Recently bought an old road bike converting to fixed. It used cottered and measured 70 mm. Any idea how to help me convert it to square tapered? Thanks

Make, model, year etc would be useful information. You need to chuck the bottom bracket and get a new one. Removing them can be a pain, the steps involved depend on the kind of BB shell, and the replacement also depends on the BB shell.

Without knowing the make of the frame it is tough to predict what the specs are for the bottom bracket shell. A 70mm shell typically indicates Italian threading (36mm x 24tpi) and both sides are right-hand threaded.

Double check your measurements as there used to be an obscure 71mm spec for old Raleigh frames, and the more common British threading standard found on many bikes is 68mm.

Once you figure out what type of bottom bracket threading you have just buy a square taper bottom bracket to match. When buying a square taper crankset there are a couple things that need to be considered:

1. The square tapers of the bottom bracket axle come in two standards ISO and JIS. There are crank arms that come in each standard as well. While one standard can be used on the other, a better fit less prone to problems would be had if both parts matched the same standard. For the stresses of riding a fixed gear it is best to not mix and match between the standards.

2. Different crank arms require different bottom bracket axle lengths, so ensure you buy the correct length axle. If you do not have a British threaded bottom bracket shell on your frame then you may find your choices limited in this area...this is an area where e-bay and craigslist may come to the rescue with used or new but old stock parts that will help you match everything up well.

-j

It is a kalkhoff frame and is English threaded. The width of the bottom bracket however is not a standard 68 or 73.

Was able to remove it only problem is figuring out how to replace it with square tapered. Tried removing the cottered spindle and putting in a square tapered, worked, but didn't run smooth. Hoping to use a sealed cartridge that would work.

At this point my first and best advice is to take the frame to a shop that has a history (or wrenches with years of experience). They can confirm the threading and shell width as well as modify the shell width to one that's common today. (As in facing down an odd width to 68mm). Bring in the original BB parts for additional reference.

Don't be surprised if the shop questions your goals. Don't be surprised if the costs begin to get vastly more then you want or have budgeted. You're doing something that the bike business never intended to do and therefore will be "manufacturing" your own set up.

Back in the mid/late 1970s there were many more options, still not intended to do what you want, but more choices of parts to cob together. These days with Asian bikes ruling the lower cost arena the parts available are only made to fit the current dimensional standards of today.

One last thought. Phil Wood makes BB threaded rings without shoulders or lock rings on them. It's possible that they might thread far enough into the shell to secure a PW BB tapered square unit or the right axle taper and length that the cranks need. Sit down before looking at those costs... Andy.

Or, just use a VO threadless. https://store.velo-orange.com/index.p...-brackets.html I think there's a good chance you have a Swiss shell.

At what price point would you accept the idea of finding a different frame?

Good idea with the threadless bb

Wouldn't like to spend more then 50 fixing this problem.

It could also just be an out of spec English shell. The only thing that will happen is that the BB will be off center by 1mm and the "adjustable cup" will be 2mm deeper into the shell.

That is if you are sure it is English threaded.

Should I get a 68 or 73 bottom bracket then?

Are you familiar with kalkhoff bikes? Many came with swiss BB shells. According to Sheldon, a BSC cup may seem like it's fitting a swiss shell, but it'll be a bit loose.

OP, if I were in your shoes, I'd find some spare/freebie BSC cups and see if they fit properly before I bought anything.... Do you have a lot of spares available? Is there a co op by you? Bike nerd friends?

You say it's not 68 or 73, then what is it? Also where are you located? My LBS rethreaded my BB from nonstandard to English for $20. For my 78mm wide BB, I used a Shimano UN72, 73 width and 112 axle bought off eBay.

"Are you familiar with kalkhoff bikes? Many came with swiss BB shells. According to Sheldon, a BSC cup may seem like it's fitting a swiss shell, but it'll be a bit loose." surreal

This is why the first statement in my post was to go to a shop that can figure this out. Andy.