Chafe-proof firewall penetration

We were out and about tonight, so only limited time to do shop stuff, but my shipment from McMaster came in and I couldn’t resist having a go at my penetration solution. I ended up ordering slightly different tubing than what I originally expected to line this. At first I was looking at some nylon 6/6 tubing that was only 1/32” thick, but the minimum length was 5’. Instead, after some poking around, I realized that double that wall thickness would still give me a larger OD than the snap bushing I was using previously. So instead I ended up ordering some 3/4” OD, 1/16” wall thickness Delrin tubing – and I only had to order a foot, In addition to cutting down on wastage, this also reducing the shipping cost a lot.

The fun thing here is that the stainless penetration is a little less than 3/4” OD, not including the bit of taper on the forward end. So I needed to turn down the Delrin to get the press fit I wanted. Though I should say that “turning down” implies a lot more precision than what I did, which was to figure a way to chuck a short piece of tubing with the drill and then gradually sand down the OD while repeatedly checking the fit in the penetration. Once I had the fit I wanted, I cut off excess tubing length on the backside, carefully sanded and rounded both ends of the Delrin, and finally pressed it into the stainless tube.

The resulting piece is just long enough to protrude about 1/8” on both ends of the fitting:


And a test fit confirms that it’ll accommodate all the wires needed here with room to spare:

I do think I’m going to remove the insert and add some glue before reinserting it, just to be extremely sure that it’s not going to slide out. Probably overkill, but…that might as well be the name of this project at this point.

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