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Tufnol or Acetal for hollow bolt

Full size diesel gear indicator

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Brian G18/02/2022 09:22:38
840 forum posts
37 photos


My son needs to replace the threaded hollow bolt that acts as the insulator for the contact that makes a light come on in the cab of a full-size Planet diesel loco. There are two of these contacts in the locomotive gearbox, one for forward, one for reverse. The bolt is just under 3 inches long and has an external thread of 7/8" BSF and an internal bore threaded 1/2" BSF.

The original part appears to be tufnol, but he wondered if acetal might be used instead? We can buy either material but are unsure which would perform best. Any ideas?

Brian G

Martin Connelly18/02/2022 09:38:57
2137 forum posts
222 photos

Tufnol comes in different types to suit different conditions. Is there any obvious thread (as in cotton thread or similar) in the ones that have failed? If so which way is it laid? Is there any sign of it being layered like a piece of slate? This may give some indication as to the best material. I would be inclined to stick to what the original design used to cater for temperature, pressure and contaminants.

Martin C

David George 118/02/2022 09:47:12
1839 forum posts
503 photos

Tufnol is the one to use. It is more stable, threads very well, and as long as you get right type and round section. The round section has the internal layers in the right direction. Wale is the correct one to use for this item.


Edited By David George 1 on 18/02/2022 09:48:37

Brian G18/02/2022 10:51:30
840 forum posts
37 photos

Thanks Martin, David. The original appears to be normal round laid Tufnol bar, not made from flat. I also believe Tufnol is the way to go, one failure in 70 years (from accidental damage) shows it works, and it keeps things original.


noel shelley18/02/2022 10:53:28
1345 forum posts
21 photos

Like the others - TUFNOL- acetal will be to soft ! Give me a PM I may be able to help you. Noel

Andrew Johnston18/02/2022 11:06:34
6602 forum posts
701 photos
Posted by David George 1 on 18/02/2022 09:47:12:



It's the lowest grade of tufnol that uses fabric, as opposed to cheaper versions that use paper.


Jeff Dayman18/02/2022 13:10:27
2225 forum posts
47 photos

A good candidate material for the switch contact would be PPS or polyphenylene sulfide rather than acetal. Brand names are Ryton or Supec. You have some in your car or truck now, if you have halogen headlamps, the light brown mantles/connector housings behind the bulbs are R4 grade PPS with 30 percent glass fibre fill. PPS is primarily made as an injection moulding resin but it can be bought in sheet and rod form from plastics suppliers.

Tufnol reinforced phenolic is inferior to PPS in pretty much every respect except the fan club. To be fair, it WAS the only game in town for this sort of part until PPS and PSU were developed in the 1960's. By the 1980's for mass most production items, PPS and DuPont's Rynite thermoplastic polyester were the preferred materials in high performance electrical switching and high heat lamp mounting applications. The large US controls firm I worked for in the 1980's -1990's moulded about 25 tons of PPS and Rynite parts yearly at the plant I worked at. Great resins.

Brian G28/04/2022 15:05:58
840 forum posts
37 photos

I just wanted to say thanks to everybody for their advice on this. We ended up getting a bar of Tufnol Whale which my son machined to make the replacement parts. Here is a photo showing the original and replacement switches, with one in place in the Planet gearbox. I wonder if they will darken over time to the colour of the original?

Brian G

tufnol switch body for planet.jpeg

David George 129/04/2022 19:45:14
1839 forum posts
503 photos

Nice to see a result of finnished job.


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