Mainspring Strain Screw Question

[fleabus101]

Just bought a 1947-48 post war K22.
My normal K L N mainspring screws dont
thread into this k22 well. Basically they appear
to be diff thread pitch than the ones I use
for My pre model 50s N frame main spring
screws. The usuall ones wont start at all and the orig looking screw has longer threaded body than the later screws currently widely used.. So do the WW2 & Victory frames And post war K frames use an earlier screw and pitch variation(pitch) thread type? My screw
looks like Victory long threaded strain screws advertised on ebay.
Thanks for the help..
Randy..

 

[armorer951]

I thought the mainspring screws were all 8-32, but I'm clueless.

 

[MWC2068]

I too thought that all strain screws were 8-32. Perhaps yours has been shade treed? You can take the one that is in there to a machine shop or to a millwright and have them check the one you have with a thread pitch gage, but I would like to think that you might have enough "meat" left in the existing hole to run an 8-32 tap back through it and clean up the threads so you can use a standard strain screw.

 

[fleabus101]

Im just really wondering..with the victory type long threaded type screw in this one..if these early post war frames have a diff pitch. If no one informs me diff Im gonna order a victory
screw off ebay and try it out..
The seller estatejewels4u2 sells new old stock
S&W parts in the old manilla pkgs. The strain
screws out of the pkg shows them to fit
victorys" k22s k38s and m&p types.
The 1947-48 k22 I just bought from an
older collector doesnt appear with its
long thread screw looks to be original.
So I'll try another stock long thread screw
first. Im reluctant to change the gun
prematurely.

 

[fleabus101]

Added info

I just received an orig 40s era K22, victory, etc
40s strain screw I bought online. As suspected from the auction picture, its the same longer
threaded screw and fits the strain screw threads perfectly like my orig screw. So I dont think the frames
strain screw threads are altered. I retried several current K L N strain screws and none
start at all. There seems to be a diff in early
post war and back "Made in USA" Marked
frames. At least in my 1947 era K22.
Any good info on the thread differences
would be appreciated. Its not a working
issue, just seeking info on the differences.
Thanks a Bunch.
Randy..

 

[sjb66coronet]

I don't know what type of gage or measuring device they have but i have brought some oddball screws to my local Fastenal store and they were able to identify the thread pitch for me and get a replacement.

 

[Hondo44]

You don't need a thread gauge: the first thing to do is take your original strain screw and one of the newer ones and 'nest' the threads together.

Whether they're the same screw pitch or not you'll find out immediately THAT THEY ARE!

The next thing you do is measure the diameter of the threads with a caliper. And you'll find out they're not the same size; the K22 screw is .150" and the later strain screws are .160".

What you've run into is: S&W standardized it's screw sizes.

In the early to mid 1940s a specification change was made for the standardization of tolerances for screw diameters. A change from the old S&W proprietary 'smaller minimum diameter' tolerance to the larger minimum standardized tolerance (no, threads per inch did not change). Therefore, there CAN BE a difference between 'pre-change' and 'post-change' screws and they CAN measure different diameters. Sometimes they are interchangeable and sometimes not. For example: after 3/4/42 K & N stock screws became #5-44 NF with an upper OD tolerance of .125" and usually measure .122"+or-.

As usual, the changes did not take place immediately; first, old frames with the smaller screw hole were used up.

Another little tidbit about K frame 5th screw:

• Change order April 19, 1943 to shorten the K frame bug screws (upper side plate screw; the 5th screw) to .181". It appears the K bug screw was on a different evolutionary trajectory than the other frame sizes and may explain the different K bug screw diameter from other frame bug screws during a specific time frame as well.

Hope this helps,

 

[fleabus101]

Well Hello Jim
Thanks for that primer on the strain screw
tolerences and differences you can run into.
I didnt think I had a buggered frame. 2 obivious
orig screws suggested another answer. Your
factory change standardization information makes perfect
sense having the screws in hand, and with
your info trying a dial caliper on their dimensions.
Thanks very much sharing the info I was
looking for.
Best Randy...

 

[chief38]

It is also possible that somewhere along the line someone changed the thread pitch either by using an incorrect Tap or forcing in an incorrect screw. You never know!

 

[Johnnu2]

You can ALWAYS count on the info from Hondo44; he REALLY knows his sit...

J.

 

[Hondo44]

Hi Randy,

I checked my pre war K22 which has the .150" diameter; my 1952 masterpiece has the new .160" diameter.

You were right all along, your k22 was not messed with.

 

[Hondo44]

You can ALWAYS count on the info from Hondo44; he REALLY knows his sit...

J.

Sometimes, if I can manage to type it correctly!