From a lot of reading and asking I think more than once, the threads on various runs of D4 may or may not be compatible with the D4v2. It depends on the run. Good luck, though!
Regarding flashlights that can run on different size batteries: I watched a few of reviews on different flashlights, and the same reviewer shows he gets more lumens from a 21700 battery compared to a 26650 from the same torch. How is that so?
First you need a light that is sensitive to the battery power such as one with a FET based driver. Then you pick a strong 21700 battery and a weak 26650 battery and there you go.
Ok, thank you. I thought a 26650 should pull more lumens, and longer run times. I didn’t even think about the mAh aspect. I love this forum, I am learning a lot.
Battery manufacturers can use different chemical mixes to optimize the battery. For instance you can look at the Samsung 30T, 40T and 50E. The 30T has the lowest capacity but the highest amperage and the 50E has the highest capacity but the lowest amperage. Then the 40T is right in the middle.
The 26650 container holds about 50% more volume than the 21700, but they are only made by smaller battery companies and they don’t usually have the best mixes. There is only one 26650 that has 5750mah and the rest are lower.
So each size battery has a range of capacity and amperage. The person who did that test you mentioned probably chose a more powerful 21700 and a less powerful 26650.
Thank you for explaining it.
Probably worth noting a 30T will pull more amps than any 26650. Until it runs dead quicker, anyway.
He wasn’t using 26650s that were as high-drain as the 21700s he compared them to.
Thought I would update this. I ordered the 18350 tube for the d4v2 from mtn hoping the threads would work out, and they did. Fits my d4v1 perfectly, and makes it scary tiny.
My favorite genre of movies are horror, and thriller/suspense types. As soon as a movie character pulls out a flashlight, I lose focus on the movie plot, and immediately start analyzing the torch. Does anybody else here do the same?
One thing that I have picked up on in movies, is when a modern day movie that is based on the past (1950s - 1990s for example), the flashlights are always really bright, and have a good amount of throw. Nothing like what flashlights were really like back in the day. I never picked up on this until I got into flashlights this year. LOL.
Wellp, watching old-timey teevee shows like “The Saint” 
, you can see what they had to do to get any light to show at all.
Eg, he’d pull out a penlight (wonderful how even Simon Templar EDCed a small light!) and shine around the room to try to find the hidden safe, etc. Someone off-screen would be shining a small spotlight around the room, trying to follow where Simon was shining his light. You could even see the eerie off-angle shadows being cast, but hey, that was high-tek back then.
My Lumintop FW3A arrived today. I only have one 18650 battery, and it is a Sofirn 3.7v 3000mAh button top (very small button at that, but never the less…). It works in the Sorfirn SP32A V2, of course, and it works in the Convoy C8+. So, I put it in the FW3A, and no go.
I read the owners manual and sure enough, an unprotected 18650 flat top with high drain (10A or more) is recommended. Ok fine, I have a Samsung 35E 18650 unprotected flat top battery scheduled to arrive tomorrow from the Liion store. Now I am learning a lesson about Li-ion batteries, so I go online to double check what exactly I ordered. Yep, it is a unprotected flat top 18650 battery, but it states it is only 8A, but also says up to 13A manufacturer pulse rating. Another problem I just caught was the SAMSUNG 35E is not recommended for high drain devices, and it measures a length of 65.3 mm, and the Lumintop manuals says the maximum supported length is 65 mm.
Question: Can I use the Samsung 35E in the Lumintop? Or did I just learn an expensive lesson in li-ion batteries.
I am currently using 35E’s in 8 different FW3’s, so yes, you might get a little more output from a 30Q but not for very long as it will heat up very fast and step down.
The Sofirn should work, it just may not hold up to turbo. Unless you’re saying it didn’t fit?
The 35E should be fine, though I’d personally recommend a 30Q.
The Samsung 35E is perfect for this light. 35E, LG MJ-1 and Sanyo GA are the perfect top three. The extra capacity is impressive, and there is no chance your eyes will notice any difference in brightness vs a higher drain, lower capacity cell. Exxellent choice.
The Sofirn fit just fine, but the flashlight would not turn on.
Thank you for all the replies. I will use the Samsung 35E with no worries.
“Why is a carrot more orange than an orange?” 
I think the one thing your missing is that the FW3A manual just doesn’t go into enough detail. It should say something like “unprotected 18650 flat top with high drain (10A or more) is recommended for maximum output”.
The reason the 35E is not recommended for high drain devices is it’s chemical mix is optimized for max capacity. This means it’s internal resistance is a bit higher than say a “high drain” cell so it won’t pull as many amps. You can read more about battery ratings on this older post and the one below it.
It’s probably too long. The extra length is not letting the inner tube make contact.