AR-10/LR-308 rear takedown pin detent spring and TM lower

[nick]

I'm assembling a Nick Mod 4 lower (which started it's life as a Tactical Machining 80% lower, and it based on LR-308 pattern ), and I've encountered a problem which, I'm sure, others have encountered before, given that most of the parts I used are common - I'm using a DPMS .308 LPK. So, to make the long story short, the spring for the rear takedown pin's detente is too short. It's the same length as the spring for the front pivot pin's detente, and the same length as those for AR-15 (most parts are the same between the LPKs). Which is just the right length for AR-15, but is too short for LR-308.

I solved this problem by inserting another detente behind the spring (luckily, I have a couple of those "oh, shoot" AR kits with spare small parts, since my cats keep stealing the small parts). However, shouldn't LPKs contain ALL the parts for assembling a lower? Looking at the parts list included with the LPK, those springs are supposed to be the same, as far as that list is concerned. So, what am I missing?

[osis32]

I think that hole is threaded a's meant to have a retaining allen screw holding everything in making stock installation easier. It's been a while since I installed mine so I'll be corrected if I'm wrong

[nick]

Well, yes, the hole's threaded and all (and it's threaded on all of my AR-15 lowers, too), but that doesn't solve the issue of the spring being too short. Unless, of course, the threads go over .5" in (the spring is short by about that much). Come to think of it, that might be the case, I'll check it.

[high_revs]

weird that the rear take down spring is too short. You sure you didn't mix it with safety selector spring? Kinda odd that same spring as front take down would be too short. Been awhile since I assembled but I just put together my armalite pattern ar10 and had to think twice about that one.

I think the thread comment might refer to safe selector detent?

[nick]

weird that the rear take down spring is too short. You sure you didn't mix it with safety selector spring? Kinda odd that same spring as front take down would be too short. Been awhile since I assembled but I just put together my armalite pattern ar10 and had to think twice about that one.

I think the thread comment might refer to safe selector detent?

I'm quite sure I didn't mix those up. They're different diameter and all.

Were both detente springs the same length in your LPK? Which LPK did you use? Was it for an AR-10, or LR-308?

The distance between the rear of the FCG pocket and the front of the buffer tube threads is greater on an LR-308 than on an AR-15, so that would explain the spring being too short. However, you'd think that warranted a longer spring... I must be missing something.

[high_revs]

mine were same lengths as were my ar15s. I use armalites. 2 for my ar10, 2 for my ar15s. Prior, I used rra and stag. All were same length between front and rear take down iirc. Hence they're grouped together in build instructions usually. Too bad the front would be a pita to remove to compare.

maybe post some pics here?

can't recall if the ar15 detent springs are same as lr308s, in case you want to compare against your ar15

[nick]

mine were same lengths as were my ar15s. I use armalites. 2 for my ar10, 2 for my ar15s. Prior, I used rra and stag. All were same length between front and rear take down iirc. Hence they're grouped together in build instructions usually. Too bad the front would be a pita to remove to compare.

maybe post some pics here?

can't recall if the ar15 detent springs are same as lr308s, in case you want to compare against your ar15

They're the same length. I have the same DPMS LPK, and the springs in it are the same length as the AR-15 ones I have.

And as I wanted to compare the springs, I figured out an easy way to remove the front detente spring. Whittle a wooden toothpick to the right diameter, and use it to press on the detente. Then turn the pivot pin, breaking the toothpick. Took about 3 min.

[high_revs]

is this issue something akin to dpms pattern 308? Haven't heard of it myself.I did notice on n6 lower, my take downspring hole is right by the safety dent hole towards the back vs under the buffer tube like ar15. Can't recAll though if my n4 lower was the same.

[Nathan Krynn]

I think that hole is threaded a's meant to have a retaining allen screw holding everything in making stock installation easier. It's been a while since I installed mine so I'll be corrected if I'm wrong

This is the correct answer. We include a dowel and 2 set screws. One of the set screws is for the take down pin detent and spring hole. It is threaded very far and you just use that to to spring tension you want.

They are the same spring as the AR-15.

You do not need to put something in the hole but the detent, spring, and set screw.

[nick]

This is the correct answer. We include a dowel and 2 set screws. One of the set screws is for the take down pin detent and spring hole. It is threaded very far and you just use that to to spring tension you want.

They are the same spring as the AR-15.

You do not need to put something in the hole but the detent, spring, and set screw.

Thanks, Nathan. Didn't realize it was threaded that far. Well, at least I put the dowel where it belonged (bolt catch). Had to file it down a bit, as I didn't want it to be stuck in that hole. Otherwise it would be impossible to extract later. It's probably due to anodizing.

I think, I'll still keep my setup though. The screw was a tight fit after anodizing (I got the raw 80% receiver, and anodized it after the milling).

[Nathan Krynn]

NP I saw this so much we now include an instruction sheet for the take down pin and bolt catch.

If it works leave it, trust me I have seen worse.

EDIT: it is threaded so far and such a weird spot on both we had to have a tap made that is custom at $200+ each tap.

[high_revs]

this is great! i remember a tutorial back then on ar15's on how to thread that hole so that it's not a pita to have the spring/detent going sproing on you when you remove the stock and end plate. LOL

i hope this is a trend starting among manufacturers of lowers.