Made a Gamo pivot bolt and the gun's owner........

Made a Gamo pivot bolt and the gun's owner........

Joined: July 24th, 2010, 1:44 am

January 17th, 2011, 2:52 am #1

said it actually works! LOL

A fellow for whom I've been doing a bit of airgun parts work told me about his Gamo pivot bolt seizing up so tight that it was pretty much destroyed by removing. As I understand, Gamo doesn't sell replacement parts, so I offered to attempt fabricating one just to see if I could do it. The issue is that the head of the Gamo pivot bolt has a 1 degree taper, as determined by measuring the bolt and laying it out on my CAD software. With such a shallow taper even relatively minor measuring/fabrication errors would make the bolt unuseable. are a few pics..........

Fabricated bolt and damaged factory bolt with sleeve.

It was a bit of a tedious project for a "seat of the pants" machinist like me with a bench top lathe and dial calipers, but it was also a fun project!
Last edited by nced1 on January 17th, 2011, 3:02 am, edited 1 time in total.

Joined: December 15th, 2009, 2:25 pm

January 17th, 2011, 1:17 pm #2

nice work.

Joined: July 24th, 2010, 1:44 am

January 17th, 2011, 2:26 pm #3

socket head cap screw.

The fabrication steps I used was...

1. Hacksawed off the threads
2. Chucked the bolt in the lathe and skim cut the OD of the head to give a flat surface to grip with the 6 jaw chuck.
3. Chucked the bolt (head inside chuck which is why the shank of the cap screw was so long) then faced and center drilled the hacksawed end.
4. Chucked the turned head of the bolt and supported the center drilled end in a dead center.
5. With a thin parting tool the base of the bolt head was flattened and the shank was undercut a few thou.
6. Turned the shank of the bolt till it was .001 too large to fit the pivot bolt sleeve.
7. Using fine emery I "lapped" the dead center supported end of the bolt till the pivot bolt sleeve would barely slide on the end of the bolt shank. The pivot bolt sleeve became my "size gage" and I kept reducing the rest of the bolt shank with a 3/8" wide strip of fine emery till the sleeve was a close slip fit to the whole length of the shank.
8. With the thin parting tool I established the bolt shank length and under cut the root diameter of the thread a bit.
9. Turned the end of the bolt as needed for a 6x1mm thread
10. Using a die in a die stock along with pressure from my tail stock ram to keep the die square I threaded the "thread end", then cut the thread to length.
11. Chucked the bolt by the lapped shank so the bolt head was close to the chuck.
12. Machined the bolt head to thickness and rounded the face a bit per the factory bolt.
13. Adjusted the tailstock so that using the tailstock screw would advance the tool at a 1 degree angle.
14. Machine the taper on the bolt head till the "small diameter" matched the original Gamo bolt.

A couple side notes....
In the past I've ALWAYS used a good live center when supporting the "far end" of the workpiece but this time I used a dead center so the cross slide could be brought closer to the work keeping a short tool extrnsion (my live center was too large to keep a short tool extension). Ya know what.....I really liked using the lubed dead center better than the live center. Think I'm gonna hafta order a carbide tipped one in the future.

With my first attempt at making a Gamo pivot bolt I screwed up two.....
The first when I tried to machine a "slip fit to the sleeve" shank diameter. The problem was that my bench top machine wasn't quite ridgid enough (compound,cross slide etc) to hold such a fine tolerance so I had to "sneal up" on the diameter by "lapping" with fine emery cloth.

The second when I took too much of a cut when cutting the tapered head to diameter making the head too small to bind in a Gamo receiver fork.

Oh well......lessons learned and still learning!

Joined: December 15th, 2009, 2:25 pm

January 17th, 2011, 2:33 pm #4

that seems like a lot of work, i wish there was an easier way.
i could use a couple of those.

Joined: July 24th, 2010, 1:44 am

January 17th, 2011, 5:49 pm #5

there needed to be a certain sequence to construction, for example....once the "head taper" was cut I don't have a parallel surface for chucking to make the rest of the cuts, so that cut was done last. was a bummer to screw up the final cut after all the tedious fitting (like I did on one attempt).

Actually, while a bit tedious it's a pretty easy project, especially using one of the 7x12 Asian lathes with the electronic speed control (and light cuts with a sharp bit). My other lathe started out as a 7x10 Harbor Freight model that I modified by adding a 14" bed and other pieces (like a quick change tool post) from The Little Machine Shop. I would have used my smaller lathe for this small project however the last time it blew a fuse in the HF lathe the electronic control board also gave up the ghost. LOL Some day I'll replace the board but I've just been using my LatheMaster since, even for the small stuff like this threaded insert to repair stripped threads in the receiver of an old R9 that had the stock screws torqued too tight too often.

LOL...after cutting and installing these I found some commercial 5mm inserts at the McMaster Carr site and they worked perfectly for restoring the threads of an old R10.

Anywhoo.....give it a try, you might be surprised!

Here are a couple pics of what I consider my small and smaller lathe, even though the LatheMaster weighs about 350 pounds.....

Setting up the HF lathe for barrel crowning after chopping.

Another barrel that was home chopped, crowned and choked. The final result is pretty good after a home choke with a vice and clamp block (The HF steady rest broke with use (porous casting ?) so I made up a couple "support plates" from steel mending plates LOL

I'm still using this barrel.

I got annoyed trying to alaign my HF tail stock using the hokey "remove the tailstock, loosen the screw, tweak the base position, tighten the screw, replace the TS to see if the adjustment will work" tailstock I made this hokey jack screw and set screw arrangement that actually worked pretty good.

Might seem that the LatheMaster is a heavyweight (compared to the HF lathe) but it's still not ridgid enough to take any but light cuts and maintain close dimentional parts. Wish I had pockets deep enough to afford a larger lathe with at least a 1 1/4" spindle bore!

Joined: December 15th, 2009, 2:25 pm

January 17th, 2011, 6:01 pm #6

can chop barrels, i just chuck up the muzzle and spin the rest.
i am thinking of trying the pivot bolt, i would like to change the pivot a little.
like on the Hunter Extreme no threads just a pin, also turn and fit precise bushings for both sides of the breech block.
i think good tension on the barrel and good alignment can be found with bushings instead of tightening up sloppiness.

Joined: December 3rd, 2010, 5:56 pm

January 19th, 2011, 7:10 pm #7

FWIW, on those mini's ya gotta spend lots of attention to all the gibs. That lathe is capable of holding .001" and better IF the gibs from the carriage, cross, compound slide are adjusted correctly. Usually the carriage and cross slide because they can be a real PITA to adjust right get the least amount of attention so they get sloppy and everything after that is just plumb luck with soft material, sharp bits, and close to the chuck holds. Anything too far out from the chuck grip even with good dead centers just magnify issues. Bottomline is forget about hardened steel and tight tolerances if gib adjustments are out of whack has been my experience. Of course going real slow and sneaky helps too. lol

Last edited by AKSevenDave on January 19th, 2011, 7:10 pm, edited 1 time in total.

Joined: December 15th, 2009, 2:25 pm

January 20th, 2011, 2:34 am #8

"sneak up on er"
i run my gibs tight and when i am not using the compound or the cross slide i lock down the gibs altogether.
i am happy with this lathe, putting the five inch chuck on er was the best thing for it.
i tuned the speed control and the overload so it may actually be able to break a tool instead of stalling out.