External threading question

[o0norton0o]

I have a carburetor part that I need to modify. It's called a "jet holder". It's threaded inside and outside. I can buy the stock jet holder to modify it for my needs, and I only have to modify a small part of it rather than manufacture the entire modified part.

Internally, the jet holder is threaded to hold the needle jet on one end and hold the main jet on the other. Then the assembled jet holder has external threads to thread it into the bottom of the carburetor. Hopefully the pictures help visualization.

The question I have is about the modification I need to do. I need to cut and thread the lower, unthreaded part of the stock jet holder so a brass sleeve that I have to make (and don't think I need any advice to make it) can thread on to the bottom of the jet holder. The problem is that the unthreaded side is very short and only 4 or 5 threads worth of space that go right up to the hex part of the jet holder. So, essentially a die wouldn't reach all the way to the hex, and I think it needs as many threads as I can make to hold sufficiently...

So, how would I thread all the way down to the base of the hex when I modify the stock jet holder?

stock jet holder below that I need to machine



Here's a modified jet holder below that I am looking to duplicate



For the sake of context, here's an image of what the modification changes make to the carburetor assembly. I'll explain briefly below the assembly picture

So, The carb bowl is normally held in place with the ears on the bowl which bend over time causing the gasket to leak. By threading the unthreaded bottom half of the jet holder, I can make the brass piece in picture 2 to hold the float bowl to the bottom of the carburetor securely without warping the face of the float bowl... The system was developed some years ago by someone else who has since dropped off the internet. I was lucky to get the parts that I want to copy from someone who bought his system some years ago.

The problem I'm having is how would I cut threads on such a short stub all the way to the tiny remainder of the hex that I need to leave in place?? I know taps have "bottoming taps" to get to the bottom of holes. Is there such a thing as a "bottoming die"??? All the research that I have done suggest that in the old days dies only had "lead in" taper on one side so it had to start with that side facing the work. Also I read that even if you could find a die with only lead in on one side, that you couldn't reverse the die to use it backwards to fully cut a second pass all the way down to the hex shoulder. Modern dies have lead in taper on both sides (it seems) so either way you use them they can't cut full threads all the way down to a shoulder.

Is there a method or a special die that cuts all the way to the shoulder to thread such a small stub? Thank you all in advance for your opinions and advise.

 

[f350ca]

Any die I've owned only had the lead in on one side Have often flipped them over to thread close to a shoulder.

Greg

 

[Huub Buis]

My "modern" metric dies all have only one side with a lead in.

• I would cut the thread on a lathe using a full profile insert threading tool (can cut only 1 pitch). That threading insert will cut up to the shoulder.
• I could grind a HSS threading tool for this pitch that also will cut up to the shoulder and make the thread on the lathe.
• Using a threading mill for the required pitch, I could mill this thread on the CNC router. The CNC router will also cut up to the shoulder.

I

 

[piper184]

Flipping the die over to get close to a shoulder is one of my go to moves for this kind of thing.
Another approach might be to single point thread it on the lathe. I would chuck the part and manually use a wrench on the spindle or chuck to power the lathe. That way you have absolute control over the situation. It will be slow, but you don't have a lot of threads to cut.

 

[Makintrax73]

How long is that unthreaded part? It looks damn short. I would think a nub that small is going to be impossible to start a die on. I'd take the manual powered lathe tact that Piper is talking about.

If I had to start with a die I'd remake the part longer, then flip the die, then shorten it after it was threaded.

 

[o0norton0o]

Thank you all for your responses. I'm going to look for a die with a single side lead in, then hope to flip it to thread down to the shoulder.

To answer makintrax73's question, the length of threads I need to make is 1/2" and I can't make it out of a longer piece because the existing threaded parts of the carburetor are the very rarely used 19 tpi so I am modifying an existing part so as not to have to reproduce that 19 tpi part of the jet holder, and the inside threading.

My modification comes down to cutting the shoulder to narrow the existing hex, turning the remaining 1/2" below the hex down to size and threading it with 20 tpi. to match the brass piece that you see in picture #2 which I have to make and thread in a corresponding pitch to the jet holder, so they can be threaded together when assembled.

Huub, I'm not sure what a full insert threading tool is, but I'm going to google it to find out...

thanks again for all your help

 

[pdentrem]

Single point at least to rough out the threads and/or finish threads. Turning by hand will not be an issue on such fine threads.
Long term, it may be worth the time of acquiring or making the appropriate 19 tpi tap. This would free up a lot of time in making the part.
Pierre

 

[Nutfarmer]

Single point thread with the spindle in reverse from the chuck out. Joe Pie has a good UTube video out that explains it better than I can. It avoids threading a right hand thread towards the chuck.

 

[Chewy]

Just curious. Your picture and description says 19 tpi. Is it British Standard Pipe Parallel thread ? The dies are available for as little as $8. You could take a piece of brass or aluminum a tad longer then the piece is shown and just tap both sides. That way if you don't get it right the first time, you could just make another. Cutting that small amount of threads on the adapter shown is doable, I would single point Jo Pie style with carriage staying engaged and turning the chuck by hand. Slow but probably the best way for that short of distance.

 

[o0norton0o]

Single point thread with the spindle in reverse from the chuck out. Joe Pie has a good UTube video out that explains it better than I can. It avoids threading a right hand thread towards the chuck.

I've seen that video, made a chuck locking device for my lathe, and even talked to Joe Pie on the phone about cutting upside down and away from the headstock 6 months ago for a different project. (super nice guy btw) I didn't think of cutting the threads upside down and away by turning the chuck by hand which is the only way possible for a metric lathe like mine. The thread dial on a metric lathe is really hard to use correctly because metric isn't based off of TPI so disengaging the half nuts and rolling the carriage backward doesn't always work with the thread dial.... I'm sure some of the more expert machinists here will say that they can do it easily,..... but I've tried it and it's obliterated a few pieces from my last project, so I stuck to leaving the half nuts engaged and backing up the carriage to make multiple passes... Backing up to the shoulder under power is pretty dicey IMO...

As far as finding a 19 tpi parallel die, I could only find it at Tracy tools in the UK. They don't have any dies left in stock and they only had a single bottoming tap in 19 tpi. From what I have read, 19 tpi is more common as a tapered pipe thread, not parallel. If you can direct me to a site that sells a 19 tpi tap and die that would be awesome..

So, the reason that I just changed the thread size to 20 tpi (or I might go 18 tpi) is because I can buy the jet holder with 3 out of the 4 threads already finished. That 4th thread on the jet holder can be anything because I have to make the other piece that it threads into also, so as long as they match then it work... I also used a thread gauge on the existing modified jet holder that I have and it's 18 tpi, not 19 tpi, so the person who made this also abandoned the 19 tpi where he had to make both pieces that thread together

Thanks for all the help. Discussing all the possible solutions with people who know much more than I do is incredibly helpful..