Advice for modding a Surefire M951 (led and 18650)

hikoka

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Nov 4, 2019
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Hello all, long time lurker, first time poster! I've been collecting a few flashlights here and there for a few years but recently I've started thinking about modding and upgrading a few older ones. I'm pretty handy with a lathe but still learning the finer points of drop-ins, drivers, thermal considerations, etc.

The flashlight I would most like to upgrade is a used M951 I picked up and I'm hoping someone can give me guidance on doing it right or if some ideas are more trouble than it's worth.

1st, I'd like to bore it out to accept an 18650, and it looks like there's lots of material through the whole body to do that, except around the tail-cap threads. I don't think the fit with my batteries will be too snug if I bore the light body to 0.730", and I measure the minor thread diameter at about 0.830". That means I've only got about 0.050" between the wall and the minor thread diameter, so would this still be strong enough? I figure it would be, I won't be cranking the tail cap on with a wrench, but I didn't want to unduly weaken things.

2nd question would be ideas about drop-in's, if they exist for this purpose. The Malkoff looks awesome, but with a 3.4v minimum voltage I figure it's designed for 2 CR123's and not a single lithium cell. Does a decent dropin exist that would have a ~3.0 minimum voltage? Am I borrowing trouble if I try and make my own with a driver, reflector, pill etc. that is smaller than a p60 and make my own housing and pot it? It would be a fun R&D winter project for me if it's not too difficult. I suppose I should mention that I have my Streamlight TLR-1 as a weapon light, and I don't need this to be a 'bet your life on it' reliable light, though if there's a good way to make it so I'll have fun trying.

I'm hoping this is all feasible, but I'm sure I'll learn something even if it's not. Thanks!
 

thermal guy

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To answer your second question. Malkoff Dropins work exceptional with a single lithium ion. Most of us run ours like that.
 

hikoka

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Oh good, that makes the LED upgrade much easier. I guess I was worried for nothing on the tube diameter, I just measured my Olight M21 and it's only about 0.040" wall thickness between the inside of the tube and the minor thread diameter. It's certainly plenty strong, so boring this light out should be no problem.

I"ll get an M61 ordered soon and report back with pictures of boring it out.
 

mfrey

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Sep 7, 2007
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These help a lot.

The stock LU60 adaptor is not proportioned well to fit contemporary drop-ins like the Malkoff. Most find that they have to do some dremeling to make them fit. For 18650 use, you'll have t have the body bored out. PrecisionWorks did a great job on mine but the service is not cheap.
 

StagMoose

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Apr 19, 2020
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You could skip the boring out and use 16650s if you were open to that. May make the whole process easier. Just get a new Malkoff head, put a battery in there and poof! New light. Or use the Malkoff Adapter and use another SF scout head of you wanted to stay with SF.
 

draftpick

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Dec 14, 2022
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Oh good, that makes the LED upgrade much easier. I guess I was worried for nothing on the tube diameter, I just measured my Olight M21 and it's only about 0.040" wall thickness between the inside of the tube and the minor thread diameter. It's certainly plenty strong, so boring this light out should be no problem.

I"ll get an M61 ordered soon and report back with pictures of boring it out.
Sorry for the old thread did you ever bore out the m951 and get a drop in?
 
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