When is a bolt "old"?

[1_click_off]

If I built with used parts I would have a field gauge. Wear patterns between the bolt and extension are unique. When you start mixing used parts from different rifles it encourages accelerated wear. Enough to hurt anything??? Maybe maybe not.

Agree, my dad always instilled in me that parts that wear together should get replaced together. I was about 8 the first time I heard it when helping him replace the races and bearings on his front hubs. Being a kid, I asked why he doesn’t leave the races and just replace the bearings. He also explained why you never mix up the bearings while repacking them. Has always stuck with me. I would get a new bolt and carry the 2746rd as a spare.

[Uni-Vibe]

Quit guessing. This is exact science. Get a headspace gauge and check, every time you swap a bolt. It's only 60,000 psi in there.

[Joe Mamma]

Offtopic, tangent, would you mind sharing your entire Preventative Maintenance regimen stem-to-stern for us who are still in the "baby's first steps" stages?

Diamondback, send me your email address via PM, and I will be happy to send it to you as a pdf document (or some other format if you want).

I have thought about posting the information from time to time because I have never seen a good preventive maintenance and parts replacement schedule *anywhere*. That's why I created my own.

I'm sure many would find it helpful. But I'm also sure it would start all sorts of arguing (because these things always do on the internet), and I don't want to deal with that. So I don't plan on posting it anywhere.

Joe Mamma

[lysander]

5,000 rounds when used in carbine length gas systems,
7,500 rounds when used in mid-length gas systems, and
10,000 rounds when used in rifle gas systems.

Must be nice to have deep pockets.

The Army studies on M4 reliability give the bolt 10,000 to 12,000 rounds MRBF, M16 bolt 13,000 to 15,000 MRBF. On average they replace the bolt every other trip to the depot for overhaul (one tour is approximately 6000 rounds).

And the bolt is not the top replacement item. The top 4 items are:

Barrels
Bolts
Ejector springs
Hammer springs

All of the other parts last in excess of 30,000 rounds.

I just inspect bolts for cracks with every cleaning. If you are paraniod about cracked lugs, replace with the barrel, it makes headspacing easier too.

[titsonritz]

A bolt is too old/worn when it will not pass a field gauge test on a new barrel.

[Joe Mamma]

Must be nice to have deep pockets.

The Army studies on M4 reliability give the bolt 10,000 to 12,000 rounds MRBF, M16 bolt 13,000 to 15,000 MRBF. On average they replace the bolt every other trip to the depot for overhaul (one tour is approximately 6000 rounds).

And the bolt is not the top replacement item. The top 4 items are:

Barrels
Bolts
Ejector springs
Hammer springs

All of the other parts last in excess of 30,000 rounds.

I just inspect bolts for cracks with every cleaning. If you are paraniod about cracked lugs, replace with the barrel, it makes headspacing easier too.

Deep pockets, ha ha.

So let's look at my numbers in the context of the numbers in your post:

With a rifle length gas system, you say it is on average 13,000-15,000 before a bolt failure (based on the numbers you quoted). So if someone plans to go to 14,000 rounds on the same bolt, half the time they will not make it because they will have a bolt failure. Replacing it at 10,000 rounds as preventative maintenance seems like a very good time to do it.

With a carbine length gas system, it is more complicated. The M4 military carbine length gas system is used with a 14.5 inch length barrel. Outside of a military setting (in the United States), a carbine length gas system is almost always used with a 16 inch barrel. The slightly longer barrel, and specifically the significantly longer dwell time (about 20% longer if my memory is correct), is much harder on the bolt. It's one reason why there were often extraction problems with 16 inch barrels until different inserts, o-rings, and better extractor springs came into common use. Another reason is larger than M4 spec gas ports on 16” barrels (to help cycling some lower quality consumer ammo) which is also harder on bolts.

Again, the 10,000 to 12,000 round number means (theoretically, in this data set, etc.) that if you try to go to 11,000 rounds, half the time you will not make it because of bolt failure. So as preventative maintenance, someone should go for what number, maybe 8,000 rounds? And that would be for a more gentle military carbine system with a 14.5" barrel.

But with a carbine length gas system and the much more common 16" barrel and often larger gas ports (which have been shown to be significantly harder on bolts than the military M4 setup) that round count number would be significantly less. Also, if you spend some time looking around, you will see evidence from reliable sources (military and civilian) of bolts breaking with carbine length gas systems at a significantly lower numbers (and no I am not going to research that for you). So the 5,000 round number still sounds very good.

One thing I did not mention is that my parts replacement schedule has a 1,000 round “cushion” built in to everything. This means that if you should replace a part at let's say 2,500 rounds, you can probably comfortably go to 3,500 rounds. I know that sounds like a big difference. But this covers a situation where it's right before you should replace the part, but you unexpectedly want to grab it and go to a shooting class, weekend of shooting with friends, SHTF situation, or whatever without replacing it. I think 1,000 rounds is a reasonable (and reasonably necessary) amount.

Regarding your “top 4” list, I am not sure if you are suggesting barrels should be replaced before bolts or if you simply cut and pasted from an alphabetical list. But I think it would be wrong to replace barrels before bolts unless you consider things like rate of fire, barrel material (stainless, chrome moly, chrome lining, nitriding, etc.) ammo type, etc.

Joe Mamma

[opngrnd]

Is Forster the only headspace gauge name in town or can you still find the ones where you don't remove the ejector. Also, when do guys get your extractors, extractor pins, ejector roll pins, and ejectors? I will go with Sprinco on the springs since I have mostly has good luck with them.

Lastly, any special tools you recommend for the bolt assembly/disassembly? Special roll pin punches, etc.

[Uni-Vibe]

Is Forster the only headspace gauge name in town or can you still find the ones where you don't remove the ejector. Also, when do guys get your extractors, extractor pins, ejector roll pins, and ejectors? I will go with Sprinco on the springs since I have mostly has good luck with them.

Lastly, any special tools you recommend for the bolt assembly/disassembly? Special roll pin punches, etc.

I've seen some guys that take a Dremel to a headspace gauge, and fix it so you can leave the ejector in.

I get my small AR parts from BCM.

[sinister]

The USAMU changes M16 competition rifle bolts usually at the end of the third barrel's life, or around 8,000 to 12,000 rounds. Any longer and you start seeing bolt lugs crack and break off.

[opngrnd]

A lot of excellent posts in this thread. I can see why some people just pop a new bolt in every 5k and keep driving. I do have a new BCG inbound on an order that was placed before I started this thread. My hope is to find out if the bolt is in spec or not. After I catch up on a few more spare parts(funny how they turn into builds), I'll grab a few gauges and see how it looks. Being that it's a spare, time is on my side. I'm still having trouble figuring out where to find the roll pins for the ejector assembly, though buying a JP ejector kit seems like it would do the job.