European Scope Mounts, the FN-Sako, and Thread Compatibility

Discussion in 'Sako Mausers, Hi-Powers and Magnum Mausers' started by icebear, Dec 19, 2018.

  1. icebear

    icebear Well-Known Member

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    I just had an educational experience with my custom FN-Sako .300 H&H, and I thought the information was worth passing along. I recently mounted European scope bases on the rifle, using the screws that came with the bases. The mount setup is an East European knockoff of the EAW swing mount. Despite the use of Loctite and a torque wrench, the mounts came loose while I was trying to zero it at the range. When I took the bases off, I discovered that the mounting screws were M3.5, while the FN action was drilled and tapped for 6-48. I replaced the screws and I'm confident that the problem will not recur.

    6-48 and M3.5 are both oddball screw sizes that are in common use for firearms and seldom seen anywhere else. The threads are very, very similar in diameter and pitch, but they are not the same. A screw of one type can be forced into the other thread, but it will damage the screw and it may not hold. Sako ring screws are M3.5. At least one aftermarket parts vendor sells Ruger ring screws, which are 6-48, as suitable for both Ruger and Sako. That's a dirty lie; don't fall for it. The screws will go in, but you'll get a bright edge on the threads where they get recut by the threads on the rings, which are made of harder metal than the screws.


    If you have a rifle built for the European market and drilled and tapped for scope mounts, it is quite likely that the threads are M3.5 rather than 6-48. Likewise if you have a mount setup that was meant for European sale. Best to check anything you aren't sure of.

    300 New Scope.JPG
     
    Hillcountry and robinpeck like this.

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