I was trying to get a friend's 1924 vintage M&P back in to shooting condition and discovered that it was missing the side plate-mounted plunger and spring which are rather essential for safe operation. In fact, because there is nothing to retract the hammer block, you would think that it shouldn't fire at all (it does anyway).

Until I purchased a 1919-ish M&P myself last year, I wasn't even aware there was a difference in the hammer block on the early models. As usual, my thanks to many of you on this forum for the information on the engineering changes etc. However, I was still puzzled as to exactly how this early 4th change hand spring/hammer block combination deal really worked. I managed to find an early model donor side plate which still had the spring and plunger on ebay (the seller just happens to be local to me) and I thought I would share some tidbits I learned in this process.

First, a little background (thanks, Jim); with the 4th change to the Model of 1905 in 1915, (1917 on the .32 HE 3rd Model) the hand spring was eliminated and the side plate hammer block mechanism was added (with the exception of Target models apparently). This new arrangement lasted until about 1926.

The hammer block in the side plate appears at first glance to be similar to the later hammer block but it lacks the tab on the side. Instead, it is quite a bit thicker in the middle and has a 0.125" wide cut-out across the middle. About where the later hammer block tab cut is in the side plate on post-1926 guns, there is a hole for a spring and plunger which retracts the hammer block and provides the tension for the hand.

Early side plate left (1919), Later side plate right (1941)


Early side plate.



Later side plate.


The early 4th change trigger and hand are not interchangeable with later models as there is only one pin on the hand and no slot for the hand spring pin on the trigger. (Something else I didn't know).

Hands: early left, later right


Side view: early rear, later front


Also, the rebound pin on the older trigger is square where the later pin is round. The later trigger has a U.S. patent stamp on the underside and is serrated where the early trigger is smooth.

Early left, later right


The hammers seem to be almost identical except that the earlier hammer has a straight forward plane and the later model is slightly curved. The sear is a little thicker on the later hammer as well. Dimensionally, other than the curved front, they are the same.

Early left, later right


The plunger is approximately 0.10" in diameter and 0.710" long. It is rounded on one end and the last 0.155" on the other end is reduced to 0.065" in diameter to fit inside the 0.64" long spring, similar to the spring and plunger in the cylinder locking bolt.

Plunger


There is a sloped cut-out on the underside of the plunger which tapers down from front to back and flattens out where lays across the corresponding cut-out on the hammer block and then ends in a vertical cut. The bottom of the plunger, from that point back towards the spring, is flattened, presumably to prevent it from rolling.

Plunger, spring, and hammer block (the old hammer block was broken in the middle)


The really clever part is how the plunger interacts with the hammer block. As the plunger is depressed by the angled edge of the hand as it moves up, the sloped cut slides across the hammer block pushing it back in to the side plate cut. When the trigger returns after the hammer is released, the hand moves back down allowing the plunger to come forward which, in turn, allows the hammer block to spring back out. The vertical cut edge on the underside of the plunger is caught by the edge of the hammer block when it is in the normal raised position and thus retains the plunger. To remove it, push in hard on the middle of the hammer block to allow the edge of the plunger cut-out to clear the edge of the hammer block and release the plunger. The side plate hammer block cut is dished out a bit to allow for the additional thickness in the middle of the hammer block.

After about 1926, the hand spring returned to the trigger and the hammer block was actuated by a ramp on the hand which pushed against a tab on the hammer block (2nd style?). This configuration remained until the evolution of the new (3rd style?) rebound slide actuated hammer block in 1948.
(Correction: this was in 1945, not 48.)

I don't know exactly why this was changed back to the "pre-4th change" hand spring, but I suspect the plunger was more prone to sticking or maybe it didn't reliably engage the hand. If anyone knows, please share. I'm also curious why there wasn't a formal nomenclature change since this seems to be a fairly significant engineering change to me due to the non-interchangeability of parts if nothing else.

I hope this was informative.

BTW - if anyone has any ideas about how to get the hammer block out of the side plate without breaking it, I'm running out of ideas. The new side plate actually will fit (more of less) but not quite good enough.

Rick

Maybe one reason for the change was that the plunger style hammer block sometimes makes the sideplate difficult to install because the plunger hits the side of the hand. I had one .32 HE apart recently that had a small spot ground into a tapered ramp to ease the pin back, but I don't know if it was factory or a post factory mod.
I think that the later style hammer block with the ramp one the rear of the hand and the tab on the hammer block is simpler, more reliable and probably cheaper to manufacture.

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Tom
1560

Wow! Impressive data and photos!

I had trouble getting the side plate on my .32 HE back on as well. I thought it was just me!
Cost probably was a big factor in the change. The later config is much simpler.

This needs to be up in the Expert Commentary section. Excellent description with pictures to match.

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H Richard
SWCA1967 SWHF244

Glad I found this posting. Very informative. I just bought a 1905, 4th Change, 1st Variation with the sideplate mounted hammer block and plunger, which, unfortunately, isn't in proper operating order.

Is it just me or is there a reason I am unable to view the pictures? Just a big triangle with exclamation mark inside it. Very interesting post and I would like to view the pictures. We're they moved??

What he said!

Ralph Tremaine

I'll see if I can find the pics and reload them.

I try not to use third party image posting if possible. It is easy and permanent if you load up to this site directly. Go through Manage Attachments tab and the images will always be attached to your post for others to see down the road.

After you search to find informative threads like this one, only to see that the images have been taken down, it can get very frustrating for future users. This and other Forums have lost valuable information over the years.

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Gary
SWCA 2515

pic reloaded - sorry for the delay!