How does one stretch worn cap threads?

[MementoMori]

What I'm about to ask may be a question for Ron Zorn directly, but if anyone else has experience with this please let me know.

 

I purchased a large box of broken vacumatics recently and have been experimenting with learning new repair techniques. Of the pens I purchased, I've diagnosed several as having worn cap and/or barrel threads. I have seen several posts referencing an operation wherein the barrel threads are stretched in order to engage the threads of the cap better. Can anyone describe exactly how this is done, or am I just going to have to play around with it myself to figure it out? So far as I can tell a mandrel is used and the pen barrel is heated to within an inch of its life, but that's all I've got so far. Below are a few questions that I've been unable to satisfactorily answer:

 

- Are those of you doing this kind of work professionally using custom made mandrels? 

- At what point do you know the barrel is sufficiently stretched? I imagine the amount of stretching to be done is a relatively small measurement. What kind of measurements are you taking before, during, and after the operation?

- Is the stretching done in one shot, or incrementally? My gut tells me that repeated extreme heat applied to celluloid would drastically increase the risk of damage as compared to a single heating/stretching operation.

 

I am well aware that this is a tricky bit of repair work and acknowledge that, as many posts I encountered said, "it's best left to a professional," but I want to learn how to do this repair (and more, but I'll save that for now).

Edited January 29 by MementoMori

[VacNut]

It is tricky, and I understand a custom mandrel is used that threads into the barrel, so that you do not flatten the interior threads of the barrel that accepts the section. If you find barrels that have loose section, it may be because the interior threads have been flattened by the process. 
You may consider practicing on a junk barrel or send it out.

Does the vac have sentimental value? If not it, may be easier to source a different barrel. There are literally thousands of barrels available.

I have it done by a very experienced technician on difficult to find Maxima and Oversize Vacs colors, although there are more and more barrels becoming available.

[FarmBoy]

In order

 

-yes

-The cap works.  None

-yes

 

[MementoMori]

Thank you farmboy. Is a custom mandrel made for each size of vac? I'm assuming stainless steel or similar. Do you have one you could post pictures of?
 

6 hours ago, VacNut said:

It is tricky, and I understand a custom mandrel is used that threads into the barrel, so that you do not flatten the interior threads of the barrel that accepts the section. If you find barrels that have loose section, it may be because the interior threads have been flattened by the process. 
You may consider practicing on a junk barrel or send it out.

Does the vac have sentimental value? If not it, may be easier to source a different barrel. There are literally thousands of barrels available.

I have it done by a very experienced technician on difficult to find Maxima and Oversize Vacs colors, although there are more and more barrels becoming available.

Good point re: the section threads.
 

These pens are of absolutely no sentimental value. I bought parts of an estate collection and just want to practice doing certain repairs on the junk pens while keeping the good ones. 

[Nethermark]

Probably a dumb question, and undoubtedly, FarmBoy will tell me so: If you stretch the barrel threads, don't you than also increase the inside width of the barrel? Would one than not need a new section? Or do you only stretch part of the barrel threads, so the section will still fit?

 

I typically first see, whether the inner cap can be shortened a bit. I have also sometime decreases the circumference of the cap.

[MementoMori]

3 hours ago, Nethermark said:

I typically first see, whether the inner cap can be shortened a bit. I have also sometime decreases the circumference of the cap.

I imagine that involves using a lathe and a boring bar, yes? I have some junk caps I was going to practice swaging cap bands on, might have to add this to my list of things to try. I'd read that shortening the inner cap on a vacumatic isn't always the best approach because there isn't a lot of room for the nib to begin with, not to mention the inner caps don't come out. 

What's your method for decreasing the circumference of the cap itself? 

 

Thanks for the suggestions

Edited January 30 by MementoMori

[VacNut]

3 hours ago, Nethermark said:

Probably a dumb question, and undoubtedly, FarmBoy will tell me so: If you stretch the barrel threads, don't you than also increase the inside width of the barrel? Would one than not need a new section? Or do you only stretch part of the barrel threads, so the section will still fit?

 

I typically first see, whether the inner cap can be shortened a bit. I have also sometime decreases the circumference of the cap.

This was the tried and true method in the past, but no longer the preferred method. There are plenty of barrels with shortened threads. One should remember to look for them when buying a Vacumatic.

 

It is a fine balance between stretching the barrel sufficient to engage the cap threads without making the barrel diameter too large to not engage the section threads.

 

it is a task not for the faint of heart that requires precision. I have not tried it, but have been told it is not simple unless you have  the correct setup. I do not have the setup to make a threaded mandrel so I have not tried the process, albeit I have had pen barrels stretched. 
 

On more common Vacs, a replacement barrel is more economical, but it is also good to learn a new skill.

 

Swagging cap bands is another one of those skills that require a good set up to avoid ripples on the bands

[Nethermark]

4 hours ago, MementoMori said:

I imagine that involves using a lathe and a boring bar, yes?

Preferably, but I don't have one. However, I only do it on caps where the inner cap can be taken out and then I can do it by hand (not perfect I suppose)

 

4 hours ago, MementoMori said:

What's your method for decreasing the circumference of the cap itself? 

I heat the cap and roll it over a even surface or just between my fingers. You have to be carefull, because one risks creating a dent. On the other hand, I have plenty of caps which are shrunk below where the inner cap is.

[FarmBoy]

17 hours ago, Nethermark said:

Probably a dumb question, and undoubtedly, FarmBoy will tell me so: If you stretch the barrel threads, don't you than also increase the inside width of the barrel? Would one than not need a new section? Or do you only stretch part of the barrel threads, so the section will still fit?

 

I typically first see, whether the inner cap can be shortened a bit. I have also sometime decreases the circumference of the cap.

You hit on the crux of the project.  It is a balancing act.  Go to far and you get a slip fit section.

[FarmBoy]

I'm going to add that most of the people that do or did this task do/did not have it high on list when looking for things to do pen related.

[MementoMori]

13 hours ago, FarmBoy said:

I'm going to add that most of the people that do or did this task do/did not have it high on list when looking for things to do pen related.

Any suggestions as to areas I should focus on instead? Loose cap bands are sort of my white whale at the moment since I see them all the time. Just waiting on some PEEK stock to make bushings for some ER25 collets per fountainbel's drawings. I figured why not try fixing worn caps/barrels at the same time.

 

I primarily collect Sheaffer's and Parker and have gotten pretty good at refurbing filling systems, vac fillers included, so I am looking to expand my skills in other areas. 

Edited January 31 by MementoMori

[GlenV]

Good luck with those efforts I'd like to hear for one how it goes. I still just use a piece of scotch tape in the cap as is so effective for a user pen...lazy me, on the other hand I've been trying to learn my metal lathe uses better...

[MementoMori]

34 minutes ago, GlenV said:

Good luck with those efforts I'd like to hear for one how it goes. I still just use a piece of scotch tape in the cap as is so effective for a user pen...lazy me, on the other hand I've been trying to learn my metal lathe uses better...

Now if only I'd thought of that... in all honesty if I were simply fixing these just to use day to day I'd probably do that. But I seem to have a knack for over complicating things for myself, and as I said learning a new repair skill seems like a good thing to do, even if it is esoteric.

Edited January 31 by MementoMori

[FarmBoy]

If you have a lathe, bore out the old cap threads, sleeve the cap and cut new size appropriate threads. 

[GlenV]

I need more taps… making inside threads for the cap with the threading done with the lathe looks math intensive..

[FarmBoy]

Just like cutting external threads but backwards. 
 

doubt you will find taps even close to correct

[VacNut]

Bezos will sell you collets for a lathe, but I am guessing you could try them for swaging bands. You will likely also need a threaded mandrel for the cap, so that you do not crush the cap. The collets are likely the incorrect diameter but it could be a good experiment.

it would be difficult to compress the collets on all sides without a lathe

[MementoMori]

3 hours ago, VacNut said:

Bezos will sell you collets for a lathe, but I am guessing you could try them for swaging bands. You will likely also need a threaded mandrel for the cap, so that you do not crush the cap. The collets are likely the incorrect diameter but it could be a good experiment.

it would be difficult to compress the collets on all sides without a lathe

Yes, I have two lathes. A small watchmaker's lathe that I use for relatively imprecise operations and a bigger, albeit still relatively small, harbor freight benchtop lathe. Currently I don't use it for much besides making replacement vac filler rods.

[es9]

7 hours ago, MementoMori said:

Yes, I have two lathes. A small watchmaker's lathe that I use for relatively imprecise operations and a bigger, albeit still relatively small, harbor freight benchtop lathe. Currently I don't use it for much besides making replacement vac filler rods.

have you found it effective at turning steel? 

[GlenV]

On 1/31/2025 at 7:09 PM, FarmBoy said:

Just like cutting external threads but backwards. 
 

doubt you will find taps even close to correct

Well it’s tricky since most manufacturers don’t use standard sizes, but I have found that many are 36 tpi so probably can get close for some caps. The barrel external threads are mostly 4 start which don’t exist in a die I think unless special ordered… you could get a set made for a particular pen but would cost more than my bench top lathe…I’m a noob at this but just made an eyedropper in BHR super fun, slip cap though.