Bottom bracket sizes and chainsets

[RichM81]

I currently have a hybrid bike Kona Dew Plus 2010 and the chainset is seized on, also the bottom bracket is quite worn. When I get them off there is a very good chance that both will need to be replaced, and would like to have the parts available so I can replace straight away.

To replace my bottom bracket I have found out that a Shimano UN-55 is a replacement for the RPM-7420 that is currently fitted. The size I need is 68 x 110. As I am wanting to replace the chainset as well I was looking at Shimano FC-M311 but it needs to be fitted to a 123mm square taper bottom bracket.

Can you please advise me if I am correct in thinking if I fit a UN-55 68 x 122.5mm bottom bracket, it will fit as long as I fit it with the correct chainset? Also that it wouldn’t offset the chainset from the front derailleur out of limits.

Hopefully all of this makes sense and you can help me.
Many Thanks

 

[Ajax Bay]

Yes, the chainset and the UN-55 68-122.5 will be right for each other.
If you struggle to get the left side of the current BB off (it may be plastic and they can break - make sure the splined tool is kept firmly pressed into the BB) then go round to the other side which will be metal. But the right side part is normally tighter. Use the frame as a lever with the splined tool facing up in a secure vice.

 

[Nigelnightmare]

Wot e ^^^ said.

 

[RichM81]

Yes, the chainset and the UN-55 68-122.5 will be right for each other.
If you struggle to get the left side of the current BB off (it may be plastic and they can break - make sure the splined tool is kept firmly pressed into the BB) then go round to the other side which will be metal. But the right side part is normally tighter. Use the frame as a lever with the splined tool facing up in a secure vice.

Thank you so much, you email the shops that sell them and them never get back to you. I was worried that the chainset will then be too far out for the front derailleur to change gear properly, but I guess the square opening is just a fraction larger. Thanks again

 

[Ajax Bay]

I can't tell you that your current FD can reach across. You can look yourself. Measure how far from seat tube edge the cage will go now (for the current largest chainring). Unwind the top limit screw ("H") - count the number of turns - and release the cable clamp bolt. Now manually push the cage as far as it will go, and measure again. That will give you an idea of how much scope there is for your chainline to move out. The new chainset won't necessarily actually make much difference - the designated BB axle length is so that, once the chainset is fitted, the chainline stays within sensible parameters for two main reasons: FD reach (discussed above) and 'Q factor': to do with the width your body prefers your pedalling feet to be (MSK and efficiency factors).

 

[PpPete]

Don't forget the right-hand side of the BB has a left-hand thread. It unscrews clockwise.
If you don't have a vice to hold the tool (or the space around the vice to put the bike) .... I hold the tool into the recalcitrant BB with one hand, hold the bike steady with the other and recruit a small child on the end of an 8 foot scaffold pole (which fits over the end of my 1/2" square drive wrench).
The look of delight when small child realises that they have assisted in something parent could not do solo is beyond price, not to mention the practical learning.

 

[RichM81]

Don't forget the right-hand side of the BB has a left-hand thread. It unscrews clockwise.
If you don't have a vice to hold the tool (or the space around the vice to put the bike) .... I hold the tool into the recalcitrant BB with one hand, hold the bike steady with the other and recruit a small child on the end of an 8 foot scaffold pole (which fits over the end of my 1/2" square drive wrench).
The look of delight when small child realises that they have assisted in something parent could not do solo is beyond price, not to mention the practical learning.

Good to know, thanks