[How do I?] Going to cut some threads for first time and have a question or two

[Lone Watie]

I need to fabricate a union for my brake line where it passes through a mounting point on my motorcycle. The major diameter of the aluminum piece I'm replicating is 3/4" and the pitch is 16 TPI. To what diameter should I turn down my work piece before I start cutting the threads? This will be my first attempt at cutting threads. Should I use the diameter listed on the drill/tap guide, which would be .6875 for aluminum? Thanks.

 

[Lone Watie]

I think I found answer in Machinery's HB Pocket Companion - For a Class 2A 3/4"x16 external, the major diameter max would be .7485 and the min would be .7391. Does this mean I turn my piece down to .7485 - .7391?

 

[Lone Watie]

Asking how to thread something and mentioning brake lines in the same post... You said "union". Is this a hydraulic line? In aluminum. Can't we thread something else first? This doesn't sound like a good idea, but it's your neck. I would turn it .010 under and then cut threads until things look right, then try the nut until it fits. Once it goes on about a half-turn, you're just a couple thou away from finished. Sounds like you're making a bulkhead fitting. They can't be that expensive to buy, eh?

Thanks for the reply and guidance. The original piece I'm replicating is also aluminum. But, with that said, the part of the piece I need to cut a thread for is for the mounting nut for the union onto the frame. It will not be touching any brake fluid. The internal threading for the 3/8"x24 I.F. fitting I can do with a normal tap and should be no problem. Yes, it sort of is a bulkhead fitting, only the 'bulkhead' in this case is where my original rear master cylinder was mounted on my motorcycle. I've since installed floorboards on the bike and relocated the master cylinder to the front. The stock master cylinder was mounted near the front part of the rear tire on the frame. In order to accommodate my brake light switch, I had to use a piece from the original master cylinder to act as a union, only the piece is not very long and the threaded ends allow the fittings to contact each other, which will cause a leak potentially. In order to avoid that, I need a longer piece. Yes, I could probably buy a fitting that will work just fine, but if I can make it, and it works, then that will be good.

 

[ScrapMetal]

You said that this will be cutting threads for the first time. I would highly recommend that you grab a bunch of scrap rod, tube or whatever you have handy and practice threading on it first. Trying to thread on a piece that has already been sized/machined is just a waste of the time you spent getting that piece to that point. You will most likely save time in the long run by spending a lot of time practicing before trying to thread on a completed part.

JMHO - Hope it helps,

-Ron

 

[Lone Watie]

You said that this will be cutting threads for the first time. I would highly recommend that you grab a bunch of scrap rod, tube or whatever you have handy and practice threading on it first. Trying to thread on a piece that has already been sized/machined is just a waste of the time you spent getting that piece to that point. You will most likely save time in the long run by spending a lot of time practicing before trying to thread on a completed part.

JMHO - Hope it helps,

-Ron

I'm definitely going to practice. I was just curious for the final product what to use for a starting diameter for the work piece. I should have mentioned that in my first post.

 

[Lone Watie]

It's hard to visualize, but you said "mounting". It would be awful to help someone and then read about his brakes failing in the news later on...

Btw, that .010 is off the diameter (.005 each side) and is a WAG intended to keep the aluminum from galling on assembly. It might be excessive. Can you measure the old one?

The handbook is probably indicating an OD and minor root diameter. That root is influenced by a sharp Vee, or flat. Not having fancy tools I generally use a sharp Vee and cut to fit. The sharp Vee reduces the cross section and weakens it a little, but putting a proper flat on the end is more work and good for just that pitch. The book is describing a proper form, but it may not be practical in our shops. Use your best judgment.

I appreciate the concern. As I mentioned before, the part I will be threading on the lathe will not have hydraulic fluid going through it, but the internal opening which I'll do with a tap will have fluid going through it. Of course, before I road test anything, I'll pressure and leak check it thoroughly. If something doesn't seat well, I'll know, and then I'll come up with another plan, as in a pre-fab bulkhead fitting you mentioned earlier.

Thanks for the guidance on the handbook, in other words, in a perfect world, those are the numbers I'd use, but my home shop (if you could see it) is not a perfect world. I've got about 20 things going on in the garage right now. I just finished welding an extension onto a new fender for the bike, which was a big hump. The brake line is not on the front burner yet, but getting there soon (hopefully).

 

[Tony Wells]

You didn't say how long this will be, but if it's not over 1 1/2 - 2" or so in OAL, I'd just go buy a 3/4-16 bolt and cut off the head and drill and tap the ID for your 3/8-24. I presume the brake lines have a 3/8-24 male thread? How do they seal? Is the factory fitting set up with a angled face in the bottom of the threaded bore to seat against?

This stuff is kind of critcial on brakes.

 

[Lone Watie]

You didn't say how long this will be, but if it's not over 1 1/2 - 2" or so in OAL, I'd just go buy a 3/4-16 bolt and cut off the head and drill and tap the ID for your 3/8-24. I presume the brake lines have a 3/8-24 male thread? How do they seal? Is the factory fitting set up with a angled face in the bottom of the threaded bore to seat against?

This stuff is kind of critcial on brakes.

The factory fitting (which used to be the main bore of the stock master cylinder which has been shortened) is fitted for a 10mm banjo bolt on the caliper side, which uses the banjo bolt washers for sealing, and is threaded with the 3/8"x24, and the other side was not threaded at all as it used to be inside the master cylinder. I've since threaded it the same, but with your info above, I may have to go with 1/8"x27 NPT with an appropriate fitting to accommodate sealing up the line. Good info, I'm learning more and more about brake lines every day. A simple 3/8x24 won't allow the line to seal up properly. Someone else on a harley forum I belong to had to use the 1/8x27 set up, and now I understand why. Either way, I can do what you suggested as far as buying the bolt and drilling/tapping, or make my piece and instead of threading for 3/8"x24 on the inside on one end, simply tap with 1/8x27 pipe tap, and tap for the banjo fitting on the other end. Thanks for the ideas/advice.