Glad to see steady progress on this!
Please get a price quote on bare aluminum too!
please? 0:)
I’m not a fan of that. It results in a light that’s oversize for what it does: fatter than it needs to be for AA and longer than it needs to be for CR123. It would be much better to simply have a light designed and optimized for one battery type.
Great idea. 
The way I see it the answer is No. The 18350 + 15mm (nominal) extension(s) limit what needs to built to three items:
- Head (all identical) (this may have sub components, but all heads will still be the same)
- 15mm extension(s) (all identical but designed to fit together) Male front, Female rear thread.
- Tail cap (all identical) Male thread to fit (head or any number of extensions)
So there are three identical parts, each with a price and folks can choose how many if any extensions they want to buy.
Remember the machine shop is working for us not us for them. Once the design is set the machines can run any of the parts easily but fewer parts is less design work and higher volumes typically this is what machine shops like.
The trick here is to minimize the types of parts and to maximize the volume of parts; unique tubes will affect the volume of the run and may result in higher prices. Having a single extender that can be used in multiples will keep the cost of the longer versions as low as possible.
And the added volume (because lots of folks want longer versions) will keep the total volume of heads and tails up, thus keeping the cost down.
WWEFANS please check your math; see my request for checking my math (post #31) so that we are all on the same page.
Also your three tube approach will work and may allow for S series tail caps to be used, but I am not sure how important that is as opposed to the more important use of the standardized guts from the S series.
Let the machine shop come back with pricing Lets call option one “Plan A 3 Tube” and the other option “Plan B 1+2 Tubes”
It looks like we need pricing for
18350
18500
18650
Lights for each plan so two sets of pricing
It may turn out that Plan A is the lower cost, but lets have the machine shop(s) tell us that.
Very similar numbers here for an S2+ compared with the S2+/18350, both from Simon.
Both pills seem to have 22x1.25 (mm) threading, but the S2+ pill has 21.75mm outer diameter while the S2+/18350 pill has 21.90mm outer diameter (accuracy is only 0.05mm here). So difference in OD is 0.15mm which equals .006”. But the threading pitch seems to be the same.
So the S2+ pill can be screwed into the S2+/18350 head (feels slightly more wobbly but sits tight in the end) while the S2+/18350 pill won’t fit the S2. Same with head and body, S2 body fits the S2+/18350 head but not vice versa.
Simon was asked whether the S2+ series hosts parts are interchangeable and he could not guarantee it, due to different S2+ batches.
@WWEFANS
Well done so far, I’m highly interested. All depending on price, of course - but I like the idea of a quality host which can use all three sizes from 18350 over 18500 to 18650. I’m a great fan of the 18500 as well. A good IMR 18500 is only 15mm longer than an 18350 but has about twice the capacity and can deliver 3+ ampere without batting an eye. Compared to 18650 lights it’s ~20mm shorter (most 18650 lights fit protected cells) and this makes 18500 far more pocketable.
The tube length of the S2+ is 48mm, by the way, threads ignored.
Way to go 
let's assume, the 18350, front thread,middle tube and rear thread are all 15mm. so it's 45mm long.
Plan A: 45mm(15+15+15) ti for 18350. 60mm(15+30+15) for 18500
75mm(15+45+15) for 18650.
Total: 180mm long Ti material.
B: 45mm for 18350. extension 1 for 18500, 15mm front female thread,15mm middle tube,15mm rear male thread. another extension for 18650.
Total: 135mm long Ti material
EDIT:
45mm tube wont save too much. But this costs a problem. you will have 5 parts when using 18650.
head,3 tubes,tail. Does it look odd? especially when the knurlings don't match well.
It was suggested to change the S2+ tail into a closed one,which will be better tail-standing.
with two holes for lanyard.
Yes , please .
You have a point, this is a design trade-off. Modularity vs optimisation. Nothing wrong with either philosophy.
But that would mean a decreased access to the button.
Since I need to access the button much more than I need to tailstand the lamp, I’d prefer the current semi-open design; which still allows for tailstanding when needed.
I have confirmed that the S2 pill I have now threads into the S2+ head, after a light sanding of the tips of the threads.
It only took a couple of minutes, drawing the pill across some 220 grit sandpaper.
The S2 pill did not have any noticeable ‘slop’ when put back into it’s original S2 head.
thanks for the finding.
WWEFANS , I think what Ronin42 meant , and I’m all for it , is a head , a tail and then ; 2 extensions for 18350 , 3 extensions for 18500 , or 4 extensions for 18650 , all identical.I admit , that would affect knurling choice , or demand higher machining precision for alignment.
I think I prefer a separate tube for 18350 with separate tubes for the other sizes. Just looks more uniform than 3 total extenders.
I vote against 18350 + 2*15mm adapters. Two 15mm adapters will look worse than one 30mm adapter and 18500 batteries are very uncommon compared to 16340 and 18650.
I don’t mind either way. So long as we get 18500’s.
I don’t think two extenders would be that obvious, not if the knurling has a band at the joins. It’s not as if an L2M looks wrong with the extra extender.
The reason that extensions look bad is usually because they look as if they are added later in a different design. But if designed well, either you hardly see them, or the sections contribute to a nice design.
I would like the separate tube for each size battery. If they are threaded the same as Convoy I will get more use out of them by legoing a Convoy with the Ti tube. And me using 18500 batteries is probably not going to happen so I’ll be giving that tube to someone on the forum.
And make it a limited run! With serial numbers!
WOW!
++1