Certain year group of Keuffel & Esser's cursors for duplex rules are known to crumble as the years go by. It is known among the collectors as "KERCS" (K&E Rotten Cursor Syndrome.)

I own a few of those and wanted to do some repairs as much as I could.

Cursor parts suffering KERCS. Cursor bars crumble down like cray.

• Only cursor bars are affected. Frames, glasses and screws are re-useable.
• We must rebuild cursor bars.
• The tension spring within the cursor is completely pulverized due to oxidation. So, this has to be remade.
• The original screws are "inch screws." Taps for inch screws, for example, are expensive outside of US and England.

• I decided to use Delrin because this "Engineering Plastic" material is easily handled by normal machining process as if it is metal.
• Actually, Delrin is Dupon's product name. It's generic name is "Acetal", and Mr.Yukio Kubota, a collector pal of mine, kindly got a piece of this material for me. Thanks.

• I cut out this piece from a sheet of 0.2mm thick phosphor bronze. This thickness seems to give just the right strength as a spring.

• The original structure was that the metal female screws were cast inside the cursor bar plastic. This structure is impossible to recreate. My initial plan was to "self tap" the screws in the cursor bars. After several "tapping", however, the original screw wore down and become unusable, So I had to use a real tap to cut a female screw into the cursor bar.
• The inch taps are hard to get outside of US or England. So I decided to use ISO screws and associated tools.
• Mr.Kimutra kindly sent me a set of screws of an appropriate size, 1.7mm x 2.5. Although they are Phillips headed, they seem to be just fine, perhaps because they are black.

   
(Left: Half assembled cursor, Right: Restored K&E 4081-3)

(Another rule, restored K&E 4088-3. I used brass screws for this one.)

Conclusions

• Somewhere I read that Delrin could cause allergic reaction. So I hardened myself with a mask and saved every small pieces of dust of Delrin in a bag. Luckily., I did not develop any skin rashes. Good grief.
• The newly refurbished rule is in great shape. Delrin glides smoothly over celluloid body.
• I had to remake cursor bars several times. The "yield" was about 1 out of three including initial failures. Good grief.

Addendum: Use of Inch Sized Screws

On my inquiry to the Slide Rule Mailing List, Dave Martindale kindly told me that the screws for K&E cursors were Unified No.1-72 and gave me suggested metric drill sizes for making tap holes. I was also able to locate a shop in Japan which stocks Inch taps. Now I can repair cursor using original screws.

Restored K&E 4081-3