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Universal scope mount drillings

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Iamscotticus View Drop Down
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    Posted: Mar 26 2020 at 11:00am
Hi forum.

Im wanting to attach a Weaver M 48409 side mount to a Universal US Carbine M1.
The rifle is a Universal SN 460784.
The rifle has the side mount holes drilled and tapped from Universal with the plastic cover.  
It has the dual spring rods and slide lock back toggle switch.
The rifle has never had a mount installed and in very good condition, shot very little if any.
The holes in the receiver and the mount align.
The Weaver side mount screws did not thread into the receiver by hand and appear to be slightly too large to fit.
I measured the inside holes with a digital caliper and got an average of .135.
I measured the screw diameters and got an average of .16.
The tips of the screws range from flat ground to slightly beveled.
I couldn't get any screw to start after catching the first thread.
I have some tap sets I can attempt chasing the holes.
Does anyone know what the thread pitch should be?

Any advice?
Thanks in advance
handy little rifle needs upgrades
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jackp1028 Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 11:34am
Hi and welcome to the forum from New Mexico!

I had the same issues with a later generation Universal. The Weaver screws are *8-40 UNF. I don't know what the threads in the receiver are. They're not *8-40. I ended up chasing the threads out with a *8-32 UNF and replacing the Weaver base screws with socket head cap screws. The screw head will fit tightly in the base counterbore. You may have to route the stock where the heads stick out. You will need a bottom tap for the forward hole as it screws into a blind hole in the receiver. This picture was taken before I got my bottom tap. Be sure the screws are ground enough so they don't interfere with the bolt lug.

*Note this is a correction. Previously stated #6 was an error. Should have been #8.






Edited by jackp1028 - Apr 01 2020 at 12:32pm
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Iamscotticus View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 11:49am
Thanks, Jack.
You got the short spacing mount. I could only find the long. that's OK. I plan to put an Ultra dot in it.  It should be well supported!

I have one problem, the rifle isn't mine.  
Its Father-in-law, who used an M1 in the S. Vietnamese army before fleeing to USA, one of the boat people.  
This mount is going to be a surprise.  I don't think he will mind any non-reversible modification, but cutting the stock and tapping for the cap-heads isn't going to happen. 
I have to find the right thread for those holes.
Thanks for the fast reply.
Scott
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jackp1028 Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 12:42pm
Let us know what screws you end up using to fit the existing threads. Post pictures if possible.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 2:30pm
Lowe’s or Home Depot might go nuts if you tried this, but a mom-n-pop or small local hardware store likely won’t. Just ask them before hand if you can bring in your action to try and find some screws. Most hardware stores have a large selection and even thread gauges in some cases.

Edited by floydthecat - Mar 26 2020 at 4:35pm
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Post Options Post Options   Thanks (1) Thanks(1)   Quote shadycon Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 4:42pm
Have you tried places like Midway or Brownells, they have real gun screws?
M1's are FUN!!!
TSMG's are more FUN!!!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 8:14pm
Update,
 I went to a smaller hardware store, the kind that has everything.  They also have the most eclectic collection of antique rifles collected from all over the world hanging on display.  And a decent US service rifle collection too.
Turner Hardware Hampstead Road Houston, Tx.
I found the rearmost hole is drilled a tad larger than the others and an M4 screw threaded in better than anything else we tried (we, yes I had some help, whether I wanted it or not). However, even the M4 has a slight in-and-out play.  It is a very thin wall that only has about three threads.  But the M4 still will not start into the others.  The M4s I found were countersunk hex drive.  I am slightly tempted to run a M4 tap in the other holes.  Not sure yet.
I have some ways I can look at the threads with high magnification to look for signs of damage.
I am very skeptical that Universal in the 80's would be tapping anything in metric.

Also, I bought the wolf springs for dual rods.  I hope these will ease stress on the crack-prome charge handle.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote jackp1028 Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 8:44pm
I suspect it's a *8-48. I doubt it's metric. Whatever you end up using, I believe it's important to have a tight fit between the screw head and the base countersink. You will not be able to tighten the screws enough to prevent the base from slipping if there is any clearance at all between the screw and the holes in the base. It will shift under recoil and loosen. That's why I used the socket head cap screws. A *#8 Fillister head screw, typically used for firearms may not be tight enough in the Weaver countersink.

Note this is a correction. Previously stated #6 should have been #8.

Edited by jackp1028 - Apr 01 2020 at 12:36pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Mar 26 2020 at 10:50pm
My 6-32 tap threaded in but loosely.  My 32 and 36 pitch gages seemed to have the best bite.
40 or smaller seemed too small.
I will have to try some screws tomorrow.
But its getting to a point of utter hysteria around here over this virus and Im not wanting to go out too much as the numbers of sick increase.  Allergies aren't helping things either.
Is it ragweed, pollen or Corona virus giving me that headache?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote New2brass Quote  Post ReplyReply Direct Link To This Post Posted: Mar 27 2020 at 12:50pm
It seems for scopes or gun related the 40TPI is a standard even thought it is not necessarily a common (NC or NF) screw size 

National Fine (NF) would be 6-40 or one size bigger 8-36
National coarse (NC) would be 6-32 or one size bigger 8-32
There is a slew of special sizes. 

I mention the No. 8 size for two reasons. If you had to go bigger it is the next size.
 Also 4mm is .157 of an inch and yo said it started going in. No. 8 might be your size.
No 6. is 0.138 and No. 8 is .164 diameters

With the receiver being relatively thin a higher number of threads makes sense, however you may get less torque before pulling out the threads.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Mar 27 2020 at 3:23pm
Thanks for the info.
I tried a 6-32 screw and it slid through. 
Looks like 8 I thought I got an an 8 to try but picked up another 6.
I am beginning to think that this may be a bespoke size that a Smith will need to identify,  and possibly cut screws for.
Minimum of $200 work, I expect. 
I could just cut the Weaver screws to length using  the #6s as mules, and jam the weavers in  with red loctite,  if I want to hamfist it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Mar 30 2020 at 1:17pm
Well,
I went to the hardware store that had an assortment of Weaver base screws and picked 4 of each size in #6 and #8.
Nothing worked.  I tried 6 32, 6 40, 6 48, 8 36, 8 40.
What I don't have is 8 32.  I did note that the hole threads like the 32 feeler and they look coarse.
So, my money is on 8 32.  I ordered flat head hex and torx from some online sellers.  I'll let you know what I end up using.
Of coarse, I had to buy bags of 50 but they weren't too expensive.  Let me know if you need some.
I plan on returning the Weavers back to the HW store since I paid &1.29 per screw and I got about 20 in different lengths.
The Weavers have reduced head diameters of .233 to fit in .25 base holes.  Mine are all Torx T10 and T15 in lengths of 1/4, 3/8, and 1/2.
Somebody LMK if you want to buy some or all of them.

One thing I noticed about McMaster-Carr is they list machine screw lengths that include the head height.  So a 1/4 screw will have 1/16 thread depth, which works perfectly for the receiver.
The only caveat is I will be turning down the head diameter to fit.  For that I found a 3" threaded stand-off to hold the screw in.  I need to be on the correct side of the sanding disc!  Fun times ahead!

I also took some pics of some of the rifles on display at Turner.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Mar 31 2020 at 8:28pm
Crickets..
Y'all are a tough audience!
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Post Options Post Options   Thanks (1) Thanks(1)   Quote New2brass Quote  Post ReplyReply Direct Link To This Post Posted: Mar 31 2020 at 9:11pm
whoa, jungle carbine with bayo. Did not know they made a bayo!

As to the screws, did you look at special threads online?
I have old catalogs that list them.

It is possible you need to chase the threads.

You could try melting in low temp metal or hard plastic to get better measurements. 

I forgot the name of the metal but is also used to slug a chamber 
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Post Options Post Options   Thanks (1) Thanks(1)   Quote jackp1028 Quote  Post ReplyReply Direct Link To This Post Posted: Apr 01 2020 at 12:25pm
Note to all. After reading the preceding posts, I realized that I misstated the thread size that I used to attach the Weaver scope base to the Universal receiver. I used 8-32 screws, not 6-32. Also, the Weaver screws are 8-40, not 6-40. Anywhere I said #6, I meant to say #8. I apologize to forum members for any confusion I may have caused and will note the correction in the individual posts.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Apr 01 2020 at 4:14pm
Thats a great idea.  Probably soft lead.  What would I insert in the center, presumably to pour the metal around, to serve as the removal core?  Something with flat sides?  It would be really thin.
The 8-32 screws are coming in today.  I should have something to say later.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote arthur Quote  Post ReplyReply Direct Link To This Post Posted: Apr 01 2020 at 8:19pm
   Dan, 

BROWNELLS - CERROSAFE® CHAMBER CASTING ALLOY

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Post Options Post Options   Thanks (0) Thanks(0)   Quote New2brass Quote  Post ReplyReply Direct Link To This Post Posted: Apr 01 2020 at 11:31pm
Thank you Arthur!

It is a bismuth based metal and some of the combinations will melt in tea water and firm up quick. There are non brand name that have higher and lower melt temps.
It is soft enough that you can trim overflow with a razor knife. you can back it up with a penny or a piece of metal. Might want to use a short piece of brass tube on the outside as a mold so you can grip it.

But if you have the screws try them first. Might even be cheaper in the long run to tap them to a bigger size with the heads you need
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iamscotticus Quote  Post ReplyReply Direct Link To This Post Posted: Apr 01 2020 at 11:56pm
I tried the 8/32 screws but those did not thread either.
There may be damaged lead threads.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RClark9595 Quote  Post ReplyReply Direct Link To This Post Posted: May 04 2020 at 7:44am
I have a late Universal with a weaver side mount installed on my rifle. For me the screws that came with the mount worked fine except a little long and interfered with the bolt. The thing I want to point out is this wasn't designed to use any standard size threads, these screws are what is called resistance screws, they are made to go in very tight because the guns recoil can loosen the screws very easily, I even had to put lock-tite 721 on the threads to keep the screws tight. This is also even more so with the two large take down screws, After only five shots the scope was flopping around and the screws loose, had to use the lock-tite there also. So what I am saying is if the screws turn hard but do go in, then they are correct, if you replace the screws with ones that go in easy you're going to be having a lot of loose scope issues. With the correct screws you can't screw them in by hand and with a screw driver it takes some effort, this is how it is meant to be from weaver.
Ron

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F4 Combat Crew Training
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