The outside diameter of the o-ring installed in the groove takes into account the thickness of 1.38mm to equal 50.10mm fitting into the 50mm inside head diameter. A thicker o-ring would create a larger, tighter fitting or not fitting outside diameter.
The o-ring durometer (density, harder vs softer) also comes into play with fitment.
Not sure if the number of visible rows means much since one of my Q8’s has 5 rows while the other one has 4 and based on the screws they are both second batch. Both came with a few tiny chips in the anodizing but nothing I haven’t seen in other lights and firearms. One has already survived an unplanned drop test 
Yes, I accidentally dropped a lighted Q8 yesterday from 1 meter height onto an oak floor without any damage. I hope that is exemplary for the Q8.
I had to change my top o-ring after I indexed my battery tube, the stock o-ring was too thick and I was one flat away from making contact with it.
But that dosen’t explain what is happening to your STOCK light now does it??
Got a Vernier so you can take some measurements??
Mine (1st batch) works fine, and my measurements are done with my digital calipers towards the discussion of Solar Lumen’s issues.
These batch 2's are definitely machined slightly different in the area of the o-ring. I got a bunch of batch 1 and batch 2 Q8's here, and consistently the batch 2 o-rings stick out further. I'll take measurements but the machining and the o-ring positioning is different by eyeball.
Just make sure the o-ring is well lubed - makes a huge difference. Can't believe how hard it is to tighten when bone dry.
> nothing worked except without any o-ring
So is the problem that the inside diameter of the head isn’t cut out sufficiently and the O-ring is hitting a ridge/’shelf’ that it should be sliding under?
One or two thousandths makes a big difference on the battery tube because of how close they machined it, that o-ring groove is non forgiving, with that tall rear shoulder.
Even over lubed and my thin o-ring takes a bit of rotational force to make contact still.
Just got in a batch 2 light and measured compared to batch 1:
Funny because on batch 1, the top row of knurling is sanded or cut down a little, while the batch 2's don't have that. so this is directly below the o-ring groove. This light came from a BLFer, so not sure what was done to the threads, but they were the driest I've seen - think he might have used a cleaning agent. So it was really hard to tighten. After moderate amount of Nyogel, it's almost as easy as a batch 1 -- much improved.
Not even a dent in the floor? 
I have not looked for that actually, I will be back home on friday and will have a look. But oak is pretty hard and even if there is a dent it will add character to the floor
Confirming with Batch 1 lights — the top half of the first row of knurling (little diamonds) is cut down a bit — the row that disappears under the edge of the light head when tightened.
Makes me think there was a production line making heads, and another line making battery tubes, feeding a third line that was assembling them.
And in one of the first two lines, some guy missed a step and made some pieces with a little too much material to fit.
Yeah, That’s what I say about my truck.
Wow this is so interesting, subtle but distinct differences between batches. I am curious how they are in a year if they are still being made (which I so hope!)
If you think your o-ring is too thick to work, you could try a thinner one. You might be able to find it at the hardware store. I know my local Ace hardware carries lots of o-rings. A more likely place to have it would be a local hydraulic repair shop. They carry huge assortments of o-rings. Then it would be water proof again.
If you can’t find a thinner O-ring, or the next size down is too thin, you could try a smaller diameter one that needs a good stretch to fit, which will thin it. Not ideal, but, might be worth a try.
The correct size o-ring can be determined by measuring the diameter of the groove in the battery tube, and the diameter of the mating surface of the head. Subtract one from the other and divide by two.
Then Google e.g. “O ring groove design” and choose an appropriate one for a low pressure seal application. That should get you in the right ball-park.
Possibly the correct o-ring will not fill the width of the groove in the tube. Nevertheless it should give the best seal. A too thick O-ring will not seal as well as the proper size, will wear badly, and generally be unsatisfactory. If it is stiff to tighten, the fit is probably incorrect. Only a little compression is necessary.
I would like another Q8. I recall seeing some fairly recent posts that our original codes will still work.
Can anyone verify that?
They did on the evening of the 19th. Just give it a try & see.
The more i play with the Q8, the more i love it. 
its the perfect soda can light with some balls. I have some mod ideas in mind to do soon, including resistoring back the switch LEDs, (for less drain and more of a firefly glow) and may swap them for amber, Yellow, or warm white micro LEDs. ( and re-wire the original green ones to come on when the light is on. Also add a lanyard ring to the tripod hole. There seems to be enough space between the driver & the LED base for a TP4056 USB Charger, and may mod one into it like the lantern prototype has for charging the Q8 from my solar chargers on my long back country trips.