[jfk]

I've been given the opportunity to assist with an SP89 upgrade. This will cover barrel removal, paddle mag install, clip/pin trigger frame, 3 lug barrel re-install, and refinishing. Here's a few pics of the project. I'll update/add more pics/text as time allows.


I forgot to take pics of the barrel removal even though it was straight forward. I cheat when removing the barrel pin. Instead of trying to clamp the receiver into a vise/jig and pounding the heck out of the barrel pin, I use an undersized end mill to drill out the barrel pin. The remainder of the barrel pin pops out easily. I use only carbide end mills (1/8" or 3/32" work well) since the barrel pin is incredibly hard. HSS and Colbalt end mills will not work and the bits will dull easily.


By dirtyeight


I placed the receiver (upside down) into the vise and milled out the lower paddle pocket area and took out the rectangular metal piece.



By dirtyeight




By dirtyeight


After cleaning out the magazine release area of burrs/machine marks, I used a clipped/pinned lower to determine where the dowel pin hole needs to be drilled. I used an automatic center punch to mark where the hole needs to be drilled.




By dirtyeight



The receiver is drilled with an undersized colbalt drill bit. The drill bit goes about half way through the other side of the ledge. DO NOT drill the hole all the way through!!! The hole is then reamed out with an 1/8" reamer.



By dirtyeight



By dirtyeight




The dowel pin is cut/trimmed to the point where it sticks out about the width of a dime shaved in half (very little material sticking out). Here's a few pics of the paddle release before welding. Notice the wide pocket area where the paddle will flop back and forth. This are will be cleaned up.



By dirtyeight



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The paddle bushing is made out of steel round stock. The diameter is turned down a few thousandths smaller than the inner diameter of the paddle. The hole in the busing is also drilled slightly oversized.

WHY would the holes be drilled oversized? The oversized holes will compensate for any wandering of the drill bits or a miscalculated pilot hole. I've read several posts where a paddle magazine release binds or needs to be broken in.

The dowel pin hole is precisely 1/8". This hole is not oversized. The bushing is the key to a smooth install. TIME FOR BED, good night.

 

[edman99]

Thanks for the info jfk.

While you have it in the shop could you take a quick measurement? What is the distance from the rear of the sight base to the rear edge of the receiver? Some of us have a 9mm flat and we may want to modify it to "K" dimensions.

Thanks again for your efforts.

 

[kobra]

Awesome job! Is that HK mug for sale?

 

[jfk]

Thanks for the info jfk.

While you have it in the shop could you take a quick measurement? What is the distance from the rear of the sight base to the rear edge of the receiver? Some of us have a 9mm flat and we may want to modify it to "K" dimensions.

Thanks again for your efforts.

The overall length of the SP89 receiver from the front edge of the trunion to the rear is 8 1/2". 1" of the receiver (cocking tube area) needs to be trimmed off in an "L" shape. Your JFK MP5k cocking tube will go into this area. Make sure the hanger is lined up with a forearm before you tack weld and final weld the tube in place. Make sure you use the heatsink I sent with your parts to make sure there is no burn through and the heat sink will keep your tube lined up. Here's an 80% cast receiver that was made into a K model.

 

[jfk]

Awesome job! Is that HK mug for sale?

Yeah and the coffee inside comes with it. First drink of the morning with a dash of German flair! HAHAHA

 

[AGG]

Nice work thus far!!!

 

[kobra]

That's a real German HK SP89 getting a rebarrel and paddle mag installed, jfk does quality work!
jfk- can you do the same for my old HK 43 also?

 

[jfk]

Here's the receiver with the ledge area abrasive blasted. Before I weld the dowel pin into place, I install the mag release parts and make sure the fit is correct.



By dirtyeight




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I make any adjustments to the bushing at this point (light sanding/polishing, etc.) if needed. If the paddle is loose, that's not a problem (read further down). The receiver is blasted with an air hose and sprayed with aerosol brake cleaner.

The TIG welder is set to 60-70 amps. The heat is applied to the dowel pin. When the pin begins to melt, I move the torch in a small circular motion towards the edge of the receiver. Once the metal pools together, I back off on the foot pedal and allow the weld to cool. Keep the torch over the weld while the gases continue to come out. The shielding gas prevents air from the weld for a few seconds. This keeps the welds from being contaminated and cracking/breaking.



By dirtyeight



By dirtyeight



The amount of metal to be dressed up and cleaned is very minimal since the pin was shortened. I use a Dremel sander (60 grit) on the very tip of the pin. I change the grit to 120 when the pin nears the ledge. I then switch to a cratex polishing bit. Clean the ledge in one direction to prevent hiddeous marks from showing up during refinish.



By dirtyeight


The paddle is installed now. What about the big gap where the ledge was machined out? The paddle will now flap back and forth and look amatuer. No problem. Here's what I did:



By dirtyeight


The metal piece in the above picture was a part of a Vector .223 receiver that was removed during a 53 build. I keep parts/pieces lying around and this was ideal for the realease area. The thickness is very close to each other. With a few whack from a hammer, it looked like this:



By dirtyeight



This piece of flattened out metal is cut, trimmed and shaped.



By dirtyeight



By dirtyeight



By dirtyeight


The "U" shaped piece is dressed up on a belt sander andbent in a vise.



By dirtyeight



By dirtyeight




I place this piece up against the paddle, which still has the rest of the release parts installed. I place this up where there is no movement. This piece will keep the paddle from excessive movement and rattle. The sides are welded onto the receiver.



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The back portion is tapped down until there is no gap between the ledge and the modified sheet metal piece. The back seam is welded.



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By dirtyeight



The welds are cleaned up.



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That's it for now. The next post will be clip/pin modifications to the trigger frame. I'm waiting for the barrel to come in. Off to work I go.

 

[kobra]

Please Use the HK MP5K 4 position / 2rd trigger frame for the next example.

 

[ghilliebear2000]

great job jfk look great ...

 

[edman99]

Thanks again for the info and for this great thread jfk.

 

[11C ABN]

Great Information

JFK,

Thank you for the information, I know that I really appreciate you taking the time to inform us HK members about the nuances of building and /or modifying our HK weapons system.

I would also like to add that of the builds that I'm currently in the process of and/or the completed projects that i have had completed your Items that were purchased and or the items that you manufactured and/or machined for me were 2nd to none.

 

[dcondiff]

Great job, looks very nice.

Duuuuuuuuuuude where did you get that coffee cup

 

[kobra]

Great job, looks very nice.

Duuuuuuuuuuude where did you get that coffee cup

hey...I got 1st dibs on that cup! hehe jk!

 

[jfk]

Clipped and pinned. My first attempt was several years ago with hand tools (Dremel, file, etc.). It was horrible. I wasted my money and tossed the frame into the garbage can. I decided some jigs/tools were needed.


By dirtyeight



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This chunk of aluminum was patterned after the trigger box. It's installed into an upright jig to support the trigger frame while machining. Here's my wannabe jig with a few trigger frames in the machining process:



By dirtyeight



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The ears on the trigger frame have a small seam right in the middle. Machining the plastic to the seam line seems to work perfectly. I use a wider end mill (3/8") to cut most of the middle area and a long skimmy (3/16") end mill to finish thinning the ears. The top edge of the frame does not need to be cut. Here's what it looks like:



By dirtyeight



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How does it fit?



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Okay, waiting for the barrel now to get this project finished.

 

[kobra]

Awesome job!...again!

Will let you know when the PDW Barrel arrives ASAP!

 

[csbrit007]

Awesome work Jimmy! Just what I've been waiting for.

 

[Lukas Koukal]

Can´t wait for pics of further progress, keep it up!

 

[kobra]

MP5K PDW barrel has arrived from Mike @ Rim Country MFG!

 

[kobra]

WTS: HK SP89 barrel

The Factory HK SP89 barrel will be up for sale. Please pm me for info and price on the barrel.