OK - sorry it took me a while but was helping #1 kid with some college homework.

Bottom line is that you can probably do this with off-the-shelf ANSI stuff and off-the-shelf regular hardware-store Unified Metric stuff. By the way, 5/8-28 is a defined, albeit uncommon, thread size.

Short versions:

1. USA: Use a 9/16" drill in the end you want to thread. It would be much better to bore in a lathe or ream. Run a standard UNF tap in: 5/8 - 18. With decent quality drill and tap and care, you should get about 87% thread, which is pretty tight. Wall thickness at bottom of internal thread will be about 0.040", which is pretty close to what the factory leaves at the tail cap end. Cut and file a good quality 5/8 - 18 bolt, file a lead (taper the first thread smaller), and turn it in. Don't plan on using this end to open the capsule because a) it's potentially a pretty snug thread and b) galvanic corrosion between the steel and aluminum will weld it in (you can use a cadmium plated bolt but I would NOT - cadmium is very toxic. Try automotive bolts if you want a good galvanized 5/8-18 bolt and forget about using junky "home improvement store" bolts. You can TRY to get a nylon bolt from your local hardware stores - that would be much better than a steel bolt for this purpose. [You may use a 37/64" drill for about 75% thread and that will hand-turn easily enough] Rummage the hardware store for a standard O-ring to make this end more water tight or - my suggestion - epoxy the darn thing in place. NOTE - it is going to be darned difficult to "ream" this out with a twist drill bit, so be careful!

2. Metric: Well, this really would work perfectly fine with a 15mm x 1.0mm bolt (no drilling or reaming) BUT as you may know, that is NOT a standard size - no such thing as a standard 15mm bolt. (At least, not commonly available.) All these SI suggestions should result in a 75% thread. But a 16mm x 2mm should work and no drilling should be required, based on the inside diameter of three samples here. You can run a 14mm bit in the end you wish to thread, just in case. Better alternatives would be a 16mm x 1.5 mm (use a 14.5mm drill or reamer) or best, a 16mm x 1.0 mm (use a 15mm drill bit or reamer). Frankly, I'm not very comfortable with the 16mm x 2.0mm solution, though it should be OK. Same comments as above, substituting an appropriate metric bolt and again, BE CAREFUL if you use a twist drill bit.

BOTH: WD-40 or ATF (automatic transmission fluid) will work OK for a cutting fluid for both drilling and tapping. Use plenty. Aluminum galls easily and can be "gummy", even in heat treated alloys.

THE REST OF THE STORY:

I suspect that the samples that I have are made from a standard size 6061 T6 aluminum tubing stock, although they MIGHT be a slightly customized extrusion. By way of example: One supplier's standard sizes includes a 6061 T6 tubing with OD 0.750" ± 0.004", ID 0.584" ± 0.004", wall thickness 0.083" ± 0.004" The samples I have were very consistant in OD: 0.710" Basic wall thickness ranged from 0.078" to 0.080", and you can do the math on the range for ID. The wall thickness (measured at thread crest height) is less, of course - look carefully inside your mini-mag to see what I'm talking about or do the math on the threading diameters. I also note that the threaded portion has VERY consistant wall thickness, which I expected.

The end caps are very consistant in major diameter: 0.620" And I agree that the thread is probably 28 tpi, although it COULD be 27 tpi (standard thread for 1/8" pipe) - not enough threads on the cap for me to be certain and I am way too lazy to use my thread wires and micrometer for this measurement - I just use thread gauges to check the pitch.

I think these are CNC machined from standard 6061 T6 aluminum tube and that is why the dimensions are so consistant on the machined portions and why there are slight variations on the un-machined ID.

Quick note for the thread-comparison-impaired: 28 threads per inch (tpi) is close enough to 0.90mm thread, so this is pretty fine for a 16mm "bolt". 18 tpi is about 1.4mm thread - call it 1.5mm. 11 tpi, the standard coarse size for a 5/8" bolt, is about 2.3mm thread.

16mm is about 0.630" and 15mm is about 0.591" - recall that the factory plug is a precise 5/8" minus the radiused or flat tips on the thread tops for an actual diameter of 0.620". So a 16mm bolt will leave slightly thinner walls at the bottom of the internal threads than the factory end cap - about 0.040" vs an approximate 0.045" - not enough to be concerned about.

If I just had to do this and I didn't have a lathe, I think I would lean towards the 16mm x 2.0mm as this is should require no drilling or reaming. My second choice would be either 16mm x 1.5mm or 5/8 - 18, using appropriate drills or reamers (14.5mm or 37/64 for the 16mm and 9/16 or 37/64 for the 5/8"). The closest readily available solution over-all to factory would be a 16mm x 1.0mm using appropriate drills or reamers (15mm or 19/32).

Other's comments about headsize is correct - you will want to reduce the diameter of a hexhead bolt substantially (chuck bolt in drill press before trimming length and carefully use a file - if you bought a grade 8 bolt, you're gonna be there a while!) A 5/8" slotted fillister head would not be too bad - head OD is about 0.827" - 0.875" A slotted round head is huge (0.944" - 1.000" but due to shape fairly easily reduced. Forget truss head - monsters. Pan head also very big. Eyes too tired to look up socket head diameters.

So... I would NOT use the head of the bolt. After you thread the body, run a bolt in finger-snug, cut it off a bit long, slot the end with a hacksaw to suit, and file edges smooth. Remove with a screwdriver. Put epoxy or RTV or thread locker on it, run it back in with a screwdriver, and let it become permanent.

And with that in mind... I go back to my previous post: Epoxy in a short piece of aluminum rod. Cross drill and drive a pin thru for belt-and-suspenders approach. If you like, drill and tap the center of the plug for a small eyebolt (threadlock or epoxy it in) so you can attach this end to another a key ring instead of the factory end. This just isn't worth the work to make a threaded plug IMHO.

Long post, eh? I could have machined up a couple in the time it took me to peck this out...

Let us know what you do and how it turns out, please!

HTH,

Tom