TB20 Exhaust Clamps
These use an inconol (a high temperature steel) strip inside them, to provide a reasonably gas tight seal without the use of sealants.
However, it is very easy to damage this strip by careless tightening of the clamp, and this is a very common problem.
The best procedure is as follows:
Take the clamp off the exhaust system. With your reading glasses on :) carefully form the ends of the metal strip inside the clamp so it forms a very clean circle without any distortion, and so that its ends exactly face each other as the clamp is compressed.
But you cannot simply tighten up the clamp now, because the two ends of the strip will meet head on and buckle!
So, you need a second person (ideally) who watches the ends of the strip as they approach each other (as the clamp screw is tightened up, by you, very slowly) and just before they meet he needs to push one of the ends inwards a little bit, so they just pass each other. The metal strip has a bit of flexibility in it. This pushing needs to be done with a narrow tool of some kind because the gap in the clamp is only a few mm. The following pic shows the overlap
Once the two strip ends have passed each other, continue to tighten up the clamp a little bit more, as necessary for the exhaust system to be properly secured. But the clamp does not need to be tightened until it is closed onto itself!! That is another common mistake.
Then you get a gas-tight connection - well as "gas tight" as this will ever be... there will always be a very small amount of gas leakage.
I've had to "fix" a lot of these clamps. Very few are beyond recovery, which is just as well given their price of about $120 (2011). The only ones which are beyond recovery are those where the inconol strip burnt right through but I have never seen this and 700hrs old clamps (engine operated at peak EGT or LOP continuously) had the strips in perfect condition. But even if the strip was damaged, one could (unofficially) buy some inconol strip and fit it inside the clamp...
A contact which can fit a new Inconol strip into clamps which would have otherwise been discarded is here:
Dawley Aviation, Inc, 140 Industrial Drive, Burlington, WI, USA (1-800-338-5420; FAX=262-763-3735) email = email@example.com
I used this company in March 2019 and they did an excellent job.
It is desirable to lubricate the stanless steel clamp bolt with some high temperature grease e.g. the stuff for spark plug threads. The Socata price of this bolt/nut/washer is eye watering even by the normal aviation part standards but there is nothing special about it. Looking at the corrosion of the washer, it is made either of mild steel or an inferior grade of stainless and it rusts much faster than the other two parts.
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This page last edited 27th March 2019