Hello again,
How do i check which type of thread my frame has, and how do i choose the correct bottom bracket. I have an old bianchi frame and i want to fit a sugino rd2 messenger crank, so im guessing i need square taper bb. Any info on how to choose and measuring the correct bb wouldnbe great.
Thanks again

On the Classic and Vintage subforum you will find an expert, Bianchigirl, on all things Bianchi. You might also ask on the mechanics sub forum.

How do i check which type of thread my frame has? I'm a "trial-and-error" guy, which often doesn't work out as well as I hope it will. Asking here is probably a better strategy.

How do i choose the correct bottom bracket? According to the Sugino website, their Sugino RD2 Messenger crankset requires a 103mm square-taper (JIS) bottom bracket.

The bottom bracket matches the crankset, not the bike's frame.

One caveat: Bianchi had frames built in both Italy and Japan. If the OP's frame is an Italian-made frame, it will likely need an Italian thread bottom bracket. If it's a Japanese built frame, it will use an English thread bottom bracket.

If the bike has a bb, the thread size might be stamped on the cups.

https://www.jabetc.com/products/prec...ant=8747117509

A thread pitch gauge can be a handy tool, but not in this case. A threaded bottom bracket on a Bianchi is going to have either English or Italian threading, and both standards use 24 tpi threads.

There are a few ways to tell English and Italian bottom brackets apart, though:

• English bottom bracket shells are 68 mm across. Italian bottom bracket shells are 70 mm across.
• The drive-side cup on an Italian bottom bracket is right-hand threaded, meaning you'd turn it clockwise to tighten. The drive-side cup on an English bottom bracket is left-hand threaded.
• The diameter of an Italian bottom bracket is slightly larger than an English bottom bracket. English bottom brackets are 1.37 inches across (about 34.8 mm) while Italian bottom brackets are 36 mm.

Anyway, as mentioned above, if the bike already has a bottom bracket installed, it probably has markings that can be used to identify it -- either by model or by size/threading. Or, if the frame has some identifiers like a model name or tubing type, that might be enough to identify what bottom bracket it'll use. (If the tubing was made by a Japanese company like Tange or Ishiwata, the bottom bracket will almost certainly be English threaded.)

Or you could use the tool designed specifically for finding the pitch of threaded parts.

If the old BB is in the frame, read the numbers and tell us what they are.

How do find the pitch if the bottom bracket is installed? Once you take it out, you will now if it is English or Italian by the way it threads out. Your tool really doesn't help. A rule would be simpler way as SkyDog75 says, just measure the bottom bracket shell.

John - English and American threads are slightly different though they have the same number of threads per inch and the same diameter. Japan makes American threaded bottom brackets though over the counter most American threaded BB's are referred to as English. The difference is the shape and depth of the threads.

If he has that he simply has to read it. Italian BB's are in mm and American threads are in inches.

And your point is, what, exactly?

Measuring the thread pitch doesn't help in this particular case. Being a Bianchi, this bike's bottom bracket is going to be either Italian or English, both of which use the same 24 tpi pitch. Since the thread pitch can't be used to differentiate an Italian bottom bracket from an English one, we'd need to identify the bottom bracket type via other means like the cup diameter, shell width, or fixed cup threading direction.

My point is that if you're going to talk about something at least know what you're talking about. "English" threaded bottom brackets are in fact SAE threads. (American Society of Automotive Engineers)

*plonk*

Oh, the irony. ;-)

The spec as a whole -- dimensions, thread pitch, and thread direction -- were the common specs adopted by most British builders, so the British designation is appropriate in that sense.

But to delve into specifics a bit, the thread angle & profile for a British bottom bracket happens to be defined by the British Standard Cycle screw thread standard, which was originally codified by the (British) Cycle Engineers' Institute.

Well, maybe not "originally"... According to Sheldon Brown, the threading specs used by BSA (Birmingham Small Arms) were largely adopted and codified to become the British Standard Cycle spec. BSA was a British company that built motorcycles and bicycles in addition to their namesake business.

If you take a "real" English threaded bottom bracket assembly and try to thread it into an American bottom bracket it will fit but the threads will be deformed and the tops of the threads cut off - it will actually have the threads reformed to American threads. This actually has no effects save that because you're reforming the threads it will screw in with much more force than you would expect.

This is why I'm trying to make people understand this - People who are not mechanics at heart and try to combine the two may not understand that they do not screw together as like threads would. And because the tops of the threads are being sheered off the English threads you may have to back off and clean the threads of detritus. Luckily "English threads" means American threads these days and you don't have these sorts of troubles save on older steel bikes with unsealed bottom brackets. But they do turn up now and again.

Sources I've read seem to indicate that (1) British bottom brackets use BSC threading, and (2) even if that wasn't the case, SAE and BSC thread profiles are compatible. The attached diagram shows CEI (which later became BSC) and American threadforms, and it seems plain to me that a BSC part should thread right into American threads.

If you can cite something that differs, I'd be interested to read it.

Pardon me - I should have mentioned that there are two British threads - the triangular tops and the rounded tops neither of these is strictly compatible with the American flat topped threads. And as I noted they can cause questions among people who aren't expecting the force necessary to put them together the first time.

https://sizes.com/tools/thread_american.htm

But if you've put these together before you'd know that they do not feel compatible the first time together.

Take for instance an XT pedal inserted into a new crank. They thread in the entire way easily by hand. And you need a wrench only to tighten them. This is not the case with mixed English/American threads. Look, I've twisted wrenches on bicycles since the 80's and watched the changes. I still think of the Peugeot PX-10 as one of the best handling bikes I've ridden. But you couldn't use one now because of the French threaded bottom brackets. So if you're a steel man you go with a Basso Loto.

At least we can agree on #$%& French threading! ;-)

...but it didn't stop me from fixing up the UO-8 in my garage. And if I ever come across a bargain on a PX-10 in my size, it's gonna be mine.