Where to draw the line with force? (.156 pin and .154 receiver bore)

So I finally got around to slapping together my White Oak upper and scope onto one of the PSA lowers I recently picked up (thanks again, MDS member rfawcs!) and it was with no mean amount of glee which I pulled the “G” High Speed Natl Match (HS NM) trigger assembly off the pegboard rack above my bench. I mean, it’s only been hanging there since this past March …

Everything is going smoothly that is until … wait a minute …. Unfortunately, the pins that came with the assembly are intentionally oversized and measure .156.

The trigger and hammer pin bores in my receiver measure .154 and the pins that were in the milspec PSA assembly I pulled out of the lower measure .1545 - nevertheless, they go in and out snug but, with little actual force required. Hammer and punch taps yes but, no “pounding” blows.

At the point I was beating on the .156 steel pins with way, way too much force and they still were not going to go into the .154 hole in the aluminum I stopped and called “G”

The guy that answered the phone confirmed the pins they include with their High Speed Natl Match assemblies are even more oversized than the oversized pins they include with their other target, match and DMR assemblies … he said he’d gladly go through a mess of pins in front of him and find two that are .155 and send them to me.

But an hour later, I get an email from another employee, equally pleasant and no doubt way more experienced than I, who actually suggests I use whatever force necessary to drive the .156 steel pins through the .154 holes - in an aluminum receiver …

At this point I’m crossing my fingers the first employee actually follows through on his promise and sends me some pins under .156 diameter

I mean, I think (hope) each of us has at some point in the past realized, “Even though ‘as snug as physically possible’ will benefit whatever it is I’m assembling, I am at a point right now where one of the two parts I am trying to smash together is going to be destroyed!”

Q - have any of you ever encountered “pin to receiver bore” conflicts that caused you to take a “timeout” and find other pins?

Just curious if any one has an alternative I may be missing….

Sometimes a bigger hammer is not the answer. I would have visions of bent/cracked receivers, a ridge on the inside of one hole causing drag on the trigger with pins you might not be able to get out to remove the ridge. I would ream the receiver holes for the larger pins or use the smaller pins. The hammer hook and sear face will settle on one another.

Why not buy a set of pins from Brownells or Midway or Midwest?

I'm going into my parts box and digging out standard pins. I'm trying those pins in the receiver and then I'm trying them in the FCG. If they are not compatible with one or the other, I'm digging out another lower to try and find out who(what) is out of spec and then proceed from there. Caveman approach.

Magnumite said:

Sometimes a bigger hammer is not the answer. ….

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Great minds, ha. And moreover, it’s occurred to me there is a point of diminished returns, that is, once a pin is beyond, “that tight” (or, said another way, “so tight”) there is no mechanical (or accuracy) advantage to a pin being in a bore “too tight.”

John from MD said:

Why not buy a set of pins from Brownells or Midway or Midwest?

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Well, for starters the fire control group in question runs in the neighborhood of, if not north of $300 so, I am motivated to allow the mfr an oppty to send me a set of pins closer to .155 but, if that first employee happens not to follow up on his assurance he would send me some .155 pins then, I can either go the Brownells or MGW route but, before that, I have probably on the order of 3 dozen pins I can pull out of other rifles or out of the box of milspec fire control groups I’ve replaced over the years in the likelihood I’ll find what I need. What outrider refers to below as the, “caveman approach,” which both describes and fits me perfectly

The way my time is, I believe it will actually be quicker to wait the 5 or so days the mfr employee said it will take for pins he will measure and hand select for me to get here though …

Putting an oversize steel pin into a relatively thin aluminum plate is not a good idea. Google the term interference fit. If you are having trouble getting the smaller pins in, try heating the receiver and freezing the pins.

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Try putting a little hair around the holes? Just kidding

Are they out of spec for the trigger or just out of spec for the receiver.

Well, assuming “spec” is .154 (right?) the pins are out of spec for both.

No thanks to anyone at Geissele, I wound up obtaining a set of slightly over-sized (~.155) anti-walk pins that went in without using excessive force ….

I have yet to talk to anyone that isn’t in disbelief Geissele wasn’t willing to simply replace the .156 pins (and it appears they may actually be in excess of .156!!!!) … instead, they just keep (kept) insisting, “You just gotta beat on your receiver harder!”

The two gunsmiths I took the pins and receiver to both shook their heads and concurred: “You let any jackass over the phone try and berate you into beating those over-sized .156 steel pins into those .154 aluminum holes and you deserve everything you wind up with!”

Now I can say, I have encountered the Geissele customer service, “we-walk-on-water wall” that I’ve heard so many other shooters (competitors and military team shooters) describe over the years:

“Geissele:
- first rate design
- second rate execution
- Third World customer service.

I own - and have installed at present - a half dozen other Geissele triggers (mostly SSA’s) but, paying close to $250 for this High Speed National Match has been a real eye opener. No more Geissele for me; any one of the four LaRue MBST-2S I received last month - for $87 @ - is every bit as good a trigger as the Geissele HSNM, for CMP 4-1/2lb reqt.

Oh well, what’s the saying:

“A le$$on bought is worth two lessons learned!”

A little snug is great but you definitely don't want them so large that you have to pound them in.

I'd have chucked the oversize pins in a drill and polished them with oil and fine grit sandpaper until they fit properly.

You can do a force calculation. I think your somewhere just below 43,000 PSI with the 2 thou interference fit with those pins. The problem you will have is you are most likely going to permanently deform those holes and be forced to always use .156 pins. Normal fit for alignment purposes is 0.0005 inch/inch of diameter. Most people just go with 0.0005 and call it a day rather than do any calcs. They will have tolerances and a 1 thou in/in isnt horrible. Once you start getting into 2+, things start to happen. I would say 2 thou on a 0.154 hole & pin is excessive. Remember everything has a tolerance and acceptable limits. I dont know the materials and just guessing on some numbers. I am also running dirty calcs in a spreadsheet off the top of my head.

I have had the pleasure of doing some firsts for companies with interference fits on large diameter shafts. I had a guy say oh, we do this all the time and completely fudge up a shaft that took almost a year to manufacture and cost $250k, Germans engineers hate to say they dont know or are wrong. All because they didnt do calcs and just assumed it was like any other install. I then had to go before the CEO's of both mine and our customers companies and a full assortment of engineers to explain how it will work and provide a detailed engineering review. I got to supervise the next attempt with instructions of, "You got one shot, dont fudge it up." I had a test shaft made and did a test run with the exact staff who was going to do it. We took it on and off without issue following my exact instructions. We got it on and the team lead said, "Thank god. Now we can do it for real." I said, "Nope, lets take it off and do it again." His eyes went wide and we did it again without issue and the shaft was perfect. We got it on without an issue the day of.

I'll just sneak this in here, AR hammer and tigger pins are designed and are required to rotate freely within the receiver(OP not withstanding as he is dealing with an unusual problem).

To throw something else in the mix. slightly undersize pins allow the trigger components to align under spring pressure compensting for slight misalignments in the receiver.

FWIW, I destroyed a 1911 hammer once trying to coax through a large pin in the hammer strut pin hole. It broke when I went to remove the pin after it went in a short distance. It was not a cheap hammer.

outrider58 said:

I'll just sneak this in here, AR hammer and tigger pins are designed and are required to rotate freely within the receiver(OP not withstanding as he is dealing with an unusual problem).

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THIS. Anti rotation trigger pins are a problem waiting to happen.


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In order to create the most consistent meeting of the trigger/sear interface, pin clearance/slop in the hammer and trigger pins must be reduced to minimum. As trigger pulls get lighter and shorter, alignment becomes more critical, so makers of light, short triggers supply pins designed to eliminate slop.

You won't notice the inconsistency created by this slop in a 7# GI trigger that feels like you're pushing a truck sideways across a gravel road, but you will in a 3# single stage that moves about 1/16th of an inch to break.

Many rifles have hammer/trigger pin bores that are worn and go a little big. There is also variation in pins and a bore can be slightly swaged up, or a small amount of the anodized layer can be scraped off by slightly large pins or inept alignment. A custom trigger approach to "one size fits all" and delivering best performance is to use a pin size that is large enough to overcome the additional clearance of a worn/swaged/scraped pin bore to ensure consistent alignment of the trigger/sear surfaces. Hence the 0.156" pins.

Yes, they go overboard with too-large pins on a cherry receiver, but if you go to a smaller pin in response, you bring back the additional clearances between the trigger assy bore and the hammer assy bore, losing your *most* consistent trigger pull and potentially whatever performance level is specified by the trigger maker. This is why the makers have a deaf ear to using smaller pins.

I would take exception to their CS advice to just force it (never a good option), and use the correct sized reamer to mate the pins bores to the pins. A $300 trigger is worth more than most lowers. If you are going to dedicate a lower to an expensive match grade trigger, it makes more sense to ream the receiver bores for a light interference fit to the manufacturer-supplied pins.

Yes, I realize one would normally modify the least expensive/most expendable part (pins), but think this one through...

E.Shell said:

In order to create the most consistent meeting of the trigger/sear interface, pin clearance/slop in the hammer and trigger pins must be reduced to minimum. As trigger pulls get lighter and shorter, alignment becomes more critical, so makers of light, short triggers supply pins designed to eliminate slop.

You won't notice the inconsistency created by this slop in a 7# GI trigger that feels like you're pushing a truck sideways across a gravel road, but you will in a 3# single stage that moves about 1/16th of an inch to break.

Many rifles have hammer/trigger pin bores that are worn and go a little big. There is also variation in pins and a bore can be slightly swaged up, or a small amount of the anodized layer can be scraped off by slightly large pins or inept alignment. A custom trigger approach to "one size fits all" and delivering best performance is to use a pin size that is large enough to overcome the additional clearance of a worn/swaged/scraped pin bore to ensure consistent alignment of the trigger/sear surfaces. Hence the 0.156" pins.

Yes, they go overboard with too-large pins on a cherry receiver, but if you go to a smaller pin in response, you bring back the additional clearances between the trigger assy bore and the hammer assy bore, losing your *most* consistent trigger pull and potentially whatever performance level is specified by the trigger maker. This is why the makers have a deaf ear to using smaller pins.

I would take exception to their CS advice to just force it (never a good option), and use the correct sized reamer to mate the pins bores to the pins. A $300 trigger is worth more than most lowers. If you are going to dedicate a lower to an expensive match grade trigger, it makes more sense to ream the receiver bores for a light interference fit to the manufacturer-supplied pins.

Yes, I realize one would normally modify the least expensive/most expendable part (pins), but think this one through...

Click to expand...