Replacement crankset for old 7 speed

[Gixxerman]

I am repairing an old bike for a friend. They managed to strip the pedal threads by carrying on with a loose pedal.
It is a square taper attachment.
Having looked around on t'internet, I have found a few that seem suitable.
e.g. https://www.tredz.co.uk/.Shimano-FC-M311-Altus-Square-Taper-Chainset_74973.htm
However, they all come with warnings about only fitting certain BB sizes.
For example, the above says that it fits 123mm square taper BB.
To what does this 123mm refer to?
There is nothing on the BB on the bike that is 123mm in size (see attached drawing).
The warnings seem to be about chain line.
So how do I ensure that I get a crankset that fits?

 

[Hugh Manatee]

It'll be axle length. Older ones tended to have longer axles.

Sheldon has a good database.

https://sheldonbrown.com/bbsize.html

 

[Rusty Nails]

The correct BB size/chainset is important for easy gear changing.

I am not sure the name of it but there is a website that gives a table of chainset/BB compatibility.

A bit of searching should find it.

 

[Ajax Bay]

For example, the above says that it fits 123mm square taper BB.
To what does this 123mm refer to?
There is nothing on the BB on the bike that is 123mm in size (see attached drawing).

You've already identified (aiui) that the BB is a square taper (and it'll be JIS). You need to take a view on the number of teeth you want the chainrings to have eg 24-34-42 or 28-38-48.
In your diagram the 119 figure is the axle length, compared to the 123mm spec for the chainset you have linked.
I'd take the approach, given this is an 'old bike', that you just fit the chainset you buy and accept that the chainline will not be perfect. Ideally the middle chainring and the middle sprocket should be aligned (the 'chainline'). Fitting the chainset onto the 119mm BB will shift the middle chainring in 2mm - less than half the distance between two adjacent sprockets. It will be fine. You could look around for other new chainsets which require a 119mm BB axle.
165/170/172.5/175 numbers refer to the crank length btw.
I'd be tempted to find a second hand MTB chainset rather than a new one (or at least an unbranded one), but this would depend on your attitiude and that of your friend.
For example this one: https://www.ebay.co.uk/c/2256019552?iid=173769184327 or
https://www.ebay.co.uk/itm/MTB-TRIP...-TEETH-170mm-SHIMANO-COMPATIBLE/392269668299?

 

[Chris S]

Don't use an axle length shorter than the original. The chainring may rub against the chainstay, assuming you can get it on in the first place.

 

[Ajax Bay]

Don't use an axle length shorter than the original.

Agreed. Use original BB (axle length) as I suggested.
With the current chainset on, measure the gap between the inner ring and the chainstay to give you an idea of how much tolerance (?5mm) there's likely to be.

 

[Gixxerman]

Thanks for all the help peeps. I have good news. My nephew had a scrap bike (one he bent the rear wheel on so badly, it was beyond repair and he decided not to repair the bike). This had a reasonable triple crankset on it. It is not great but it is well better than the one on the bike I am repairing. I fitted it and it works fine and all front ring gear changes work. As the large ring was a bit bigger than the original one, the front deraileur needed moving up the frame a bit (luckily with it being a pinch band mount it was possible). The chain had enough length to accommodate the larger chain ring also.

As the bike was a bit of a wreck, finding free parts was great as it really wasn't worth spending too much on. She wanted to start cycling a bit with her hubby so she got the bike off her mum, who I expect was used it a few times and threw in a shed and forgot about it, hence the state of it.

Some other horrors I did find. The pinch bolt for the handle bar mount to the stem was missing, and the bars were just held on by the threaded insert collar, which thankfully was a tight fit. If that had vibrated out, the bars would have come off the stem. The rear tyre sidewall was worn to the point that it was threadbare. The brake cables were quite stiff to operate due to lack of lube.

Again luckily, a friend who lives opposite my nephew, had 2 tyres free to a good home and also a pair of plastic mountain bike mudguards. So tyres were changed, mudguards fitted with some suitable bolts and tie wraps and it was given a thorough clean and lube. I rid it in its original state to my nephews house (about a mile) to get the bits and it was bloody awful. Apparently, she did a 12 mile ride on it last week; How she did that in the state it was in is beyond me (she deserves a medal!). After my work, it rides like a different bike. She is well chuffed with it. I quite enjoyed doing it to be honest. It was quite theraputic it the current climate. There is something satisfying about taking a wreck and making it something good again. Total cost £0.00, plus a bit of oil, grease, a few nuts, bolts and washers I had lying about, some tie wraps and 6 hours effort.

 

[Ajax Bay]

There is something satisfying about taking a wreck and making it something good again. Total cost £0.00, plus a bit of oil, grease, a few nuts, bolts and washers I had lying about, some tie wraps and 6 hours effort.

Bravo! Think you owe us all a picture of the fettled bike! So we can share you rightful pride.