Help Fitting New Great Plains Rifle Hammer

[Acorn Mush]

I need some help fitting a new percussion hammer to the lock of my Lyman GPR. The old hammer nose is showing wear, as in a depression in the cup face where it has been hitting the caps for the last 35 years. Thought I would fit a new hammer and modify the old one for use with musket caps in the event I run short of #11s.

The square hole in the new hammer, just received from Track of the Wolf, is about .004" - .006" smaller than the original hammer. It doesn't even begin to start on the square tumbler shank. My diamond grit files are about 1/2" wide, so that option is out. My carborundum stones are also too big. I do have a set of small jeweler's-type files, but I doubt they would cut through the hammer's casehardening.

What would you more expert guys recommend in order to address this problem? Any advice would be greatly appreciated. Thanks in advance.

 

[Phil Coffins]

The case isn’t so hard and your jeweler file will cut it. Even if you ruin one a whole set of them are cheap. Work only on two adjacent flats, not all four.

 

[The Crisco Kid]

Buy a set of needle files from Harbor Freight for a few bucks and see how they work. As Phil says, just work on two flats. A Sharpy is good for marking the hammer to see where you need to remove the metal. Be glad it's too small to fit and not too large!

 

[Acorn Mush]

Thank you gentlemen. I appreciate your help and will give my files a try.

 

[Acorn Mush]

Just an update on fitting my new GPR hammer. I took the advice of both of you gentlemen. Bought a set of diamond needle files at Harbor Freight for $7 along with a clip-on jeweler's eye loupe for an additional 6 bucks. Used the loupe to examine the interior of the square hole in the new hammer, marked 2 adjoining sides that looked like they could benefit from some "file love" and carefully went to town. I checked fit every 5 to 10 file strokes until the hammer went almost completely down onto the tumbler shank. At that point I had to slightly relieve the inside corners to remove a bit of casting roughness. The hammer lacked about 1/64" of seating completely down on the shank but installing and tightening the hammer screw seated it fully. I greased the hole and the shank with some VERY old Gunslick grease prior to installing the hammer. The hammer seems to fit without any play yet is removeable by hand with just a few wiggles.

Thank you again Mr. Phil Coffins and Mr. Crisco Kid for your valuable suggestions. Couldn't find a "hat off" emoji but my hat is off to you.

 

[Phil Coffins]

There is no need to remove the hammer from the lock unless it is broken. I would suggest you clean off the grease and use lock tight instead.

 

[tallPercshooter]

I think he meant he greased the tumbler axle and hole in the lockplate, not the hole in the hammer.

 

[Phil Coffins]

I think he meant he greased the tumbler axle and hole in the lockplate, not the hole in the hammer.

Perhaps so, but the hammer should not be removable by hand.

 

[Acorn Mush]

I think he meant he greased the tumbler axle and hole in the lockplate, not the hole in the hammer.

Perhaps so, but the hammer should not be removable by hand.

I did grease the axle hole in the lock plate as well as the square hole in the hammer. I will take Phil’s advice and remove the grease from the hammer hole. Thanks.