, depending on how bad the damage. It might be worth running the fork by a machine shop; it could be less costly if they wear out their bits on it. If the bolt can't be removed, then I'd surely go for a replacement fork from SJS Cycles. As for the wiring, I'd go the zip-tie route or something similar -- alternatives include dots of super glue, running the wires under matching electrical tape (good on a black or red frame, which can be easily matched) or bundling several wires inside split-loom plastic conduit and securing just that one item with tape or cable/zip-ties. Cable ties really can be done nicely with a bit of thought. They're even available in matching or contrasting colors, and ready accessibility for service is a virtue on a touring bike.
Best,
Dan.
