So with the recent interest in MT-G2 mods for single cell lights like the C8 running on 2S and my own latent interest in building a stretched HD2010 I was wondering what’s available out there in terms of battery extension tubes/adapters.
I want to collect information on parts that are available on the market either individually or as part of flashlight kits that would facilitate adding extra cell length to existing designs.
I know I could just buy a 2s host and go the easy route but where’s the fun in that! 
For 26650 Lights
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Ultrafire/Fandyfire HD2010
Has anyone had success in finding a suitable extension tube for this popular light? I’ve had no luck so far but maybe someone has a 26650 light that came with an extension or adapter that fits the HD2010. Anyone apart from MRsDNF (who made his own extended tube) built a stretched 2s hd2010?
In my search for a HD2010 extension tube I ordered this a while ago.
26650 Extension Tube
No dimensions or compatibilty listed on the site so it was always going to be a shot in the dark, but it was cheap and I was feeling lucky. Of course it’s not even remotely the right size to fit a HD2010 but maybe it fits something else.
So here are the dimensions. Maybe it’s useful to someone.
O/D On Knurling: 32.57mm
O/D on grooves: Min 30mm Max 31.3mm
O-ring Groove: 28.3mm
I/D: 27.2mm
Length:75.55mm
Threaded section: ~6mm
Thread Diameter: Inner 28.9mm / Outer 29.8mm
Thread Cut: Triangular
Thread Pitch: 1.2mm Measured from thread tip to tip (not sure if this is the most useful way to measure this)
For 18650 lights
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Ultrafire C8 Extension tubes
There seem to quite a few options for the C8 and this one is a cheap Generic Ebay extension tube that I have on order.
Will update when I’ve had a chance to test it out and take measurements.
18650 Battery Extension Tube for C8
C8/C12 Extension tube:
Another, nicer looking option used by user RMM on his c12 mt-g2 is this one.
This one was also available on ebay but for me with shipping a bit too expensive.
So anyone know any more extension tubes that fit specific lights like the HD2010 or C8? Could check if an extension tube that comes with a multi 26650 light might be compatible with the HD2010 specifically or take measurements of what they have to give others an idea of what’s possible. I’d also be interested in hearing if anyone has had success finding or modifying a part to extend a popular flashlight.
Hopefully it’s useful to someone else.
At the moment I’m specifically looking for an extension tube to fit the following battery tube dimensions.
T-Mart/Fandyfire HD2010 Battery Tube dimensions:
Type: Male — Male So adapter would need to be Female — Male like the ones above.
O/D On Knurling: 34mm
O/D on grooves: Min 31.97mm Max 33.7mm
O-ring Groove: 28.82mm
I/D: 27.2mm
Threaded section: ~7.2mm
Thread Diameter: Inner 30.2mm / Outer 30.8mm (~5mm Depth?)
Thread Cut: Triangular
Thread Pitch: ~1mm Measured from thread tip to tip (not sure if this is the most useful way to measure this)
Cheers
Linus