I can't figure out what this is. It looks like each post is threaded, but why is a mystery to me. My guesses include a grounding post, a bolt/nut testing stand that's not actually in an aircraft, or a super metal black box 🤘.

What is it and what does it do?

Mystery Part

  • 2
    $\begingroup$ good question. it was in a collection of images of airframe components from an inspection, so i assumed it was. i'll try to go back to the source and verify. $\endgroup$
    – Nate Lowry
    Commented Apr 15, 2021 at 17:07
  • 7
    $\begingroup$ You guys are jumping to conclusions because you don't know what you are looking at. Answered. $\endgroup$
    – John K
    Commented Apr 15, 2021 at 19:08

1 Answer 1


It's the inner liner of a heat muff, either for cabin or carb heat (you can see the engine mount tubes right next to it and that's exactly where I'd expect to see it, and the black pucks behind are part of a nose gear shock absorber). It could be home-made, using threaded rod welded to the inner liner, or maybe it's a manufactured component with the rods swaged in place like rivets.

An outer shroud will go around it with inlet and outlet bosses for SCAT hose running to the air inlet and outlet to the cabin or carb. Sometimes springs are wound around, or some other metal that can act like cooling fins; some builders just stuff the space with stainless steel wool.

The threaded rods are to improve heat transfer from the inner shroud to the air flowing past them (the threads maximize surface area). There will be a flange out of view at each end that creates an enclosed annular chamber. The inner shroud itself typically has internal baffling to function as a muffler for the exhaust. Crude sketch of what I mean below. My own plane has this kind of thing, but without the studs, just an open space.

enter image description here

  • 10
    $\begingroup$ "the threads maximize surface area" Exactly. Nowadays I'd assume fins to be used instead, but bolts used to work just fine. This is also why the rows are shifted half a column each time, to increase heat dissipation. $\endgroup$
    – Mast
    Commented Apr 16, 2021 at 9:51
  • $\begingroup$ Great work! And thanks for the visual too! $\endgroup$
    – Nate Lowry
    Commented Apr 17, 2021 at 12:46

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