MIROKU Mainspring Installation Tool










The Miroku mainspring installation tool is a valuable timesaver when installing mainsprings in the Miroku made Over/Under shotguns.  This tool is designed for the Vee type mainsprings, but can also be used on the newer coil type mainspring Miroku's.  This also applies to the Miroku made Charles Daly labeled firearms made from 1963 to 1976.  This tool will also fit the Browning Citori type I  O/U's made by Miroku.

The tool consists of a base plate, base plate pin, spacer button plug, lever assembly that has an attached pivoting plunger, a tee handle and an initial alignment pin.
  The principle utilized is a ball and socket system with a pivoting plunger pressure lever.

This tool is designed to be used on both the 12 and 20ga firearms.
  It also can be used for both the RH and LH mainsprings, is especially valuable in use when installing the RH side.  Prior to using this tool, you will have to remove both of the sear units.  The tool’s base plate is inserted in the receiver in front of the trigger inertia weight, up into the area where the sears reside.  The knurled headed base plate pin is inserted into the sear pinholes in the protruding bosses & thru the base plate hole.  This secures the base plate in position.

By the nature of this firearm, when replacing a new mainspring for the RH side, or (bottom hammer), you will also have to remove the cocking lever pin.
   For the LH, or top barrel, you will need to remove the top lever (or at lease partially retract it) in order to get the new mainspring and hammer into the initial position in the receiver

The mainsprings themselves; the RH spring has a tail about .130" long from the pressure pad to the tail’s end, has the tail end slightly rounded.
  The LH spring tail is longer at .340" long from the pressure pad to the tail’s end, has the tail end square.  These measurements are taken on a line about equal to the center of the Vee. The normal overall distance from the pressure pad to the bend is about .1.125 for a 12ga.  This measurement from the bend is really insignificant, as the distance from the protruding right-angled tail on the short end to the end of the long end tail is the critical distance.

You may find 4 different top lever springs, depending on the vintage of these firearms. The reason for this tidbit of information here, is that each of the 4 different types will require slightly different disassembly.
  Also if you have to replace both mainsprings at the same tear down time, it may be best to replace the LH spring first.

Removal of the top lever;
  This will require removing the trigger guard which is retained by a small cross pin located about 3/16" in front of the trigger pivot pin in the lower tang.  Remove the appropriate top lever spring.  With the receiver upside down, you will now see a headless set screw in the center of the bottom part of the top lever shaft, back this set screw out. Use a 3/32" or smaller punch, with the top lever in the closed position, go thru the 3/32" hole in the RH side of the locking plate that is aligned with the center of the top lever shaft.  You can now drive out the top lever retainer/pivot ball.  This will now allow you to raise the top lever up enough, possible out of the receiver so that you can get the LH hammer out to replace it and the mainspring.

In use for the LH side;  If your gun is a trap version with the automatic safety reset linkage,  that is screwed to the locking block, you will have to remove it.  Also you will need to remove, as described previously, the top lever.  Insert the new mainspring with the hammer in position behind it as shown below on the Left.  The arrow points to the mainspring pad in position in the hammer’s mating notch.   You will notice that the top lever has been reinserted partially, rotated at 90 degrees into the receiver to somewhat support the hammer.   

Now you can place the tool lever’s ball socket hole over the base plate’s ball, with the pivoted plunger's concaved end placed in contact with the hammer's protruding ejector trip activator shaft.  Apply pressure on the lever handle.  Once the hammer is in the receiver’s slot, close to position, you can visually locate the hammer’s pivot hole thru the receiver’s pivot screw hole.  Now, using the Ell shaped alignment pin, insert it in to secure the now compressed hammer.  At this point, you can maintain tension on the tool’s lever, remove the alignment pin, insert the pivot screw.  Here is where the Tee comes into play, as you may have to twist the tool slightly to finalize alignment be able to insert the screw all the way in.  One note of caution when reinserting the pivot screw, tap it in with a screwdriver handle, not a hammer as you may strip the screw’s threads.

Left Hand hammer & spring in position 

Tool in use on Left Hand hammer & spring with frame in padded vise

The RH is basically a repeat of the LH side, other than you will need to place the 5/16" dia. spacer button into the 3/16" hole in the top rear of the tool’s base as seen in the LH picture below.
  This is needed only for the RH side in that it aligns the tool for a better straight-line purchase on the activator pin.   You will also have to drive out the cocking lever pivot cross-pin in the receiver.  Move this cocking lever down and rearward enough to get the new mainspring in front of this cocking lever.

After you have inserted the mainspring in the frame recess, with the hammer in as much of it's position as possible as shown below on the left, reinstall the cocking lever.  Place the tool's lever with the ball socket on the base's protruding ball.
  You will notice in the picture that the knurled base plate pin is always inserted from the Left Hand side so that the knurled head does not interfere with the tool’s function on this side.  Now as before, the pivoted plunger's angled concaved end can be placed in contact with the hammer's protruding ejector trip activator shaft.  Put pressure on the lever, forcing the hammer into position.  Again when the hammer pivot pinhole is close to being lined up, insert the El shaped 1/8" tool, using the tapered end, insert it into the receiver hole and thru the hammer hole.  The hammer is now captivated.  You can now get another possibly better bite, apply pressure again, withdrawing the alignment pin, then inserting the pivot screw in its place, while still holding pressure on the lever.  Now you can tighten the pivot screw in until it stops.

Right Hand hammer & spring in position 

Tool in use on Right Hand hammer & spring

Type 3 assembled action

The most encountered problem will be broken "Vee" type hammer or mainsprings.
  These springs are a "Vee" type about .200 wide with a protruding right angle tail on the short end, which puts pressure on the hammer, the other long end just bears inside and against the frame.  These mainsprings are different between the RH and LH.  The hammer pressure pad side being the same but the other tail side has two different lengths.  If you were to keep a spare, the longer top spring #2211 (LH) can be shortened if needed for the #2212 (RH).  Also the 20/28ga/410 are a slightly shortened overall version at the bend, by about .080 so you can't modify a 12ga to fit the 20ga.

As shown in the drawing below, it is advisable to check to see if the spring is compatible with the hammer configuration.
  The mainspring's pad can be lessened if it is overly long, binds when installing the spring.  If you however remove too much, the hammer will start to rebound before it bottoms out and you may get misfires.  You can grind a slight amount off the hammer at the pinch point to again gain clearance.

Diagram of relationship of Mainspring to RH Hammer

Relationship of LH hammer & Mainspring in the frame

The Four types of top lever springs; used on these firearms are described below.

We will call type (1), which uses a thin "Vee" type spring about 1.33" long, that had a tit protruding downward, and a half-moon notch cut in this tit for a retainer pin thru the lower tang.  This spring lies on and is pinned into the lower tang and runs longitudinal with the gun. 


Type (2) has a much bulkier spring, a "Vee" with one end considerably longer, that looks somewhat like a large heavy fish hook.  It is about 1.88" long and has an approx. 1/8" hole in the front for a retainer pin that the cocking lever pivot pin goes into as an anchor for this spring. 


Type (3) is a smaller "Vee" spring about 1.13" long that is positioned again just above the locking block, when in place, it runs crosswise in the firearm.  There is a slight notch on one tail of the "Vee" which holds it into a shallow notch in the RH receiver that keeps it from popping out.  The other slightly rounded tail bears on a flat in the top lever shaft.  When installing this top lever spring, the notched tail of one spring goes forward, lays in a notch in the RH receiver wall.  It may have to be fit so that the bend lays just behind and against the LH receiver wall with the RH tail notch snaping behind the RH wall.  The other, or rear tail bears against the flat on the top lever.  This spring end should be tapered or slightly radiused where it applies tension on the top lever, so as to slip on the top lever pad, but not bind and work it’s way out of position.

There also appears to be a Transition gun using the type (3) top lever spring but has coil springs for hammers and ejector hammers.

The Type (4) top lever spring is a (3) piece coil type unit and has coil type mainsprings.


Copyright © 2005 - 2017  LeeRoy Wisner  All Rights Reserved

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Originated 03-17-05, Last updated 08-13-2017
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There is not a lot of documented information on these firearms.  Wisner’s Inc has been trying to assemble as much information as possible for some time.  To look at what further information we have compiled go to our other article here.

Wisners www.wisnersinc.com/  make many parts for these firearms, and have over the years picked up factory new parts from other gunshops.