1911A1 firing pin stop question


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Ifishsum
May 31, 2008, 02:43 AM
I'm having to replace the firing pin stop on this Colt GI 1911A1 and have some questions for you 1911 experts - I ordered an STI series 70 stop and I'm in the process of fitting it to the slide. The replacement part is quite a bit thicker than the original piece. I'll be able to fit it into the slide as the steps on the edges are about the same, but it does not appear that rear of the firing pin will protrude far enough out the back to be struck by the hammer hard to fire. The original piece appears to have been filed or otherwise thinned to some degree. My main question is - do I have to also reduce the thickness of the new part as part of the fitting process, or is this an indication that my firing pin is out of spec? And if I do have to make the stop thinner, what is the preferred method?

Pictures of the FP stop and the firing pin, the original is on the left and the new STI stop is on the right:

http://home.comcast.net/~ifishsum/stop.jpg
http://home.comcast.net/~ifishsum/pin.jpg

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Drail
May 31, 2008, 04:08 PM
It's hard to tell by looking at a photo but that pin looks short. If it really does not extend completely through the new firing pin stop it is almost certainly too short. Try a new firing pin. The old stop definitely looks altered. I have no idea why someone would do that but it does look like a hack job. The firing pin needs to come through the stop and have about 1/16th in. protruding towards the hammer. I would also replace the firing pin spring in case they cut it also.

1911Tuner
May 31, 2008, 04:54 PM
The old stop isn't hacked. It's just beat up a little.

STI's stop is close to Browning's original design on the radius. It works to reduce mechanical advantage in cocking the hammer against the mainspring, and induces a little extra delay in the slide's rearward cycle, and slows its speed in recoil.

EGW offers stops with square bottoms that allow the installer to set the radius and the slide delay to suit the application.

Every pistol that I own has a very small radius...smaller than STI's and a little smaller then the original.

The "new" redius is specified at 7/32nds. The original was .078 inch...or 5/64ths. Mine are all 1/16th or a little smaller. I don't worry too much about what it is precisely.

Here's a picture to compare. One is an original that was on an old Colt Commander. It's radius is roughly 5/32nds inch. It's the one on the right. The center stop's radius is .075 inch...or just a bit smaller than the orginal spec.

http://i40.photobucket.com/albums/e243/1911Tuner/Stops2.jpg

Jim K
May 31, 2008, 05:04 PM
I wonder a bit about the advantages of various contours on the bottom of the slide stop. Most seem based on the idea that the slide moves gently backward, riding the hammer down, the way it works when the slide is retracted manually.

But in firing, the slide hits the hammer a fast, hard blow, driving it down and completely out of contact with the firing pin stop. I fail to see how this slows the slide after the hammer is out of contact, though it might have some effect initially.

Jim

1911Tuner
May 31, 2008, 05:37 PM
Jim. Yep...but it's got nada to do with how the hammer cocks...which does get slammed back, despite what any naysayers claim. It gets slammed hard, too. Pretty violent action.

Its effect is in delaying and slowing the slide and giving the bullet a couple nanoseconds longer to reach the muzzle before the slide and barrel reache the linkdown point.

The radius was changed early...effective January 1918...in response to the conscripts' complaints that the slide was too hard to hand-cycle with the hammer down...so the radius was made larger in the attempt to make things a bit easier for the poor lads.

And...as with many of the other "improvements" made...something was lost in the translation. So, now we seem to be coming full circle as many pistolsmiths are going back to the small radius stop.

My own testing has shownstrong indication of the advantages. Shock bufs that shred within 1200 rounds are still in good shape and still functioning after a little over 3,000 rounds of GI hardball equivalent ammunition. So, it's apparent that something positive is happening.

I don't much care for shock buffs, by the way. It just seemed like they'd provide the simplest testing platform.

Jim K
May 31, 2008, 06:05 PM
Well, that is what I don't quite understand. At firing time, the flat surface of the hammer is resting against the flat surface of the FP stop. The first blow to the hammer (not a gentle push) knocks the hammer back before the slide has moved more than about 1/8 inch, before the point where, in manual operation, the bottom curve of the FP stop reaches the hammer. From that point on, the hammer doesn't even remain in contact with the FP stop. So I fail to see how the shape of the bottom of the FP stop affects the hammer-slide interface much if at all.

Maybe with light loads, where the blow is less, the hammer would not be hit as hard and a different shaped FP stop might have an effect, but the high speed photos I have seen were all done with GI ball.

(BTW, the hammer being kicked free explains another "mystery", the missing firing pin. The slide strikes the hammer so it flies free while the firing pin is still forward, leaving nothing to hold the FP stop. Since recoil has begun at the same time, the stop tries to stay in place while the gun moves upward. Since the FP stop is not retained by the FP, and the hammer is not in contact with the FP stop, the FP stop is free to move. If the timing is just right, the shooter ends up with the FP stop dropped down, a jammed gun and maybe a firing pin laying on the ground.)

Jim

Ifishsum
May 31, 2008, 06:36 PM
Thanks for the input. I finished fitting the stop and the firing pin protrudes, just not as much as it did with the original stop. The pencil test works, the first time it hit the ceiling so I'll just try it as-is and see what happens. I wasn't worried so much about the bottom radius as whether the firing pin would protrude far enough through the stop.

So the radius on the new stop will likely slow down the slide's recoil a little? Is that a good thing?

1911Tuner
May 31, 2008, 06:44 PM
Leverage, Jim. Leverage and mechanical advantage. The contact point is lower on the hammer, placing the force closer to the fulcrum, requiring more force to overcome the mainspring's resistance.

Once the hammer is cocked, the effect is over, and the function between the two stops is the same.

The best demonstration is to install one along with a new standard 23-pound mainspring...to get the maximum effect...and hand-cycle the slide with the hammer down. It requires noticeably more effort to get the hammer cocked with the small radius.

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