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Discussion Starter · #1 · (Edited)
This turned into a much longer post then I intended, but I thought it best to include as much information as possible because of the highly unusual combination of LEM, V1, new style and old style parts into the same gun. This is geeky, sorry for the long read.

I'd like to convert this USPc "custom" LEM with ambi levers to V1 configuration. So I bought all the parts appropriate to convert a regular LEM into a V1, but because of the hybrid configuration of these USPcs, that turns out to be several parts too many. This may be where my problem lies.

I'm getting ahead of myself. The problem is that when I cock the hammer into single action and pull the trigger, the hammer slips off the sear and then catches on something else, causing the trigger to want to return home under double action spring tension. Continuing the pull past the break (trigger now touching the frame) will finally release the hammer. Something is causing the mechanism to revert from cocked single action back into double action immediately after the hammer slips off the sear, but before the hammer is released to fire.

This happens only if I pull the trigger just far enough to slip the sear and then let the trigger return forward with the hammer. If I pull the trigger quickly, it'll bypass whatever this hitch is and the hammer will drop in single action (but the trigger will bottom out on the frame). If I pull the trigger slowly, the single action sear will release the hammer onto this double action hitch, and pulling the trigger further will release the hammer (feels like the very last bit of a double action pull). The trigger seems to function normally in double action.

I'm no stranger to pistol guts, in fact I've seen something similar happen in a P99, but the solution was replacement parts. I'm hoping I just missed something.

I did much research before buying parts and am aware of the difference between "new," "old," "LEM" and "V1" parts. Here's a picture of the parts I've got. On the left are the replacement V1 parts. On the right are the LEM parts that came in the gun. I am reusing the flag, flag spring, hammer axle and pins from the LEM because they look identical to V1. Please note that the replacement V1 hammer, catch and sear were advertised as "old style," but all the other parts are presumably "new style." I also have a firing pin block spring to accompany the new parts, but I discovered this problem prior to installing it, while function checking the assembled frame.

Anybody got a clue as to what's going on?



 

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How confident are you that your "old" V1 sear is a V1 sear? I ask because it doesn't look exactly like the "old" V1 sear shown on HKparts.net. The minor differences may not matter and could be due to minor changes/updates to the design.

https://www.hkparts.net/shop/pc/catalog/view-2-sear-old-style_1961_detail.jpg
Bumper Auto part

Other then the possibility of an incorrect sear, I'm at a loss. I've converted a number of USPs to LEM and then back to V1, and I've never run into the issue you've described.

I will be doing the same LEM to V1 conversion when my parts arrive from HK. I'll let you know if I run into any similar issues when I perform the conversion with the following "new" HK parts:

Hammer #215692
Axle #214774
Sear #229882
V1 Detent Plate #214949
Hammer Spring #215694
 

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Discussion Starter · #3 ·
How confident are you that your "old" V1 sear is a V1 sear? I ask because it doesn't look exactly like the "old" V1 sear shown on HKparts.net. The minor differences may not matter and could be due to minor changes/updates to the design.
I did notice that, but there are three HK V1 sears on HKParts, the third one here appears to be the version I have:
https://www.hkparts.net/shop/pc/Sear-Complete-For-HK-Pistols-Old-Style-198p2689.htm
https://www.hkparts.net/shop/pc/Sear-Complete-For-HK-Pistols-198p1173.htm
https://www.hkparts.net/shop/pc/Sear-Complete-For-USP-9-Old-Style-p16991.htm

I did not buy from HKparts, but my sear is packaged: "Heckler & Koch, Sear, USP9, Made in Germany, 214180." Using that number I found the same sear elsewhere:
HECKLER & KOCH USP SEAR, USP 9MM | Brownells
Heckler And Koch USP 9mm Sear "Old Style"

If I can't figure this out, I'll try the sear you linked.
 

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I did notice that, but there are three HK V1 sears on HKParts, the third one here appears to be the version I have:
I think you are correct. I missed that one. I can't think of a reason why the parts you have wouldn't work.
 

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Were you able to resolve your issues with your USP Compact?

I received and installed my V1 parts from HK on Saturday. I had no issues with the new parts and my Classic Firearms USP Compact functions fine as a V1 DA/SA.
 

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Discussion Starter · #6 ·
Were you able to resolve your issues with your USP Compact?

I received and installed my V1 parts from HK on Saturday. I had no issues with the new parts and my Classic Firearms USP Compact functions fine as a V1 DA/SA.
Oh yes, I forgot to post an update, thanks for the reminder.

I bought a "new style" hammer, a "new style" sear and used the "new style" catch that came in the LEM USPc. The V1 DA/SA conversion works normally now, must have been something wonky with the other ones. Judging by what I was feeling, I think the trigger bar was catching on the right tine of the hammer after the sear slipped off the left tine of the hammer. Interestingly I discovered that there are two trigger bars as well, one LEM, one standard. My pistol has the LEM trigger bar, so this trigger bar may just be incompatible with the "old style" parts I tried to use.

https://www.hkparts.net/shop/pc/Trigger-Bar-HK-USP-Compact-9-40-LEM-261p16326.htm
https://www.hkparts.net/shop/pc/Trigger-Bar-HK-USP-Compact-9mm-40-S-W-p2266.htm
 
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