Yes, it seems that wrapping a sheet of copper there to make the wall thicker and the head fit more snugly might keep the led cooler. Someone did that, on a TrustFire Z8 I think.
That too, but what I was talking about is more structural.
If you disassemble the SK68 clone the body is a tube of varying thickness, the thinnest part is top half inch where the pill threads in, and the cooling fins are below that overlapping less than a 1/4 inch of the fins. The part with the star is the very thin tube the zoom slides over which requires a gap between the parts, no real cooling at all directly out from it.
I’m going to guess heat will flow from hot thin alum area to cooler thicker alum mass pretty well. That said, improving the transfer from pill to body via that red hot rod example was a good idea. Next, is there a good way to tighten thread contact/transfer between pill and body?
Don’t forget get heat away from LED via some heat sink - that is more sink mass in the pill.
And also get heat out of light via hand/air - that is transfer from pill to body and out of body.
Of course heat sink compound around the pill will help. They warm up both in the head and the fins, unlike the Z8 that warms up just at the head. So heat does transfer across the gap to the head and along the thin body to the fins. But I think both could be improved with some thin copper. Probably take out the o-ring or spring that the head slides on, wrap copper, solder it maybe?, put some heat sink compound or grease.
Thermal transfer is going to be related to the cross section of the path, which is tiny in the thin part of the tube going down to the fins, but the pill is good sized so my guess is that most heat transfer is through the base of the pill near the fins.
I don’t see a practical way to keep the zoom functioning decently and transfer much heat to it.
OTOH high output for an extended time isn’t going to work with a SK68 or any small light, simple heat vs surface area.
In this thread Review: TrustFire Z8 - XM-L Flood-to-Throw there is discussion of thermal droop in the Z8, but more likely it was battery voltage droop. For a brighter SK68, it seems one could follow the Z8’s example by using an under driven XM-L. That reduces the throw but the larger led die won’t get as hog as as smaller die at the same power would, even though the star, pill and light do. Also, the efficiency is better. The cooling of the SK is definitely better than that of the Z8.
Why not jump to the XM-L2, more efficiency isn’t it? My first attempt will be with a XP-G2, then maybe XM-L2 if that doesn’t seem to do the trick.
Yes, I assume the XM-L2 is better. From the spec sheets it is, but I tend to always start with the cheapest possibility.
Sounds like the pill in a SK98 is the same size as the SK68, maybe a drop in mod for higher output and XM-L? SK98 clones start about $12 shipped on ebay, worth it for the solder challenged.
No swap, 98 is ~4mm wider tube. 20mm star (and probably bigger driver) instead of 16mm. Mine has hollow pill like the 68, but not all do. The battery tube takes 18mm instead of 14mm , also now wondering if the driver works with single 1.5v (and spacers)?.
The lenses on my 68/98 are close to same size, but they fit in the bezel differently - the 98 lens lip fits up into a recess in the bezel instead of being full width to the body of the tube.
I now wonder if this is a design difference between the narrow and wide bezels variants we see among the 68 clones (also for 98s?). And if this limits/enhances throw between them. I think a much wider lens would fit the 98 bezel if it was reamed out wider …
EDIT: Yes, my UF-SK98 lens fits in the 68 but the bezel of the 68 is too fat to let it seat completely. The beam focuses to the point where round becomes a smaller square with rounded corners, but the die shape is not visible. The 68 lens also easily fits in the 98 bezel. The major 3 dimensions are roughly the same, but my 98 has a wider dome and the lip is narrower; the lip looks like it was sanded down to create a press fit for the recess in the bezel.
Pic below compares SK68 (XPG2), SK98(XML), & ZY-603(XML); sorry, the body tube of the ZY is upside down relative to the SKs. The ZY already has red fins, so now it needs a driver to match. 
I have a Raysoon TD398 which is an SK98 clone or 18650 size 68 clone. Everything is scaled up about in proportion. No part would fit.
Here http://www.aliexpress.com/item/Upgrade-800Lumens-9W-CREE-XM-L-T6-Mini-LED-Torch-Focus-Zoom-Flashlight-Free-Shipping/997633297.html is one with the LM-L already in it.
Here’s a homemade AMC driver for the SK68. It has Hi/Med/”firefly” and low battery detection. The next revision will probably also include thermal protection. The XR-E is getting a bit too warm at 700 mA.
Looks very neat. Of course the production lights can’t have low battery detection because they have to work on worn out alkalines.
I bought one with similar description off ebay and it was a garden variety XR-E, but once brought to the sellers attention, via three emails quoting their listing it was a free one. I think often the seller isn’t the one who makes the listing, or knows what it says in English. That and the people doing the listings tend to copy old listings.
No worries about alkalines with protection in the driver board, just never use them and run cheap unprotected LiOns.
That custom driver board looks really nice, sounds like a great option to try.
Speaking of trying, I “might” give the XM-L sk68 a shot for $10 not much to risk, and if its not as described it says full refund. I’ve never purchased via alibaba though.
Can the low battery detection be smart, as in between low Lion 3v and high NimH 1.6v no turn on, but below 1.6v works?
The listing above says “Upgrade”. So it may be intended as written. I bought a Sipik 58 with an XR-E and three modes, so before sale upgrades do happen.
I messaged and the seller and got the reply
“Jenny Zeng:
18:29 Sep 23,2013
it is ok, we change it to XM-T6 XM-L U2 and XM-L U3”
Also, there is an image that says both SK68 and Cree XML T6 on it.
> Here’s a homemade AMC driver for the SK68. It has Hi/Med/“firefly” and low battery detection.
Oboy. Sounds perfect. Directions/pointers/instructions/offer to sell some?
I need to look at AliExpress a bit before buying anything through them, but its tempting. I did some feedback checking and the first guy that gave them 6 stars gave everybody 5 stars, so taking it with a grain of salt. The U2 is a higher output?
Link to $10 shipped xm-l sk68 clone.
I have bought things from them with no problems, after looking them up in Wikipedia. It is the retail branch of Alibaba.com which is a huge wholesale site.