Enlarging firing pin hole in a hard bolt..?????
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  1. #1
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    Default Enlarging firing pin hole in a hard bolt..?????

    I have two Jap. T-99 machine guns, one extra bolt and a baggy full of firing pins, mostly broken. The bolts have a generally larger firing pin hole.....maybe by plan, maybe by corrosive primer leaks. Any way, the original firing pins are .086, some of the re-pops are that or slightly smaller. I can get a .092 drill in the firing pin hole now. Hardly enough larger to be much improvement.

    If I can come up with a drill like tool of .100 or .104, I can get some repop firing pins made to .098 or even .100???

    Any one know of a suitable diamond impregnated die grinder burr.?? Or....???

    PJH

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    Default Enlarging firing pin hole in a hard bolt..?????

    EDM then bushings? Drillrod and diamond dust in oil?


    Sent from my iPad using Tapatalk

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    +1 for EDM, if you use a proper microhole machine instead of a "hole popper" you might be all set right there.

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    A carbide burr in the proper diameter would work well or a carbide drill. The early Type 14 Nambu handguns had a small diameter tip on their firing pins. Later, when their primers were better they made the tip larger but the tip was still too small to keep them from breaking often. That is why they had an extra firing pin in the holster. The Japanese at that time didn't have parts interchangeably completely figured out and relied on a lot of hand fitting to make things work. Parts from one Nambu may of may not work in another.

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    Did you try a carbide reamer in a pin vise ? Should follow the existing hole fairly well and they come in graduated decimal sizes.

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    Had to rebuild a garage door today....not old guys forte anymore.

    OK......I havnt tried anything yet. I need to confirm that I can get oversize firing pins once I come up with a way to enlarge the pin hole in the bolt. Some of the responses are in advanced machinist lingo.... I will have to have my guy look at these and translate.

    I cant get to him until Monday. We have a VFW event in the morning that will end up taking half a day. I have 2 miles of electric fence thats dead......tomorrow afternoons job.

    PJH

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    A bolt head does not need to be that hard, only the locking lugs
    Why not bush or weld up the bolt and re drill to the proper size?
    This is commonly done on target rifles and many welded up drill rifles from m14, m1 garand, o3a3 etc simply have the bolt face trued back flat and the fp hole re drilled with no issues

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    How hard is "hard"?

    I'm not a gunsmith, just a regular ol' machinist... But MA Ford makes carbide drills and reamers that will cut up to I believe 62 HRC, available in decimal sizes, and surprisingly cheap.

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    Trying to get larger firing pin tip here.....the originals are close to .086. I have had a bunch of them break. Only trying this on a spare bolt. An MG 42 firing pin tip must be .125, (guess), tapers immediately to a much larger dia.......a ZB 26 uses a tip that is screwdriver shaped.

    Bigger is better in the case of machine guns. Jap gun is mag fed at 900 rpm........

    PJH

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    I needed to enlarge the firing pin hole in a Pattern 1941 bolt once. The 1914 Enfield uses a smaller diameter firing pin, most likely for Bredan primers. This small firing pin can sometimes pierce a Boxer primer. Luckily it can be replaced with an American Model 1917 firing pin, which corrects the problem. The only modification needed is to drill out the firing pin hole on the 1914 bolt.

    How hard could it be?? Well that 1914 bolt was hard as a diamond. In the end I used a Hirok straight flute carbide tipped drill, and that did the gob. Google "Hi Rok Drill" and you'll see that they are available from multiple vendors in all standard drill bit sizes. Just be forwarned: HiRok bits are very hard, but they're also very brittle, and won't take shock loads (like interrupted cuts).

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    Thanks for the additional tip. The existing firing pin hole is not symetrical due to some of the enlargement that currently exists is from corrosive primer leaks. Not good. "Go Slow" will certainly be valid advice here.

    PJH

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    With carbide, feed slow but high RPMs and plenty of lube. Once the cutter starts turning up a chip, keep going at that feed rate.
    You want to let the chip take away the heat, not the work piece. I've found that the best chip load is .002" per flute per revolution.
    With any small cutter or extra fine drill bit; "the faster you spin 'em, the stiffer they get". (Less prone to deflection)
    This is a good job for a lathe or milling machine where you can control the feed so that it doesn't dig in and try to screw itself in.
    Not something you want to try with a hand drill or Dremel tool.


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