Cross-threaded sub-frame

rb_stretch · Mar 30, 2003

Having struggled for an hour to put my standard silencer on after replacing the catalytic converter, I ended up cross threading the attachment point on the subframe.

I tried to redo the screw without the silencer to try and encourage the original thread back again, but seems like a lost cause.

Does anyone have a bright ideas as to what I can do? Do I need to replace the whole sub-frame? Hope not.

Thanks
RB

MikeO · Mar 30, 2003

Can't quite visualise the threaded point (& my bike's not here). Normal solutions I'd be thinking of would be either 1. drilling out the thread and tapping the next size up (& replacing the bolt with the next size up, natch) or else 2. drilling it out and putting a nut & bolt through the hole, assuming you have access to the rear....

MikeO

cookie · Mar 30, 2003

try putting the correct size tap through it (straight

) to clean up the threads. maybe only the first few threads are bolloxed.

as i only own an antique airhead, i am not familiar with the particular location or material you are dealing with. if it's ally, and it's terminally fucked, a helicoil would sort it (about £15 at engineering firm).

rb_stretch · Mar 30, 2003

On closer inspection there is a square nut that is welded to the subframe next to the left hand foot pedal.

Thread is fairly knacked all the way through so I've attached the exhaust miss-threaded and hope that it will hold.

Thanks for the help, will probably not do anything until I really have to.

RB

cookie · Mar 30, 2003

sounds like it's prolly steel then. longer bolt & nut sounds favourite.

Gecko · Mar 31, 2003

cookie said:
try putting the correct size tap through it (straight ) to clean up the threads. maybe only the first few threads are bolloxed.

THREAD HI-JACK

Cookie , what do you mean tap ? I have a similar problem on my old BSA but in my case I am sure it's just the first few threads that are damaged. Can you enlighten me in simple words of not more that 2 syllables
Cheers
Gecko

cookie · Mar 31, 2003

a tap is a tool a bit like a stud (bolt with no head) made of very hard steel with flutes or slots cut into the threaded bit to clear swarf (debris) as it cuts a thread in either a bare hole or clean existing.

given up on description. see pic

cost: about £5 for an M8 size i'd say. you'd maybe need some obscure British Cycle Thread for a BSA though.
metric & common imperial sizes available at decent hardware shop. BCT at specialist.

you put the tap in a tap holder, or you can use a very small spanner to turn the non threaded end and screw it into the remains of the existing thread.
with luck, it will be guided by the old thread & at least partially restore it.
inserting it from the good end of the existing thread (if poss) should ensure success.
5 minute job.

MikeO · Mar 31, 2003

cookie said:
inserting it from the good end of the existing thread (if poss) should ensure success.

Cookie,

sometimes, you get to read something on this board which is a revelation. I can't understand why, but I'd never considered using a tap from the good end of a knackered thread. This is probably obvious to everyone in the world except me - what a tawt....

MikeO

cookie · Mar 31, 2003

method: turn the tap clockwise until it binds, then back out to clear swarf. repeat.

tip: taps are quite brittle. never break one off in the hole. especially not in a blind hole. ever!

they cannot be drilled out by any normal means but an engineering shop should be able to remove it.

Gecko · Mar 31, 2003

Nice one cookie - I'll get looking around on the web for one the right size. I am restoring a set of front forks on an old 1965 BSA B40 so I need quite a large diameter , about and 1.5inches I suspect. Do they make em big ? No chance for me to start on the un damaged end though cause thats inside the fork leg . I think its just the first couple of threads that are a bit damaged but it may just solve my little problem. Cheers
Gecko

THE PICT · Apr 1, 2003

It also helps if you use some lubricant ie cutting fluid or WD40 etc.

Pthagonal · Apr 1, 2003

hmmm

THE PICT said:
It also helps if you use some lubricant ie cutting fluid or WD40 etc.

yes its allways good to use lubricant..

and a big plyers..

errrrrrrrrrrrrrrr

cookie · Apr 1, 2003

oh yes, use lube. oil for steel, paraffin for ally.

i doubt you'll get a tap for your fork legs though. 1.5"Ø?
they do exist but last time i went to buy one that sort of size was asked for £70 odd

get an engineering firm to sort it (if economic), a specialist in BSA's or buy a new leg.

Gecko · Apr 1, 2003

Yikes 70 quid - that's more than the bike's worth . I'll check it out. Only found metric sizes last night when I was searching. Otheriwse as suggested maybe a local engineering firm can help me out. problem will be that over here they are unlikely to have anything in Imperial - might have to strap in on the back of the GS and bring over to the hog roast and have a word with Uncle Vern

cookie · Apr 1, 2003

....and it will only work IF the original thread is not too damaged. you can't cut a thread if there's no metal to cut ie. if thread is destroyed or hole is too big. then you have to make an insert.

suspect you can get new B40 leg cheaper & easier. gotta be good thread for fork caps, if it gives way while riding it will be messy

rb_stretch · Apr 1, 2003

Interesting reading.

Unfortunately with my particular situation, the GS sub frame stops you accessing one side of the nut and you've guessed it -that is the good side.

I'll be going for the drilling out the hole and going for a bigger screw then.

THE PICT · Apr 1, 2003

How about filling the hole with weld then re-drill to the correct size and then cut a new thread.

rb_stretch · Apr 3, 2003

Final outcome of sub-frame problem

I finally got round to re-tapping the sub-frame, which I had to do from the knackered end. Couldn't believe that not only did it correct the 2/3rds of the thread that had gone, but it is now much easier to thread properly.

I can only presume it is because the bolt is now a looser fit as some of the metal has been taken away through the re-tapping.

RB

Cross-threaded sub-frame

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