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Thread: Adjustable rear CETME sight modification

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    Adjustable rear CETME sight modification

    I am going to try a modification on the rear sight and wondered if anybody else had tried it or if anybody thinks it will or won't work. What I plan to do is completely reversible with a new rear sight paddle and pin. I am going to get a drill bit that is the size of the four detent ball holes on the side of the sight housing. I believe this will be a #46 or #47 drill. Then I will drill through the holes (except for the one that has the spring and balls) to make a starting point on both sides of the paddle arms. Then the pin and paddle must be removed.The hole where the spring and balls were located would have to be epoxied and sanded flat and then installed back into the housing to mark the drill point on each side. Then the paddle would be moved to a drill press where a 1/16" diameter hole would be drilled about .100" deep on the side of each of the four paddles. After that .075" would be milled from both sides of the paddle, leaving shallow 1/16" diameter holes on both sides of each paddle. Sharp edges would then be broken. A 3-56 tap would be run through one or more sets of opposing holes on each side of the sight housing. The existing holes are the right size for that tap. This allows you to get another stock paddle at some point if desired and the detent balls would still work. Mount the narrowed paddle back in the housing and screw in a 3-56 screw on each side and opposite from each other. The screws should have a point or pin ground or filed on the end. The point on the screw would nose into the 1/16" holes on the side of the paddles thus locating the paddle in the vertical position. By backing out one screw and advancing the other you would have .075" windage adjustment on each side of center which should provide .187 degrees of correction for a total of 23.5" (11.75" left and right) of windage adjustment at 100 yards. Once you have the sight set, back out one screw and blue loctite it in. Then do the same with the other side. Between the front sight adjustment and the rear sight adjustment, one should be able to get on target. WILL IT WORK? To me, it sounds cleaner and more versatile than shave and shim.

  2. #2
    holescreek's Avatar
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    That's a lot to try to follow! I always try to make things reversible as well but sometimes I just have to commit.

    While I was reading your plan I kept seeing flashes of the Cetme L rear sight and wonder if it would be less work to do a similar conversion. My thought is to start out with a spare paddle and work on it so you can keep all your original parts in case the plan goes awry.

    From memory, all you'd need to do is solder a threaded bushing into the spare paddle. The thread size would be dictated by the pivot hole diameter in the existing sight body. Then you'd need a screw to pass through the first ear, threaded paddle, and second ear. As you rotate the screw the paddle moves side to side. on the far end of the screw you'd have an E-clip (or pin a disc on like an M16A1). All that's left is something to keep the paddle from rotating once in position. The Cetme L uses a spring steel leaf. You can likely do the same if you soldered a segmented disc on the side of the paddle that extended down far enough to engage the spring. You'd need the disc because of the short V sight on the paddle wheel.

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    holescreek's Avatar
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    Look at the photos in post #2 and #20 in this thread: http://www.militaryfirearm.com/Forum...-L-build/page2

    If you put a ball detent in the side of a paddle ear and have it catch the side of your retainer disc the problem of the paddle wheel moving is solved and you don't need to solder a disc to the side of the paddle. The whole conversion is quite feeseable and you aren't relying on set screw tension to hold position.

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    I considered the threaded paddle route but that negates ever using a stock paddle pin again as one side of the housing has a .157 hole and the other side has a .117" hole. The shoulder on the inside and the swage on the outside are what holds the pin in place. If you drill out the .117 to match the .157" you won't ever be going back to the factory pin. Of course you can fabricate a screw or a pin to do the same thing. I just thought it would be easier to get the alignment and adjustment from two small opposing screws using the holes already in place. This would likely be a one time adjustment unless you sell the rifle or change something. I've never seen a CETME L sight. The C model just has a tiny spring running through one arm of the paddle with a small steel ball on each end, captured between the spring and housing. I had to dose mine up with penetrating oil and pump both sides with a toothpick for a while to get it working better. The driver behind my plan is the lack of front sight windage to get my rifle to hit point of aim. I had to order 3-56 screws online and there will be delay getting them so while I ahve spot drilled three paddle arms on each side and tapped two sets of opposing holes, the project is on hold until I get screws.

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    I prefer HK rear sights on my builds so I picked an extra Cetme sight out of the pile and modded it with stuff I had on hand for proof of concept. The ID of the sight leaf was perfect for a 10-32 screw so I took the easy way out. The hardest part was grooving the screw for an E-clip. To keep the paddle from spinning I drilled dimples on the side of the paddle wheel and used an M3x.5 set screw on the left side under the E-clip. Back in the pile it goes!

    If you were doing it

    20200425_165309.jpg

    20200425_165359.jpg

    20200425_165409.jpg

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    Looks like it would work okay. Having to align the paddle arms is the fly in the ointment, but you have proven it can work. There are lots of ways to skin a cat and your solution is certainly viable while providing a classic single screw adjustment. If the set screw was on the other side you could have simply loctited a slightly loose fitting nut instead of the E clip. Perhaps one could use a square nut which if aligned properly, may be used to somehow to create the paddle alignment. To me, modifying the rear sight seems like a reasonable alternative to repositioning and pinning the barrel if the misalignment is reasonably small. If the Spanish had only made the receivers with a welded-on mounting base and then attached a removable sight to the base all kind of solutions may be possible.

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    Well my necessary parts came in last week and I went about making the rear sight adjustable for windage. Using a drill bit that was a snug fit in the four little holes on each side, I drilled a very shallow hole through all but the one where the detent balls were. These were just indentations for later drilling. I removed the center pin and slid out the paddle wheel, being careful not to lose the spring and balls. Then I epoxied a snug fitting steel pin made from a nail into the hole where the detent spring and balls were. The pin was just slightly longer than the width of the paddle wheel. After allowing the epoxy to cure I filed the protruding ends of the pin flush with the paddle wheel. Then, using a drill press, I drilled a hole about .100" deep in each of the six starting points I had drilled earlier. The 3-56 screws I used have a major diameter of .099" so these holes have to be smaller (like .050-.060" diameter). I then milled about .065" inch from each side of the paddle wheel. Then I tapped the bottom of the four holes on each side with a 3-56 tap. The existing holes are the correct size for this tap. I imagine there is a metric size tap that would work. Then I put put the paddle and a new center pin back in place with the paddle that previously held the detent spring on top. I started with 5/16" long screws. I ran a 3-56 nut up to the screw head and then ground the threaded end of the screws to about a 45 degree point. Backing the screw out of the nut cleans up the threads after grinding. Then I ran a screw through the hole on one side and turned it in so the point located in the small hole on the side of the paddle. This allows making a starter point in the remaining paddle. Then push the paddle wheel up against the other side of the sight housing and drill a starter point on that side. Then I removed the paddle wheel again and drilled the small holes on both sides of the last paddle. Then, using a new center pin I replaced the paddle wheel and carefully peened the small end while resting the other end of the pin on a hard surface. Then I installed a screw on both sides so by backing one out and turning the other side in, the sight can be moved left to right. My groups had been high and right so I screwed the front sight post up to bring the group down always stopping with the offset all the way to the right. Once the vertical adjustment was close I could work on any remaining windage with the rear sight. Any front sight adjustment other than a full revolution changes the windage. By experimenting with front and rear adjustment, one just might get it dead centered. For what it's worth, I used a depth micrometer to see how much vertical change occurred in a full revolution of the front sight. Mine measured .018". Based on a 23" sight radius one full revolution of the front sight should change the point of impact 2.8148" at 100 yards. With the 3-56 screws on the rear sight, one revolution should change the point of impact 2.796" per full revolution. The 3-56 screws will need blue loctite or locking nuts once they are adjusted. I don't appear to be able to upload photos.

  8. #8
    holescreek's Avatar
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    This site is the hardest to load photos onto but at least they are permanent as long as the site is alive.

    I have to open my pictures in MS Paint then click on "resize" and change the pixel count from however many thousands it is to about 600. Then save the photo with a new name. Then you can upload the smaller photo here and attach it to the post.

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    Let's see if your suggestion works for me.C308 rear sight overhead reduced (2).jpg
    Attached Images Attached Images

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