Of sears and bolts
All, this was originally posted in two parts on MCB, but Nobody wisely suggested posting here as well. So hang on, it's windy:
Some questions for you:
1. What exactly causes bolt nibble?
2. Does anyone know what type of stainless AGD used for their bolts, specifically, the L10? Am guessing 300 series, but that could be wild.
3. Does anyone know the RC hardness of the classic L7 sears? I'm not concerned about the cad coating, just the sear itself. It's plenty hard. A regular steel file will skate on it, unlike the L6 sears. A good file will cut, but not hungrily.
I ask this because all of my mags (classics) have bolt nibble, except for my L6 mag. That one, despite coming to me in nasty fashion, had an absolutely perfect sear, and a perfect bolt. The thing was used heavily before I got it, and heavily after, and there was never any hint that any wear was taking place.
On the other hand, all of my L7 mags have nibbled bolts. Some of them are just slightly nibbled, while others look like they've spent considerable time in a garbage disposal.
This all came from an episode a while back wherein I switched from the L6 sear to an L7 sear on a classic with L10 bolt.
Previously, the L6 sear and L10 bolt got along famously. Both mating surfaces were glassy smooth, case after case after case after case.
But there was another issue so I put in the L7 sear and, after one day of play, the L10 bolt was really chewed up, and so was the sear.
Now the irony is that the L7 sear was brand spanking new. Never used. And of course, the L10 bolt was perfect.
So I started really looking at my other mags, and sure enough, all the L7 combos had bolt nibble, from slight to great, with no exceptions, and all are running bone stock. Mags that came to me in superb condition had it out of the box.
So I decided to do some closer looking. First, I cleaned up the bolt lips. Don't worry, it was done correctly. I then cleaned up the sears. Then I started mixing and matching, but only firing a couple of shots before breaking them down.
Sure enough, in every case, the L7 sears (and I mean every single one of them) began to nibble at the bolts. Made no difference what frame, body, rail combo was made, they all did it.
However, the L6 sear, when applied first to these same bolts, produced no wear whatsoever. Cleaned up bolts stayed clean with the L6 sear. Made no difference whether it was the old short foamy bolt, long nose bolts, L10 bolt, all of them stayed perfect. But as soon as any of the cleaned up L7 sears were dropped in, they all got nibbled, even after a couple of shots.
This, ladies and gents, is what we call a mystery.
So I really began looking at the sears. Already noted (on MCB) that there were some significant differences between the L6 and L7 sears in terms of geometry. But I went back and really looked again, and the two are different in other notable respects. First, the mating surface of the lip catch is nearly flat on the L6, but is markedly convex on the L7. And second, the L6 sear is significantly softer than the L7.
I did several tests with cleaned up edges and noted that a single shot will not produce bolt nibble. But it will, however, cause a tiny tear-out in the sear. That tear-out will then nibble the bolt further, which will in turn return the favor, and so on until both are ugly again.
Now all this is on a very small scale. My guess is that a lot of folk don't even pay attention to it, so long as the marker functions. But that kind of stuff really bugs me. It shouldn't be happening on any scale. There is no good reason for this to be taking place. And it certainly makes no sense that a relatively inexpensive sear should destroy an expensive bolt. The cheaper of the two should be the 'disposable' item in theory, but in reality, neither should be a disposable item.
There is absolutely no good reason why these shouldn't be able to get a million cycles at least. This is a wonderful marker, and I don't tend to get sentimental about such things. It's just a shame that this should be a weak link.
And based on the complete lack of any wear evidenced by the L6 sear, the problem lies in some aspect of the L7 sear, either in design or in execution. The L6 sear has it's own design flaw in terms of geometry, and is a rare item besides, so that's out as a fix.
We know that even the L6 sears were consciously made harder than the bolt in order to determine wear. And it is certainly the case that the L7 sear is even harder.
My hypothesis is as follows: The L7 sear, possibly owing to geometry of the catch surface, and likely owing to the excessive hardness, is getting injured by the bolt on the return phase. The convex face was likely designed to more closely match the curvature of the inside of the bolt lip, but it translates to a lot more force being applied to a small contact area, creating a stress point.
The higher hardness means that the L7 sear is not as 'tough' as its older cousin, and under shock (as with bolt cycle) is therefore more prone to chipping than to simple plastic deformation.
My guess is that at some point in the cycle, probably on the return phase, the bolt is dropping down over that catch and causing a tiny chip in the edge. Then, when the bolt goes forward, that edge chip gets caught by the forward travel of the bolt, which then gouges up the top land of the sear. Under magnification the gouge looks like it has been violently torn out, rather than smoothly displaced. Those gouges look like dragon teeth, and no doubt are responsible for chewing the bolt, no pun intended.
What I want to do is to sacrifice an L7 sear and temper the hardness down to something similar to the L6. If I can get a precise hardness for the L6, and further determine what sort of steel is used for the sear, it will be a simple affair. Even if precise hardness or composition remains a mystery I can get it pretty close.
I will then see if the issue remains. If it does, I will then modify the catch geometry to match the L6. If that works, I will then modify the geometry on a fully hardened L7 sear and test again in order to isolate the problem.
What do you all think? And please, let's not have a chorus of voices warning against messing with sears. I am going to mess with some sears, rest assured. It will be done carefully, but it will be done.
I'd appreciate your wealth of knowledge to ensure it goes (more) smoothly.