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UD M42 recoil spring


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#1 sschoen

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Posted 15 June 2016 - 06:47 AM

Hope someone can help? I would like to find a replacement recoil spring assy if possible. If not a drawing to make a replacement would be great. Any help would be wonderful.
Thanks,
Scott
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#2 Wiclovis

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Posted 11 February 2021 - 09:41 AM

Were you able to get dimensional data for the recoil spring assembly? I too need one. Thanks.


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#3 DUBLBZ

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Posted 13 March 2021 - 04:39 AM

• My initial response was to the topic “UD-42 Parts Set Listing”.
• Pardon the delay but the item required for response is not stored at my residence.
• As for the ejector – the picture in topic “M 42 Parts Image” is probably as close as anything will get. Copying the picture and magnifying it and comparing the scale beside the parts should get you pretty close. The original is not removable from the receiver slot it is installed in. An interference fit pin installed flush with the lower surface in a blind hole can’t be removed without damaging something else. I found the on-line blueprints labeled as not accurate. Going to check them.
•The UD-42 recoil spring is a captured telescoping assembly with the spring on the outside. Upon assembly of four (4) pieces, the final piece is staked, so is not capable of disassembly. The end piece looks like it may have been threaded onto the small diameter rod after spring insertion and then punched (two small “center punch” marks on the end) to prevent loosening.
• The end with less (smaller ID rod) support for the spring (squiggly end) goes in the bolt.
• Total captured free length is 6.00 inches.
• Capable compression stroke length is 2.75 inches
• OD of both spring retaining ends is .370 inches – fits in the .375 inch dia hole in bolt.
• Spring OD is .350 inches with wire dia of .040 inches (which happens to be 1.0mm). Original free length unknown.
• Spring force was measured on a digital food scale set under a shop hydraulic press to allow precision spring assembly length adjustment. Spring rate was measured at 5 inches length at 10 pounds.
• Captured free length (at 6 inches) force is 6 pounds, starting when bolt is closed.  Spring rate is 4 pounds per inch (10lbs – 6lbs over one inch, 6” – 5”).
• Picture and drawings follow.
• NOTE – What might work, in lieu of a OEM style assembly, is a free spring not larger in diameter than .370” (there are 9mm, .356”, OD springs) with a free internal (to the spring) guide rod made to seat (slight Tee shape) in the spring assembly recess at rear of lower receiver. Guide rod cannot be longer than would fit in bolt spring hole with bolt rear against the lower receiver spring seat area (all the way back). Assembling this may be a bit tricky with a long semi-unsupported spring. This also would allow playing with the spring force (length) to match whatever ammo load you plan to use. I have found 1.0mm WD x 9mm OD x 305mm long (12 inches) compression springs for sale on the internet.
 

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#4 DUBLBZ

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Posted 13 March 2021 - 05:11 AM

Attached File  UD-42 Recoil Spring Pictures.ppt   2.48MB   18 downloads


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