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PF45 Rear Rail & Trigger Housing Anatomy

Contribution by Andy & MGB

Seems I'm not the only one that's been having trouble with my PF45 build. Since I now have two destroyed frames, and since hard data are so hard to come by, I did some surgery with a coping saw.

If disembodied frame parts make your stomach a bit queasy, stop now. Inspector 45 will grin at you in her sleep while you leave. 😋's tonight's cast:

Please note that the rear rail has already gone through the full "Rear Rail Fixes" process, including slightly enlarging the 'upper' (left side) pin hole. MGB has since modified his recommendations for fitting. This stamping was thinned a bit to fit into the frame on the left, but as you will see, it doesn't fit into the 2nd frame I worked with.

The rear rail stamping (RRS) is tight on the front and rear 'straight' areas (marked in yellow), but is not in contact with the shoulder in the trigger housing pocket in the top purple (Crayon, USMC Grape, One, Each) zone. The RRS does not fit all the way down into the pocket and contact the shelf in the frame - and it shouldn't if the pin hole is in the right place, especially if the top of the rail is even with the plastic on the rear of the frame. But it has to be lower than this one currently is.

Save the yellow zone for later - that's a relief for the trigger housing to settle into.

In the Rear Rail Fixes vid, MGB shows how to smooth the curve of the RRS. Without good data on how the rail is supposed to fit, and since it's nearly impossible to see the fit when looking into the top of the pocket, it can be difficult to know how much to take off. I assume (there's that word...) that we should work the curved part of the RRS until the rail stamping is snug, but so that the holes line up, and so that the top of the rail is level fore and aft and side to side, and down 'far enough' (though I really don't know how far down 'far enough' is).

Let's look at the upper area of the trigger housing (TS) and the corresponding areas in the frame. First, both sides of the TH have a fairly wide 'shoulder' - and the shoulder is the same size on both sides. These THs are a 5357-1 and a 5357-2 - they're identical.

On the right side of the frame, the notch in the shelf on the right side of the frame 2nd picture up) is large enough for the TH to fit. The left side notch, though is too small.

The 5337 trigger housing is reported to be different for the -1 and -2 variants. Mine are the same. Neither will fit into the notch in the PF45 frame. To get these to fit into my frame, I had to bevel the side of the TH:

And I had to carve an area of the shelf in the frame.

It's ugly but it worked...and since this frame is also cut, it doesn't need to get to the 'pretty' stage.

(Note that the shoulder on both sides of the frame is square, but the curve face of the trigger housing is beveled. It would be cool if the folks at Polymer80 could bevel both sides of the frame to match the TH, and also make the notches the same size and large enough to clear the top of the TH.)

The rough zone on the rear rail stamping we need to trim to fit is roughly from the arrow down:

I got a good fit by working on the front, flat, side. The curve fit the frame correctly.

I have a new frame kit ordered. I'll see if the above observations from the two frames I have (ok, the pieces 😢 of the two frames I have) fit when the new frame and rail stamping arrives.

Finally, here's a simulated piece of 2x4: ============ If any of the above is wrong, please note it below (whacking me with the board to get my attention if necessary) and I'll fix my errors.


(I'm a retired AF SNCO. While crayon jokes are expected, it doesn't mean I wouldn't call in an A-10 CAS mission if there was a Marine stranded somewhere. Unless he got there to find grape crayons, that is - then all bets are off. 🤣)

Edit...thoughts on working the rear rail stamping. As noted, I already ran through the early MGB process for prepping the rear rail - edges polished, good angle on the 'ear' that locates the trigger housing, and enlarging the left side hole so the pin would clear. The rear rail fit well enough but did require pushing down on the left side a bit to get the pin to clear. The problem with the early recommendation is that it's not enough to simply polish the curved section. I found it not to be necessary at all on my rail as the entire curved side matched the shape of the frame.

I marked the edge of the rail stamping with black sharpie ink. The actual high areas that were keeping the rail from fitting properly were on the front (flat) side of the rail, not on the curve. The only area where metal was a bit out of place on the curved side was down at the lower bend. As the first 7/10" of the 'curved' side of the frame pocket and the rail are flat, it's easy to take the extra metal off with 10 or so strokes on a sharpening stone. On the front, I found that the bend area around the 'ear' to be high, and found that the left side of the stamping was a bit wider than the right. The process I'll use when my replacement frame arrives is:

  1. Make sure rail assembly is square - bend the corners until both are square

  2. Remove high metal on curved (back) side of rail down at the bends for the 'cross bar'

  3. Spend the most time on the flat front face of the rail stamping - remove metal there until the rail drops into place easily

  4. Polish the edges




(The high area around the bends were on the inside half - the stone didn't touch the outside half, though the black ink is hard to see.)


Looks like the same info we've been working through since at least Feb 7, 2020 when I pointed out how to trim the trigger housing to fit.


Thanks, juslikehilo. It is a decent vid about the trigger housing. As we can see from the dissection photos I posted, though, there is no reason to take any material out of the central area of the trigger housing. The problem is the corner, and that much shorter 'shelf' area on one side of the frame. Clear that shelf and the housing drops in perfectly.

The problems with the rear rail continue. In earlier videos, there's a reference to re-shaping the curve. What I learned from the parts is that most of the trigger housing pocket in the frame has straight sides. Seems we still want the rear edges of the rail stamping to be straight, but the ones I have do need to be narrower. It might be easier to use a flat surface and take some material off the front of the rear rail stamping. I'll have to try that.

I updated the initial post with rail work. The curved area fit the shape of the frames perfectly - the lower 7/10" of both are flat, and then the curve starts. I was able to get a perfect fit by working the front, flat, side of the rail on a stone. It's a LOT easier to work the flat side by hand than to risk altering the curve on the back with a Dremel.


MGB says:

This is such a great discussion 👍. Thank you Andy for jumping down the rabbit hole and finding just how deep it goes. I’ll add a couple of things as I believe in sharing information as much as we can to make sure we are nailing down root causes.

First, this issue is a series of things that compound on each other to form a headache for builders and even ones in my network that have built dozens.

With these issues we are forced to treat symptoms and create work arounds, the problem of course when we treat one symptom it often leads to others that need attention as well.

  • The trigger housing is its own issue

  • The rear rail is its own issue

  • The trigger housing and the rear rail combining to fit into the frame and work in unison properly is yet another.

Simply because we can make the part fit doesn’t equal long term function. Getting the rear rail to sit and accept a pin is only half / it still needs to function and most importantly be Safe!!

What’s happening with the 45 is the frame is an absolute mess. I do not have access to drawings, but I have tested with over 2 dozen and am confident that the engineers have seriously miscalculated this mold. Maybe due to a rush to bring it to market, but we should be getting better as new releases come out and not going backwards. The last few releases (the CL, PF45, and the PF9SS) all share common mold problems. The CL was an absolute disaster - the worst blocks and rails by far. Enough of my rant. Lol

Okay for the 45 - before you get to the trigger housing you have to do the rear rail. I saw Andy you talk about shaving the front. The rear rail misalignment typically comes in 3 forms:

  1. The rail sits to high

  2. The rail is showing metal to the left of the hole.

  3. The rail is showing metal to the right of the hole.

Typically you’ll have a rail that is 1&2 or 1&3. That’s what really decides whether you shave the front of the rail or work the arc.

For number 1 we shave the bottom to get it lower and sometimes the top as well. The 45 rails specifically however have an abundance in the system that I deem unsafe. They require too much shaving and loss of material to structurally be reliable. In those instances they need to be replaced. I’ll include a photo in the next comment for you all to visually see those. Staying on number 1 here - we’ll say that you were successfully able to treat the system and get it to sit down. The other problem you will have is the locking blocks are like ramps on these and will be forced to sand to level with the rail. Because the blocks are so bad and you’ve already trimmed the bottom of the rail - well now you’ve essentially lowered the profile of the lower so much that the slide is going to sit down on that frame too much.

So here we’ve treated two symptoms (1) lowered the rear rail to sit and line up with the pins (2) leveled down to an even plane with the front block. We lowered the profile ^^^^^^^^ Remember This ^^^^^^^^

Now you attack the trigger housing. It’s not sitting in there because as we know the mold is incorrect on the left side not allowing it to sit down. So we sand down the left side and Andy the reason why I instruct users to come over to the middle in the video is because of the two dozen rails P80 sent me - half of them had an improper arm angle to secure the trigger housing. That’s an entirely different discussion but it’s why you see you many people with trigger reset issues and twisted ejectors on the 45. Coming over slightly resolved the alignment issue and I don’t have to make 5 more PF45 rail videos. Lol

So you sand it down and get it in and you are feeling great. Then assemble to only find out you’ve now opened another can of worms. Because we lowered the profile and manipulated all these parts to sit into an incorrect mode you are going to hit more symptoms that will need to be addressed.

The slide now sits on this frame lower so we could get:

  1. Ejector hitting the backplate

  2. Shark fin of the trigger bar scraping across the slide

  3. Shark fin scraping the safety plunger

  4. Block kit clearing the barrel

  5. Over a 100% sear engagement

  6. Slide stop that doesn’t release

  7. Recoil spring bottoming out on the channel

and I’m tired there is more. Lol 😂 Moral of the story when we are forced to chase systems more will come up that we have to treat.

There continued discussion on this topic if you are interested in reading here

Have a build question or just want to hang out with other freedom loving 80% Junkies? Come on over to the MGB Forum!!!

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