Replacing Kahr Rear Sight with 1928 Lyman

[rob boward]

I see where E-Sarcoinc.com is selling Lyman Thompson sights for 174.95. Is this an easy swap job a guy could do at home?

[emmagee1917]

NO !!!!

You'll need someone who is a Thompson expert to fit the proper length rivits to your sight / reciever thickness and use a proper rivet setting press . If you don't do this you'll be like me and continually be making trips every few years to the local gunshop to have it redone because they pulled loose. Be smart , spend a little extra now , and save the headache.

 

PS , Sarco was running a two-for special. If you can team up with another here , you can save some $$$$ ( if they are still doing it ) .

Chris

[SecondAmend]

Depends on the vintage of the 27A1. Older ones, as noted above, have riveted in rear sights. I have one from 2011 that has the rear sight retained by screws and torque retention compound (e.g., LocTite). I've considered swapping the one on the gun for a WWII vintage Lyman ladder sight or L-sight wth protector ears as used on the M1A1 as the ladder is sort of wishful thinking for .45 ACP ammo. Maybe someday when I have the cash to buy the replacement sight.

[rob boward]

will the holes line up ? no drilling?

[fxdrider]

From what I've read, the holes are in the same position on all of the Thompson's, and so will line up. If your current rear sight is riveted, you can either have the new sight re-riveted, or tap the holes for screws like Kahr uses on their newer production Thompson's.

[ithacaartist]

Getting a kick out of this thread because, as a yearling Newbie to anything Thompson, It's the first time on this forum I can say, "Been there, dont that."!

Here's the scoop. I bought a 1927A1 Deluxe last summer. One of its disappointments was the current production factory-installed rear sight, which had no adjustment for windage. This was a big deal because mine shot waaaay to the left of point of aim when using the standard cheapie ladder.. I came upon the Sarco deal and immediately ordered one. (Um, then I heard about their "two-fer"--alas, too late.)

The situation when it arrived was as described above. The Lyman repro's holes line up with the tappings, OK. The sight base is drilled for rivets, and the receiver is drilled and tapped for, iirc, 1/4' blued oval head screws with a 24 tpi pitch. I continued by merely drilling out the smaller rivet holes on the base to a precise clearance hole for the screws' overall diameter. Next, it was necessary to countersink each hole to the proper depth and angle. Since the factory screws have a bit of depth to the head below the curved top, it was then necessary to counterbore the holes. This is what was done to the factory cheapie, and you should make sure that the radius of the screw's head ends flush with the surface of the base, with basically no hole showing around it. De-burr the holes on the bottom of the base and check for it on top of the receiver, and use a heavy-duty Loctite as you snug, then tighten all the screws. Flip your receiver and check inside; remove any screw material that is proud, if necessary. Give the flush ends of the screws three little doinks with a center punch, spaced as evenly as you can around the juncture of the threads of holes and screws. Carefully smooth off the material kicked up by the punch. If I haven't left out a step, I think you're ready to try it out.

The reason I mentioned the thread pitch above is for those folks who have a riveted-style receiver and wish to drill and tap it, in addition to modding the sight base, to receive the factory screws. Better buy a tap for them! I suppose one could install a 1/4-28 and proceed as above, but with a standard tap. (The more I think about it, I'm sure I have the screw wrong in D. and pitch. But it's late, I'm tired, and I'm not gettin' the gun out. You please look it up... The general arrangement goes like that, anyhow.) In anticipation of buggering the heads of the screws, I ordered a new set from Kahr. When installed at the factory, the screws are locked in place by deforming their tips with a punch after they are ground flush with the inside of the receiver. Those babies are really in there, to say the least. It occurs to me now that the tips could have been carefully drilled out, which would have meant a lot less wear and tear on the holes; so I offer this idea to you now. I assure you, the screws will turn out. It's just hard, hard turning. Use a screwdriver that's properly sized, but the extreme forces will overcome even this advantage and the slots will suffer, regardless. Even if the slots weren't deformed, or you didn't care if they were, you'll never get the old ones to start back in, trust me. Just buy new screws from the factory.

Equipment for the job is minimal, and a guy could do it in a home shop .I guess it would depend on an individual's skillset and inclinations.The transition from metal fabricator to Bubba gunsmith seems to have been within my grasp, at least this time. A riveting job is something that also would not scare me off. If you're qualified, you already know it. And you know your limitations, as well.

Edited May 14, 2013 by ithacaartist

[rob boward]

Thanks for the help guys.I have ordered the sight . wish me luck.

[SecondAmend]

As posted on the FA Thompson forum:

 

http://www.machinegunboards.com/forums/index.php?s=20287a0dd6960af5a9664be4beadd581&showtopic=15452

[SecondAmend]

I did buy one of the sale 1928 WWII vintage "hand picked" Lyman ladder sights. I wanted the windage adjustment as the 27A1 I have shoots to the left. The sight is very nice. Everything was very stiff so I soaked it with penetrating oil for a couple days and it loosened up. I plan to sight in the carbine with the new sight tomorrow.

 

However, I did not replace the whole sight. Rather than go through the drilling, tapping, and more drilling, and staking to swap the whole sight, I only swapped the ladder subassemblies. The swap itself is rather straight forward, though the '28 pin was still pretty tight even with soaking. One thing to watch for is that the plunger on both of the sights has a pretty strong spring and will shoot the plunger a fair distance if you don't control it.

 

The detent operation is different between the Kahr sight (plunger with flat nose and mating flat section on cross shaft) and the 28 sight (plunger with pointed nose and mating groove in the cross shaft). To get the ladder to set at the vertical position, I left the Kahr plunger in place and filled the 28 cross shaft groove with epoxy to produce a flat mating section of the cross shaft.

 

Not as elegant as swapping the whole sight, for sure, but it gets me the windage adjustment as desired.

[58healeybn6]

I recently purchased just the Lyman rear leaf or tang leaf if you want to call it that. I drifted out the pivot pin on my 1927 and carefully removed the Kahr tang leaf, making sure not to let the plunger and spring go. The Lyman leaf fit perfectly and the pin was exactly the same diameter. The only difference is the plunger in the sight body. As has been stated, the Lyman leaf has two detent slots for the Lyman plunger and the Kahr has flat surfaces. I miked the Kahr plunger and turned a new one of the same diameter a little bit longer. An original Lyman plunger will not work, as it has a cross pin for alignment, which will not fit the Kahr sight body. The end of the new one needs to be beveled at a 45 degree slant across the end of the plunger; this will correctly orient the sight in either up or down position. Other than that, the fit was perfect and I now have a correct 1928 rear sight.

[Weasel]

You don't have to over engineer a simple replacement of the sight. When I built my semi 1928a1 SBR , I used 8p box nails, ground the heads down, cut to a length that would work. I used a piece of 1/4" steel plate for a bucking bar and pinned the inside into the recess in the receiver. They are there to stay.

I posted a picture of the job on Weaponsguild.

 

Weasel