After a bit of googleing Highpath Enginering can shorten cranks by drilling a hole next to the existing one. However that means that the min reduction is 20mm i.e. 175 to 155mm.
Anyone know if it is possible to 'cut' a 5/10mm slice out of a alu crank and weld it back together and be strong enough? (non hollowtech of course)
Elfin +1
What does that mean?
I think - and Elfin is more than capable of speaking for himself, of course, and I have no desire to misrepresent his thoughts - that in this instance the emoticon is meant to represent boggle eyed incredulity that you are even considering such madness. But I may be wrong, that just may be my take on it... 
And plenty of companies offer shorter cranks in multiple levels of bling.
Cost to do that properly would probably be more than the cost to just buy some shorter cranks.....
I think - and Elfin is more than capable of speaking for himself, of course, and I have no desire to misrepresent his thoughts - that in this instance the emoticon is meant to represent boggle eyed incredulity that you are even considering such madness. But I may be wrong, that just may be my take on it...
No it's true; I was rendered incapable of speech there for a moment. 
So, just why would you want to do it instead of just buying shorter cranks then.
I see.
I took some cranks off a brand new bike and screwed the thread in doing so. In order to really use them again(ie be able to remove them again), I need to have it heli coiled or similar. I thought I might be able to get them shortened at the same time as 175 is way to long for me.
So regardless of cost - can you weld alu safely?
Repair/Reuse/Recycle etc...
I would thought that most cranks are forged and welding them would cause them to be weaker at the weld.
I'd personally say no. A better bet may be to try and get some inserts like those on Reverse cranks (below) that are flipable to adjust crank length. I think Look may do something similar.
Yes you can weld aluminium safely, but it needs to be properly heat treated after welding to prevent stress risers in the metal. I really wouldn't want to rely on welded cranks, having seen a fair few unwelded ones fail catastrophically, once causing major damage to the riders leg.
Helicoiled would be the best of a bad lot I'd say. Unless extra material can be added where the thread has stripped, and a new thread cut.
Is it stripped all the way through? I've seen cranks which have had the first 5mm or so of the thread stripped, but then ground down so only that bit is missing. Dunno if I'd trust such things in 'combat', but might salvage the cranks for a pub bike or something.
Check out the cost of cranks first - there are some very very cheap ones.
can you weld alu safely?
Of course - however, the question you are asking is:
'So regardless of cost - can what I am asking be done safely?'
In which case the answer is, maybe, but probably not - and the cost to guarentee it was safe would be orders of magnitude greater than the base cost of new cranks.
No it's true; I was rendered incapable of speech there for a moment.
Platform shoes may be cheaper.
njee20 - they look like just the ticket. Can find it on google though - can you give me a clue?
Ok not regardless of cost but within reason. These are some new RaceFace jobbies I've stripped.
So a Deore chainset can be had for £60/70. But if i can reuse these by repairing them at the same price - it's one less thing in the land fill. I assume I could just salvage the rings and the left arm but still a pity to waste the right arm. And to get 165mm I'm moving up to XT...
Platform shoes wouldn't work - when one leg is extended the other leg is way way to compressed.
njee20 - they look like just the ticket. Can find it on google though - can you give me a clue?
No idea I'm afraid - they were from a BikeRumor article about 'Reverse Products' from the Roc d'Azur a few months back, and have been used on Stronglight (and their Bontrager branded) cranks in the past as well.
Thanks nj will keep looking.
1 - You couldn't weld just around the perimeter. To have even a fighting chance of the weld having the same strength of the parent you would have to weld across the full cross section. To do this you would need to chamfer the edge of the weld to at least 30 degrees - the amount of weld material to fill that gap up is then massive which will introduce residual stresses and distortion.
2 - You are relying on each pass of the weld being perfectly fused, which, with the best, will in the world it won't be.
3 - Cranks see massive alternating loads. Simplistically, welds are notorious initiation points for fatigue cracks due to the geometry of the welds, residual stresses and local metalurgy. It will fail at the weld, it's just a case of when 
I'll defer to the experts on the possibility, but I'd be extremely nervous of the welded crank shearing and causing some serious damage.
Also worth considering, I went from 175mm to 170mm cranks and the difference was quite noticeable. To go down to 155 you'd need legs like (Sir) Chris Hoy!
If its a race face jobbie not if its says 7075 "it cant be welded" using the usual methods
Funnly enough i've got legs like Chris Hoy but with the upper body of Fat Albert giving me the power to weight ratio of peppa pig.
Thanks bristolbiker for the tech reply I can understand.
Are the threads totally stripped, or just cross threaded?
Screwing a pedal through from the other side of the crank might just recut the thread enough to be used, some threadlock would also help.
Earl,
If a big concern for you is to make sure the duff cranks aren't taking up space in a landfill, then take them to your local scrap yard (could get a couple bob aswell) or council yard (they usually have a metal recycling bin there), that way you're doing your bit?
Out of interest, why are 175mm cranks way too long? Are you getting pedal strikes?
I am a bit of a short-arse and use 175mm cranks on one bike & 170mm on another bike. I can't tell the difference. I only got the 170mm ones to stop pedal strikes on my old Stumpy FSR (and they were £10 cheaper than 175mm). To my surprise the 5mm shorter cranks did actually eliminate almost all pedal strikes.
But, seeing as you are not talking about shortening the crank arm, just the position of the hole, this wouldn't be the issue?
I agree with everything Bristolbiker said - yes, it can be done, but it is not worth it.
Trying to keep the parent metal cool enough to avoid distortion would be a nightmare, as to give it any strength at all you'd really want continuous welds where possible.
To get it done to any kind of decent standard you'd be looking at several times the value of the cranks, and i'd want some sort of NDT carried out on the weld before I used the cranks, as you'd be amazed what an X-ray, or even die-pen will show up.
If you are set on trying to shorten them i'd say moving the hole by milling it out into a slot then fitting an insert would be the way to go, however any decent machine shop will want a fair bit of cash just for the set-up time.
Just buy some shorter cranks!
someone who can't thread a pedal shouldn't be welding cranks!!
The crank removal thread (i.e. in the middle of the chainring) is striped not the pedal thread.
If 'cross threaded' means they are not stripped all the way then i guess they are just cross threaded(result!). Will take advice from above and see if machine shop can 'rethread' instead of helicoil.
I'm 5'7 whe shoes on and have tried all sorts of crank lenghts. I definatelt prefer shorter lenghts got some old xt 165 on a geared bike and 170 on the ss. I find riding twisty singletrack much more controlled on 165/170 than 175. Can turn better and feels more engaged like having extra paws on the freehub - does that make sence?
The crank removal thread (i.e. in the middle of the chainring) is striped not the pedal thread.
wouldn't worry about it. you can always loosen the bolt a little then ride them gently to remove the crank (or whack it from behind with a mallet)
what cranks are they?
If they're solid cranks you could try bending them. It would look as if you were riding really fast as well.
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