jock
stranger
Reged: 15/05/2008
Posts: 5
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I competed in a twenty four hour enduro over the winter in which i came through all right, but my bottom bracket did not. It appears to have slackened, or been slack, I know that's pretty poor maintenance but now the threads are goosed. A new aluminium frame, of this type - it's a santa cruz and jungle aren't helpful - costs approx £1000 (give or take) so i'd actually prefer to get it fixed than throw it out. Nobody is interested, at all. Can you, or should i ask, who can repair a bottom bracket with no threading left. Does it need to be removed and replaced, or filled and rethreaded. You can fix anything that goes wrong on a car, why isn't this so for a bike?
Thanks for your advice,
Jock
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Sir_Queuesalot
Pooh-Bah
Reged: 05/12/2007
Posts: 2094
Loc: Congleton 2x as posh as Staffs
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Hi Jock,
Interesting problem you've got there.
Two things I can think of. One is something called a helicoil however, the pitch or thread for a BB is incredibly fine, must be a metric pitch of 0.5mm or less. The helicoil process involves reaming out the apeture, rethreading it to accept the coil which then creates a new thread within itself, make sense?
Weblink has some detail http://www.emhart.com/products/helicoil/heliex/design.asp
This process is commonly used if you strip out a sparkplug thread or similar but I don't know how large they go.
Only other alternative is to speak to a good metal fabricator or engineering firm, see if they can drill out the old BB shell and insert a steel sleeve, stainless of course, using a friction fit or similar.
Anyone else?
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beaker
stranger
Reged: 17/05/2008
Posts: 9
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Tricky. I agree with Sir_Queues_a_lot, I think finding the right helicoil is unlikely. The bore out-press in a tube-bore out to size-tap thread option is possible, but would involve taking making one of the most important bits of your bike thinner, and probably more trouble than it's worth as a one off job. Do you have any mates who could do a foreigner?
If it was my frame, I'd kick myself heavily for having scrapped it, then figuring I'd nothing to lose, try
Get a crankset with external bearing cups. Check that the cups will seat squarely against the BB shell. If not, then file/ream/polish/bodge the threads out inside the shell till the cups go home snugly. Then GLUE them in. Seriously. Gears are bonded to shafts all the time.
Depending on the gap, you could use Loctite 638 (maximum strength), which will deal with 0.25 mm, or if it's really bad, bodging metal, Loctite 660, which fills up to 0.5 mm.
Make sure you have good metal-metal surfaces, clean the bits with wire wool till they shine. I'd probably fit the crankset before the glue hardens, to make sure everything is all aligned. You could use the screw end cap on the non-drive crank to pull everything tight, to make sure the cups are really seated. Try not to get the glue on the bearings themselves though!
I came across these fixes when replacing suspension bearings. You can get the full range of bodging goo (sorry high performance engineering adhesives) from www.bearingboys.co.uk . Don't use the threadlock from Halfords -that's designed to come apart easily.
I can't guarantee it will work, and it will be almost impossible to reverse. I think seating the cups tight and square will be the key to success, and it might save the frame from the scrap bin.
Good luck!
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greeneye
veteran
Reged: 19/04/2008
Posts: 1272
Loc: sunderland
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i have a plan!! i am a metal worker and agree that the 2 lads are right with there opinions. i dont think the helicoil will work cos the threads are so so fine, but i dont know for sure.
im thinking of using an old frame thats made of the same material and cutting the bottom bracket housing out of it. you will be left with a tube.
take the frame and the tube to a metal work shop who has a coded welder. they will very easily be able to cut your damaged housing out and replace it with the good one.
it may be a bike frame but its still just metal.
this will mean a strip down and a new paint job but imagine the nice new looking frame you will have!!!!
this is my suggestion but maybe the hardest way to go but you will end up with a normal frame again.
p.s. you could try a second hand frame off ebay.
good luck.
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jock
stranger
Reged: 15/05/2008
Posts: 5
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Thanks for the advice gents,
the summer 2008 mag has this question in it with a recommendation, i also got this from another forum, so seems as though helicoiling and inserts are both possible.
http://www.betd.co.uk/items.asp?categoryID=236&Name=Frame+repairs
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