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Thread: Drilling holes in trunion

  1. #1
    Gunco Member JimM44's Avatar
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    Default Drilling holes in trunion

    I am making a trunion out of the milled receiver stubs, I know I have to grind down the sides to fit in a stamped receiver. When drilling the holes in the trunion I know they are 7mm and I need to use cutting oil but what kind of drill bit should I use and where is a good source for them? I also plan on making a 1/4 inch countersink, is that the correct size?

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    Gunco Veteran Steelcore1964's Avatar
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    they are alot smaller than 7mm, I would say 2-3 mm hey guys what size drill bit is that in standard? When I gwet home I will check it for you and remember to champher the holes
    That which does not kill you, makes you stronger

  3. #3
    RIP Sangrun Hunter's Avatar
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    7mm is the barrel pin size.

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    Gunco Member JimM44's Avatar
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    Default Steelcore

    Do you mean what I am calling countersink or do you mean I need to put lines on the inside of the holes?

  5. #5
    Gunco Veteran Steelcore1964's Avatar
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    the barrel pin hole is 7mm but if you mean the smaller holes where the rivets go then it's more like a 3-4 mm hole. Some of the rivet holes on the trunnion are usally "countersunk" to allow for the metal of the receiver to be partially pressed into the hole then a swellneck rivet finishes filling the hole. This makes for a MUCH stronger union and also prevents the receiver from shearing off the head of the rivet under stress, however this arrangement is usually considered to be over kill on strength because the front trunnion rivets are not under much stress anyhow.
    That which does not kill you, makes you stronger

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    Happy Camper hcpookie's Avatar
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    The original spec calls for 4mm and 4.5mm rivets in the trunion. If you make the rivet holes to be a "loose" 5/32" you'll be fine. Up to 11/64" is acceptable. The only other hole, as already mentioned, is 7mm. That turns out to be an "I" or "J" index drill bit. "I" is the deal IIRC, and "K" is what you end up using if you need to adjust headspace and re-drill the hole oversized. The oversize pin hole is really as big as you make it - depending on what materials are available to you and all that.

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  7. #7
    Gunco Rookie Les Staley's Avatar
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    This is an intresting question. looks like a job for a milling machine.. thinning the front receiver stub for a stamped receiver... I'll be following this thread.. thanks Les

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    Gunco Rookie Wizzard's Avatar
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    Enco has solid carbide 2 flute 140 deg. straight flute drills in 5/32 for $10.56 . Good for steel alloys harder than HRC-50. Check out their catalog on the web. use_enco.com

  9. #9
    Gunco Member aloharover's Avatar
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    It takes a lot of oil, and very slow speed. I used some Cobalt brand drill bits I got from Lowes
    U.S. Army (Ret)
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    Cobalt will rip through like butter.

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