First, I noticed that the cap extended after the threads ended, which seemed like a waste of space. In this picture, there is a line corresponding to the top of the threads. The cap goes on for at least another 1.5 mm, maybe 2 (small interval markings in 0.5 mm). The different coloured top part is the bevel, not the top surface. This means I can potentially increase the capacity by 3 mm.
First, I looked at other examples of metal threads - my Fenix UC40UE's threads went all the way to the end of the body (male). So it didn't seem necessary to have a flat portion without threads. People praise Fenix's build quality so I thought they would be a good reference.
Next, I looked in the Sentinel's tube to see how the female threads ended. From what I could tell, it was:
(end of body) (flat portion) (threads) (thin line) (centre of body)
It looked like the cap's top lined up with the thin line, meaning the cap's threads would end before the body's. Worst case scenario, if I ground the caps down to the threads, the body's threads would be exposed. If I say, kept a powder in my Sentinel, it could get into the threads. But then again, the threads didn't fit so closely in the first place. They were surprisingly loose, actually, given how many claims were made about their quality.
So I decided to chance it. Now, if you want to try this, it's best if you have some kind of mechanical grinding aid, maybe a belt sander. If you do it freehand on sandpaper, it's hard to keep the cap perfectly vertical, and you might end up with a curved or sloping face. Things I learned:
- Some people look down on Grade 2 titanium, but wow, it's really hard. Much harder than aluminium. This was really slow going. Gave me faith in the strength of the Sentinel, though.
- I managed to use both hands at the same time to grind both caps, saving about half the time.
- I managed to keep the face nicely flat. Yeah!
- If you're going to try this, it's best to use masking tape to protect everything you don't intend to grind, especially the threads. It saved my threads from some damage.
- I took out the o-rings, since you have to take them off for cleaning afterwards anyway. You don't want metal dust/abrasive under your o-rings.
- Once you sand away the bevels, it leaves a 90° edge on your cap. Combined with titanium's hardness, this is enough to cut skin! I didn't have a lathe to put a precise bevel back on, what could I do? The inner bevel is easy - fold a cone of sandpaper, about the size of the inner surface, then move it all around the inside. On average, every point on the edge should be sanded the same amount, resulting in a uniform bevel. For the outer bevel, take a funnel, line the inside with sandpaper, and do the same. It's not a perfect bevel, and it's very very shallow, but at least it's not sharp. Maybe I can use a cone to thin the cap and get more space...