HK4 Caliber Conversion Procedure

Before attempting this procedure, take two very important precautions:
  • Ensure that your HK4 is unloaded.
  • Select a workspace in which you will not lose any of the springs and pins that invariably fall and roll towards the nearest abyss.

Begin with the slide assembly sans barrel or mainspring.

  • Pivot the extractor slightly outwards to expose the hole that runs through it.
  • Insert an unfolded paperclip through this hole so that the extractor will remain in this position.
  • Insert a long, flatheaded screwdriver through the muzzle end of the slide and remove the cylinder head screw which retains the breechface plate.
  • Remove the breechface plate
  • Rotate the safety lever to a position halfway between "Safe" and "Fire".
  • Remove the firing pin and firing pin spring from the front of the breech. (This may require the gentle application of some needlenose pliars.)
  • Wiggle the selector lever out of the slide.
  • Remove the paperclip that is holding the extractor outwards, and wiggle the extractor out of the slide. ("Wiggle" does not have a technical connotation, but you certainly do a lot of it during this entire procedure.) Tension on the extractor is provided by two set bolts with a spring between them, and this tension should slacken once the selector lever is removed. All three of these parts should fall out of the front or rear of the slide once the selector lever and extractor are removed.

If there was ever a time to clean all of your HK4's slide components, it is now.

Do not reassemble in the reverse order. Instead:

  • Choose the proper extractor for installation. (One HK4 schematic that I have seen references 8 different extractors - 4 old style and 4 new style for each caliber - yet did not reference the universal extractor, so there may be as many as 9 extractors to choose from. Good luck.)
  • Insert the selector lever into the slide.
  • Reinsert both of the set bolts along with their compression spring through the hole in the front of the slide that is immediately behind the extractor port. The set bolts are interchangeable, but be sure that:
    • the tapered end of the rear set bolt faces rearward to make contact with the selector lever, and
    • the tapered end of the front set bolt faces forward to make contact with the extractor once it is installed.
    The compression spring and the firing pin spring are not interchangeable, so be sure to put the correct spring between the set bolts. The compression spring is shorter and stronger than the firing pin spring.
  • Press the extractor into place by using its retaining stud to force the set bolt rearwards.
  • Again, pivot the extractor slightly outwards to expose the hole that runs through it, and reinsert your handy paperclip to hold the extractor in this position, thereby allowing sufficient room to reinstall the firing pin and breechface plate.
  • Insert the firing pin through the breech with its rear, chisled tip facing downwards. The square stud on the firing pin points upwards, and properly seating the firing pin may require some patient maneuvering.
  • Slide the firing pin spring over the tip of the firing pin.
  • Press the breechface plate into position with the appropriate side facing forwards. (If you are performing a .22 conversion, then the "R" - for rimfire - stamp should face forward. If you are performing a .25, .32 or .380 conversion, then the "Z" - for centerfire - stamp should face forward. Also, be sure to insert the tip of the firing pin through the proper hole in the breechface plate: the bottom hole is used for centerfire conversions, the center hole is used for rimfire conversions and the top hole is reserved for the cylinder head screw.)
  • Insert and tighten the cylinder head screw.
  • Remove the paper clip from the extractor.
  • Reassemble the pistol.