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10-26-2014, 03:59 PM   #1
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Need nylon bushings for a shift lens

Hi,

I recently purchased a Pentax 28mm f/3.5 shift lens and needed to repair some looseness in the shift mechanism. I took care of the looseness with an adjustment to the shift dovetail. However, someone had repaired the lens earlier and replaced the shift ring in the wrong direction, i.e., it read 11mm of shift when the lens was at zero shift. So I rearranged that and noticed the nylon bushings that slide in a "female" cam groove (the groove controls the shift when the ring is turned) on the shift ring were worn. Also, one bushing had been replaced with a slotted screw and was beginning to wear the inside of the cam groove. I took a bit off the screw to prevent it from scoring the top of the cam groove. It will be a machining situation to replace the slotted screw, as the threads in the shift ring were retapped to a larger size than that used for the shouldered screw that mounts the other bushing. That shouldn't be too much of a problem, but first I need to find a pair of bushings. Attached is a photo with dimensions as close as I could measure with calipers or mm ruler.

Note that there is some wear on the bushings and the height is probably more like 5+ mm. There is 1.5mm headspace above the bushing in the cam groove. Also, the cap of the bushing mounting screw fits into a shoulder in the top of the bushing.

Any suggestions for where to find these bushings would be most welcome. Thanks in advance BAB

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10-26-2014, 04:35 PM   #2
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It looks like the wear is on one side only. How about flipping it over?

Otherwise, it sounds like a job for someone with a jewellers lathe. It should be easy to make something
10-26-2014, 04:37 PM   #3
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I have had good luck finding small parts @ McMaster-Carr

But I seriously doubt you will find that exact bushing. Best result might be something close that you can finish machining yourself.

If you want to search in person go to the biggest hardware store in your area and find the old guy in the back and hand it to him. Do not go to a big box store. Do not bother with the cheerful, happy, young guy up front that wants to help you. Find the old, crusty, codger in the back and tell him your story. Get his attention and get him interested in solving your problem, then shut up and let him think.
10-26-2014, 05:29 PM   #4
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I have made bushings on the lathe of about that size for Graflex mods, but out of 360 Brass.

I have made larger items out of Acetal plastic but it expands as it is machined away and I doubt I could hold the dimensions with those small bushes. If I got the dimensions in spec, I feel they would change when the bush was parted off its parent bar.

SurplusShed sells a gutted variable focal lens barrel which has bush/screw sets for cam slots
of 3.34mm and 3.60mm (per my measurements here). They would be too small for you if you have measured correctly. It might be worth sending them an email to see if they have others.

I an not sure how 360 brass would run in your slots. If you want to try, feel free to send me a pm.

Good luck with your restoration.

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