Installing Weaver #90 mount on Argentine Rolling Block

[ohland]

This was more fun than being attacked with a Ronco Turnip Twaddler....

My initial enthusiasm was there, since it was supposed to be a close fit. Before YOU start, I highly recommend measuring the distance from the outer sides of the existing mount screws. That will tell you a critical dimension... If you use a close fitting drill, you may end up .020 too close between centers.... Using a drill of .004 larger diameter is a skosh tight, and the center-to-center measurements are critical. In my case, the rear sight holes in the barrel were 2.030".

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First hole I drilled was the middle hole. Picked up the center with a #19 (IIRC), swapped to the drill for a #10 screw. Followed with a 3/8 mill to open the counterbore up for the #10 button head cap screw. In my case, the shortest #10 cap screw at Ace was 3/8 inch long. The Weaver #90 full thickness is .425, I measured the height of the screws in the barrel (sans mount) and determined the thread engagement was @ .125. Following more mathichism, I determined the depth for the 3/8 mill was .170 from top of mount.

NOTE: Hmm, using a large enough center drill to center the base might have overcome any slop from using a drill to center the hole. Now instead of the base being able to move around the drill (a few thousandths, but stuff adds up), we use a center drill, and the 60 degree angle (cone) centers the base. Just a thought... Too late for this base... Whatever...

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Repeated for end mounting screw. Love the smell of Tap Magic for aluminum. Sorta like cinnamon.

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Side view of base installed.

It was a pain to remount the base on the mill and indicate it so it was parallel to the head. Then I put a 2-4-6 block on the table to indicate off of, moved the table .020, and took another pass with the drill. Even though drilling an existing hole is not precise, it was close enough to work... Oh, this also stacked the deck in my favor by widening out the end screw hole on the mount so it would have a little more room between it and the octagonal barrel section...

I could have had a V8... the hole was @ .190, I could have used a 3/16 mill to extend this hole. Oh well, when alligators are snapping at your buttocks, you sorta have tunnel vision....

[Bob Busetti]

I guess I got lucky, I had a mount like that. It was a perfect fit, holes and all. Of course it was in a box of stuff I bought so some one did the work for me. But like what you did.

[ohland]

I guess I got lucky, I had a mount like that. It was a perfect fit, holes and all. Of course it was in a box of stuff I bought so some one did the work for me. But like what you did.

What were the mounting screws? I assume originals? If so, how tight was the fit when you were tightening down the #90? In my case, I had to drill the mount for a #10 screw (the rear sight was re-tapped to 10-32!), and being precision afflicted, I used a drill close to thread diameter.

So without knowing more, I'd suspect your screws were of a diameter that allowed the base to be directly attached...

[brstevns]

Guess I lucked out as well. Did not have any of the problems you had.

[country gent]

I have to ask this after 25 years as a tool and die maker. How does hole size affect a center to center dimension? Swing in hole zero swing in next hole and center to center work right down the line. Or use a bolt in each hole threaded to bottom measure over and subtract 1 bolt dia. We drilled holes on high stress parts to .005 clearence on the bolts and they almost always started and screwed in fingers first. Alot are afraid to swing a corner or edge in to dead zero. or swing center with a gage block. I have a "chair" corner block with a top and 1/2" hole on top on center of corner. In use put it on corner X-Y and indicate the hole on top. A simple hole pattern in a mill or good drill press shouldnt be a big problem or issue.

[ohland]

How does hole size affect a center to center dimension?

Good question. The center to center dimension is fixed. If someone would please get the CtC dimension of an unmodified Weaver #90, that would be fact. Consider my care-free set-up. Used the closest number drill to find center, clamped the base down, changed to proper drill for a #10, followed with a 3/8 mill for the button head.

Hmm, using a large enough center drill to center the base might have overcome any slop from using a drill to center the hole. Now instead of the base being able to move around the drill (a few thousandths, but stuff adds up), we use a center drill, and the 60 degree angle (cone) centers the base.

[ohland]

bolt in each hole threaded to bottom measure over and subtract 1 bolt dia.

Which is what I did AFTER one screw wouldn't start because my set-up allowed too much freedom of movement.

Tried my Starrett CtC points, but sticking the screws in and measuring the outside was definative.

[leadman]

I have drilled and tapped several rifles on a Sears tabletop drill press and must be pretty lucky as they worked out. I do tighten one screw some then bore sight a scope on the mount and mark it to do the next hole.
Got 2 cataracts staring in my eyes so may have to mount scopes on some more of my rifles, including 2 rolling blocks and a 71 Mauser. I do have a friend that just completed gunsmith school and does have a mill so maybe he can do it for me.

It will be interesting to see the level of accuracy that is capable of getting with your scoped RB.

[ohland]

It will be interesting to see the level of accuracy that is capable of getting with your scoped RB.

My .439 H&I will be here Friday, and making this .439 push through is just an intellectual exercise... Boy, did that Class 5 bolt turn nice colors, just like case hardening colors. If I had to do one, I'd go with a Class 2 (Rockwell B70 - B100), center drill, start with a screw machine 11mm, follow with a jobber length 11mm, take a slight tap with a 1.5 degree drill to break the edge a little, end up with a pass with a .4375 reamer. Use 7/16 TGP as the push rod.


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Some delay is happening. The old retrobate tried to open an H&I die he made up years ago. The smaller drills cut well, but the bigger one he used burned up. The die turned color in the lathe. I will be heading down to the basement to investigate. Might be able to use my Blake Co-Ax to insure my Kalamazoo Universal Fixture (5C collet / 4" chuck) tilting head is at darned close to zero degrees in relation to the mill's quill. Then chuck up the 3 jaw, indicate dead center, and try to figure out what the OR wanted to do...

[ohland]

Sears tabletop drill press and must be pretty lucky

Imagine how the old school would have done precision metalwork with the cutting tools of the day. No digital DROs. Dealing with the steels of the day...

If one analyzes the part, CONFIRMS the measurements (which I didn't do for the #90 or the rear screws), and sets up properly, the part should almost machine itself...

One book worth looking through:

Setup reduction through effective workholding 1st ed.
by Edward G. Hoffman for the Center for Manufacturing Systems at New Jersey Institute of Technology ; foreword by Wayne S. Chaneski. ISBN 10 0831130679