Assistance for Resistance

Home shop machinist.

This is a great first project if you want to learn the metal lathe. It covers the basic fundamentals of facing, turning, drilling and threading, all while making a real part out of Delrin, a very tough polymer that machines super easy. With Delrin there is no risk of breaking any tools or egos!
The dimensions shown should fit typical aviation spark plugs. Prudence dictates double-checking your plugs just to be sure.

This month’s home shop project was suggested by Matt Burch. Matt sent a couple of photos of a cool time-saving tool he made to probe the center electrode of aviation spark plugs when measuring their resistance values. Mechanics call this type of tool a “third hand,” which simply means it provides the services of a third hand. In the case of checking plug resistance with an ohm meter, this little guy eliminates all the fumbling and bumbling associated with the job.

Precision starts with facing the raw material ends (left) and turning the diameter to establish accurate measurements (right).
With the large end turned to 0.625-inch diameter, a spotting drill (left) is used to establish the precision center for the #29 (0.136-inch) tap drill (right).

The design itself is both supremely clever and ingeniously simple—an insulator with a spring and screw center conductor. The screw is a common 8-32 stainless steel flathead and the spring is a 1-inch-long by 0.160-inch-diameter right-hand-wound compression spring from a ballpoint pen. The spring can be found in many Parker and Paper Mate pens. The spring “threads” onto the end of the screw and once assembled inside the insulator, it’s pretty much on for good (think Chinese finger trap). The flathead screw provides a place for the probe to make contact, or if you have an alligator clip for your ohm meter/multimeter, a place to clamp to.

With Delrin, an 8-32 thread can be “power tapped” with zero risk of breaking the tap (left). But still give the lathe bed a quick clean and a shot of lube before power tapping (right). This helps the tailstock slide back and forth as freely as possible.
Even with a low-powered bench lathe, you can take very aggressive cuts when rough cutting Delrin (left). Light cuts are made to “sneak up” on the final dimension (right).

While the tool itself will be of little interest to owners of engines with automotive plugs (Rotax, Jabiru, et al.), this project is perfect for someone just getting into, or thinking about getting into, machine work. As an introduction to basic lathe techniques, it covers the basics of facing, OD turning, drilling and thread tapping. The material used for the insulator is Delrin, which is a DuPont brand of nylon that is particularly fun to machine. It cuts cleanly and you can take aggressive cuts even with an underpowered benchtop lathe or mill. And, where some plastics are prone to melting and gumming up drill bits and cutters, shavings shear off Delrin like peeling a potato. Even in the worst-case scenario—taking too big a cut at too fast a feed rate—Delrin simply tends to go from peeling to slivers of plastic flying off the cutter as the material fractures away from the surface. Yet even with the most aggressive cuts, the machined surface usually still looks fine. The one caveat is when drilling: It’s best to peck drill to keep the flutes from clogging with plastic.

That’s it for now. It’s time to get back in the shop and make some chips!

A file was used to chamfer the edges (left). Checking the fit with an exemplar spark plug (center). After spotting, the clearance hole for the spring was drilled (right). Since the hole is 1.125 inch deep, peck drilling was necessary.
With the screw and spring installed, the third-hand tool in action (left). The real convenience comes with having an alligator clip to secure the ohm meter connection to the center tap when testing batches of eight or more spark plugs. An alternative to using an alligator clip is to clamp the probe to the center tap using a conductive washer (center). In this case, a brass washer with a slot grips the multimeter probe (right).
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Bob Hadley
Bob Hadley is the R&D manager for a California-based consumer products company. He holds a Sport Pilot certificate and a Light-Sport Repairman certificate with inspection authorization for his Jabiru J250-SP.


  1. Cool tool! I just drew the plastic part up in CAD and then 3D printed it in both PLA and then PC-ABS. Takes about 30 minutes to print each. Can’t wait to try it out. Thanks!

  2. I just finished checking my plugs without this and it’s a PITA. I’ve got both a lathe and 3D printer and think I’ll go Chris’ route and 3Dprint it in ABS. Great idea!

  3. I’ve found it fairly easy to hold the probe inside the barrel. The difficult part is getting a good electrical connection to the corroded center tip of the plug. I have to scrape the probe on the tip to get a good reading, which should be in the range of about 1,000 to 2,000 ohms [ 1k to 2k ].
    A high resistance plug, GT than 3k-4k, can cause the voltage to rise to a point that there is a flash over in the mag distributor, which then can cause a misfire…

  4. Delrin(r) is an ACETAL resin, not nylon. You could use nylon or PVC…they work on the lathe about the same and would be acceptable for this application.

  5. Thanks for the correction and my apologies for the mischaracterization. Readers are always welcome to point out a mistake like that!


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