I'm building up a new frame. It's never had a BB installed. The BB threads appear to have been tapped before media blasting and powder coating, and the shell faced after. There is debris along the threading making it difficult to thread the BB in.

Without going to an LBS or buying a proper tap set, is there a DIY solution to just chasing the threads?

If you have an old BB shell and a bench grinder you can probably grind some grooves into to to make a sort of tap to clean the threads out ??

Have a look at a tap to see how the flutes are ground, and try to duplicate that to some degree, you could probably use a Mill bastard Nicholson file too if you do not have a hand grinder with a cutoff wheel or a bench grinder.

Bill

Unless the threads are really damaged, not just gritty and dirty, clean them out with an old toothbrush and some solvent to remove the grit. Run a pointed wood dowel or similar around the threads too.

A shop isn't going to charge you much to chase out the threads. I had this done recently on a bike where the BB had been cross threaded and it fixed the problem.

You can have a little success just using a few drops of oil and screwing in a bottom bracket cup/shell over and over but don't expect a miracle if it needs to be deburred... I've used files and dental picks to clean up threads. A proper tap is always best.

Nice idea. I think you were referring to BB cups, not the shell. I have a ton of old BB cups lying around. I can file flutes around the threads like those on a tap (though the cups are all hardened steel-- could be tough). I'll give that a try.

Yeah, even with oil it was gritty and rough.

Yes I must admit ignorance may have caused me to use the wrong term :-).

I have done this with a bolt in the last when I did not have a proper sized tap to clean out a thread, it is not hardened so will not work nearly well as a tap, but the "flutes" let it scrape "stuff" out of the threads somewhat anyway :-).

Oops I see the cups ARE hardened, so maybe it is dremel or bench grinder time :-).

Bill

New frame? Certainly not worth paying a pro.

Do the threads appear damaged? I would hope whoever was doing the blasting would be smart enough to at least stuff a rag in the openings before blasting. If it's just general crud I'd try to clean them with a brush first... I always keep a few of these from Harbor Freight in my toolbox...

Yes, much better than a toothbrush, readily available at hardware stores, sometimes even dollar stores.

Not what I meant, screw it in... screw it out... screw it in... screw it out... clean with tooth brush... add more oil... screw in/screw out.

Sometimes repetition will clean it up, assuming the threads themselves are only marginally damaged at most.

Yes, that can work:

Last time I had a BB chased at the LBS it was about $25. well worth the money.

If the threads are just dirty, why not just clean them? Chasing them risks damage, whether with a DIY kludge or a professional tool.

The crud appears to be media from the media blasting prior to powder coating. Somehow it is imbedded between the threads and baked on-- it doesn't come out with brushing using a metal brush.

The threads themselves are fine, and I was able to thread the BB in all the way. But the resistance to threading was high and whatever crud was in the valleys between the threads caused excessive compression on the cartridge-- enough to feel binding on the bearings when turning the spindle. The BB is an SKF.

If you can find one in the correct pitch for a BB shell a thread restoring file can do light cleanup and chasing and would cost less than a tap, but possibly more than an LBS' labor charge.

For something non bike related, a machinist once told me to use a regular sized tap, with the same thread pitch, as a scraper to clean up some threads that got dinged up on a fine pitch threaded ring. Should be cheaper than a full diameter tap.

$100 frame- DIY
$500 frame-maybe a cheapout isn't a great idea.
I am cheap-very cheap
On a cheap bike/frame- something worth $200 all total(entire bike-a frame that would sell for $200-maybe even 853 reynolds MTB I would not cheap out)
I might use a the back of the point of a knife blade to CAREFULLY de-burr/straighten out a few burrs - or run a modified cup thru after removing/rebending worst burrs with knife
So it all depends on what the bike frame is worth(actually sell for not what it originally sold for)
and if it is mine, or something I will sell

Sometimes you have to restrain your natural CHEAPSKATE tendencies
Lots of bike riders-are CHEAPSKATES
Yeah I am

I rarely have a use for mine but, when they're really needed, can be an invaluable tool. Intended for external threads but could work here too. Maybe you can find a metric one locally, Southpaw.

GearWrench 2228D Metric External Thread Restorer File

UPDATE: I filed a set of cups just like in John's picture below. Worked wonderfully! I oiled the threads before threading in, and all the gunk accumulated in the flutes. I repeated the process and after the second time, no more gunk!

I think what happened is that the media hadn't been adequately blown out from the voids in the frame before powder coating. Somehow, even after facing, bits of media had remained embedded in the threads. Or, when the facing got done by the frame builder, the threads of the tool didn't go past the first 10 or so threads. the SKF BB unit I installed has very deep cups that thread very far into the shell, 15-20 threads, farther than typical Shimano cartridges. After the chasing, it threaded in by hand very smoothly!

Problem solved. Thanks to all who contributed.

For what it's worth, this frame is a new frame from a very reputable and long-time local frame builder. He knows what he's doing. It's not my frame, but a friend's frame that I am building up.