I am looking at getting the KeepPower 6000mAh Protected Flat Top 26650 for my L6 that will be arriving soon.
Some people say because they are flat top cells that running them in series in the L6 will not work well and that a button top is better.
Will they fit or should I go with some other 26650?
They're made by PLB, the same OEM under the very popular LiitoKala INR26650-50A/Lii-50A. Same can, slightly wider than usual but they should fit (protected cells won't). 
Cheers 
P2660C fits quite tightly on L6 (will need to push them out from the other side, once batteries are inside)
Thankyou.
So just to double check before i spend money - Will I be safe with 26650 KeepPower 6000mAh Protected Flat Top from Illumn for my L6?
As I understand from your advice is that they will fit but will be a tight squeeze and may need to push them out the other end to get them out and also connecting correct6ly in series.
Thankyou
I got mine from an AliExpress Store, but I suppose the P2660C should be similar as long as they’re genuine? My P2660C measures 26.8mm diameter.
I dont like the idea to have to struggle and push from both sides battery, if u remove tailcap and it doesnt work then i dont want them really… 
Struggle? Seriously? A little stick and/or petroleum jelly maybe?
The alternative is likely to be a lower performance battery, much lower performing if you're not careful. 
It's your choice. 
Cheers ^:)
Remember the cells can swell a bit when charged, so if you are testing them at storage charge, you may want to fully charge them first. if you have to push them out with something, I’d say you can’t use them without increased risk if trouble.
If one is really forcing the protected cell in the tube, should one worry about shorting the strip on the side of the battery with the battery tube?
Personally I dont like any battery tube that is a neat fit for the battery to be installed. I have seen very slight lumps in the anodizing on the inside of the battery tube that has torn the heat shrink on batterys. The only reason they did not short was because of the anodizing that caused the tear in the first place.
Most Convoy lights, large or small, I purchase now I either ream or machine the battery tubes out to a bigger diameter.
Another reason batterys can be a tight fit half way down is that the battery tubes in two sections can have the threads machined of square. When the battery tubes are screwed together they end up with a bend at the intersection. This again has happened with Convoy battery tubes.
Not knocking Convoy itself as I like them, just pointing out some short comings that are not readily evident.
I would say yes, because after some time the wrapper could wear or crack if it is a tight fir. If it is tight, best to find a different battery.
According to List of battery sizes @ Wikipedia, maximum dimensions for the 26650 format are 26.5 × 65.4mm, whereas for 18650 it's 18.6 × 65.2mm (!).
To me it is clear that 26.8mm cell diameter is a little bit out of spec, but not too much. If I were the chief manager in charge behind battery tube design and specifications I'd probably add an healthy extra 5% cross section to that figure just to be safe. This means I'd bore the tubes to a diameter of 26.5 × √(105/100) = ≈27.15mm. Even if I were to only add a 3%, my bore diameter would still be 26.9mm.
Just measured a spare unused dirty pair of LiitoKala INR26650-50A cells I have inside a drawer, ∅26.8mm. 
Forcing? Struggle? You speak like if it takes a lot of effort to make the cells go inside. Aided with some sort of stick, fork or spoon you can easily slide a single cell along the entire tube to test how tight the fit is and check the wrap integrity. If fit is too tight and/or the wrap gets damaged, I'd rewrap the cell(s) and fix the tube or use other cells in that tube. Finding solutions it's all about.
Next time ask your flashlight maker to leave tight-fittings for you know what. 
Cheers
Does anyone know what happened to US availability of KP 26650 6000mah cells?
I am finding them available on Ali by special order and only in batches of 700