UN73 BB 73X113 is it OK to use a spare 1mm mega exo spacer?

gtd.

I have a old Carrera Hellcat

It has a 73mm shell and has a Shimano UN73 bottom bracket 73mmx113

I'm bulding it up as a short travel XC bike but the only decent crankset I have is a Cyclone Precision CFM 52-42-30 square taper crankset

Chainline( +/-1.5 ) 45.0mm
BB Axle Square 118mm

The inner chainring bolts catch on the pivot bearing caps.
I have a spare 1mm spacer from one of the FSA MegaEXO bottom brackets on one of my other bikes. If I fit a spacer between the shell and bb the chainring bolts clear the bearing cover fine.

Would this be ok to use or do I need to get a 118mm axled bottom bracket?

Just planning to use it over winter as a commuter.

supersonic

Should be ok, but makes ure the opposite crank does not touch the frame.

nicklouse

it depends on what the clearance is like on the other side.

but I would get the right BB.

gtd.

Thanks

Frame clearence is fine, been looking online no one seems to sell the un73 bottom brackets anymore.

will probabbly use till spring or sooner if I get a GXP or 24mm MegaExo setup.

nicklouse

why? a new SQ BB is about a fiver.

gtd.

finding a 73x118 or even 122 in stock and less than £20 all in is difficult

though I defiantly need a new bb as now I've tightened the crank bolt fully the chainring bolts are catching on the pivot bearing cap.

If I am to use this crankset I might have to use the bigger 122 axle as its a 45mm chainline instead of 47.5-50mm according to http://www.sheldonbrown.com/bbsize.html

The bikes original crankset had a chainline of 48.5mm
a 122 BB would take it to a 47mm chainline

thinking of abandoning this crankset unless I rebuild my hybrid or swap it with the Gamma Drive MegaExo off my hard tail

bikaholic

If the BCD of the triple crankset is 130/74mm, then I would recommend that you bail as there would be too many variables to juggle in order to get it functioning properly. That is, by the time you've spaced the middle chainring out for the 2-3mm chainstay clearance, the smallest and largest chainring would be too far outboard to be properly usable - the chain can't accomodate such a big angle bend.

However, if the BCD was a compact triple 110/74mm, with the middle chainring being 36T or less, then I would definitely try it - that would be my main condition for using a road crankset on a mtb.

Regarding chainline - there are two methods in which manufacturers measure it. Shimano measure their chainline from the frame center line to the inside surface of the middle cassette cog/chainring. Others, measure theirs from the frame center line to the center of the middle cassette cog/chainring.

Hence 47.5mm (Shimano) == 48.5mm (Others, eg Truvativ)

So, be consistent in whichever method you choose to use, ie use the same method in how you measure and calculate the chainline.

Also note that road frames usually have a dropout spacing of 130mm and mtb frames 135mm, hence the increased chainline from 45mm to 47.5mm (ie the cassette would be shifted outboard by 2.5mm and optimum chainline would be achieved when the middle of the cassette is parallel with the middle chainring position. Taking it out to 50mm accounts for frame/front derailleur limitations but is within range of the angle the chain is able to handle).

So, in short, use the Gamma Drive MegaExo mtb crankset instead!!!

gtd.

Its130/74 52T/42T/30T which was fine on my 03 Giant Terrago FS1 though the top tube was a bit long for me)

Just tried to fit a MegaExo 19mm axle bottom bracket.

Right (drive side) was fine but the Left (non drive side) cannot take an external cup bottom bracket as the pivot cap sticks out more on this side.

To avoid the pivot bearing cover I'd need to use 3 spacers on the left hand cup so the frame can only take an internal BB.

Back to the drawing board.

The bike originally had a UN73 (XT) 73mm/118mm Bottom bracket with a silver Truvativ FiveD crank set. Unfortunately the left crank is pedal thread is damaged beyond repair and silver FiveD cranks are discontinued.

Looks like I'm stuck with square taper or change to Octalink or ISIS