B31 rigid - broken frame

[KiwiGF]

My A10 had a cracked downtube, about halfway down.
I repaired it the same way that I lengthened the downtube of my M21 when fitting the vee twin engine.
First, if it’s a crack, then cut it right through. Then use a flap wheel to clean inside both ends of the tube. Then turn up a tube that just slides in the frame, walls a touch thicker than those of the frame (solid gains nothing except weight and makes it harder to braze). Make the tube about 3” long and mark halfway with a felt tip pen. Now drill a half inch hole right through the frame about an inch from the crack / join. Clean up with flap wheel. Push frame ends apart and slide in repair tube. Line up frame tubes and use felt mark to guide as you slide inner tube to bridge the join. Now braze through the holes and the join itself. Braze is best, due to its capillary action. Got pics of it somewhere, but can’t find at present.

Thanks, very helpful, the cast headstock has “slots” to aid in good brazing so thankfully no requirement to drill that, and the holes in the tube itself is a great idea. I’m not sure if i’ll Replace the tube or repair it yet. 

[KiwiGF]

see the ist pic didn't make it onto the page ...

Brilliant pic thanks.

[KiwiGF]

Does riding the gravel roads around the Wairarapa count as motocross?

it depends how you ride Kiwi 

From now on, in this bike, slowly....until I am 100% sure it’s fixed!

[KiwiGF]

I don’t remember much metallurgy from schooling days, but it looks like a brittle fracture to me but views from others with more knowledge appreciated. No signs of rust which indicates it’s not been cracked for long, before breaking.

You can see a “pin” on the inside of the headstock end of the downtube, but there is no sign of a pin on the outside of the headstock 

[duTch]

 I've seen those 'pins' before- I thought they were just some bad aim from the job site next door 

[Swarfcut]

Kiwi, Those pins were the problem to people who thought a frame repair was a case of heating the lug and pulling.  My assumption is that in production, the pins held the frames rigidly enough for handling, prior to brazing. Nice to see one for real, but as you say, well concealed. Without prior knowledge of the location, most folks would give up, leaving Percival Bothers and Webb the specialists in a small market.

Cheers

Swarfy.

[Greybeard]

Those pins look remarkably like carpentry nails!

[Swarfcut]

Good chance GB, with the nail making capital of Redditch just down the road from Small Heath. Quick, easy, cheap.....

Swarfy.

[duTch]

 

Those pins look remarkably like carpentry nails!

 Yep ;

 

  I've seen those 'pins' before- I thought they were just some bad aim from the job site next door 

[Greybeard]

Good chance GB, with the nail making capital of Redditch just down the road from Small Heath. Quick, easy, cheap.....

Swarfy.

BSA probably owned the nail making factory.

I imagine this scene: The frame was assembled in a jig. (Would they have fluxed the joints at this stage?) Pilot holes were drilled and the pin, (nail)  was driven in with a hammer, but leaving some protruding to make it easier to cut off. Then the carrier, loaded with frames was rolled along to the brazing shop where the excess pin was removed prior to hot 'glueing'

[RDfella]

Just a warning to anyone thinking of un-brazing a tube from a lug. Tried it a couple of times in the past, with nil success. Firstly, the pin, once found, is a bugger to remove. Won’t punch through, because it’s brazed. Difficult to drill, because it’s harder than the lug. Once you have overcome that, you need a hearth to get the lug cherry red ALL OVER to get the tube out. I’ve tried acetylene with a large nozzle and it just doesn’t keep it the right temp all over. Lastly, the area that can be a disaster – the lugs appear to be a type of wrought iron. They’re certainly not as strong as one would expect, with the result that whenever I’ve tried twisting the tube out, I’ve torn the lug. Only way I’ve ever got tubes out of lugs successfully is by machining them out – which of course will be impossible in certain circumstances.

[KiwiGF]

Just a warning to anyone thinking of un-brazing a tube from a lug. Tried it a couple of times in the past, with nil success. Firstly, the pin, once found, is a bugger to remove. Won’t punch through, because it’s brazed. Difficult to drill, because it’s harder than the lug. Once you have overcome that, you need a hearth to get the lug cherry red ALL OVER to get the tube out. I’ve tried acetylene with a large nozzle and it just doesn’t keep it the right temp all over. Lastly, the area that can be a disaster – the lugs appear to be a type of wrought iron. They’re certainly not as strong as one would expect, with the result that whenever I’ve tried twisting the tube out, I’ve torn the lug. Only way I’ve ever got tubes out of lugs successfully is by machining them out – which of course will be impossible in certain circumstances.

Add to that list that the pin is right where the frame number is! (not that one can see it). So removing it would involve filling the resultant hole and re stamping at least one character.

If I went the the un-brazing route I would use a die-grind the pin (and surrounding tube) out, to try to keep the number intact, but there is un-brazing the other end of the tube to consider (cast engine mount end) where the pin is harder to access.

I guess the consensus is the pin is purely for holding things in place whilst brazing is done, not for added strength? (Personally I can’t see it adds much strength).

[Guy Wilson]

see the ist pic didn't make it onto the page ...

Great advice, thank you..I have sprung hub T100 that has break in the frame. The scrap man was in the process of cutting it up when I found it.. I will apply your technique!
Guy

[Rex]

Those pins hark back to the days of push-bike frames and their construction. Drilled the lugs and pinned it, then put the whole construction in the hearth for an easy brazing job.

[KiwiGF]

I fitted the frame section I bought, i will fix the original frame one day and will give an update on how it was done, I’ve got an old but decent TIG welder going now (lockdown project) so it may be welded rather than brazed but no decisions yet.