Exhaust Parts Interchangeability (different mfr’s)

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GAHorn
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Exhaust Parts Interchangeability (different mfr’s)

Post by GAHorn »

Just to pass-along a consideration for those who find they need to make exhaust repairs.

There are several exhaust systems sources for the 170 and 172 aircraft… three of the most common ones are Alcorn, Knisley, and AWI.

AWI (Minnesota) has given us superb support having donated parts to our Association convention in the past and a plan is underway for a presentation by them at the Burlington, VT convention.

Knisley (California) is also excellent product support and I’ve used them at one time or other on every airplane I’ve ever owned without difficulty.


The reason for THIS post is to describe an issue that MAY arise if anyone “mixes” parts between various mfr’s…. which came to bite me this last week.

Background: I bought a 1962 C-172-C model three years ago to train our son for his Private Cert. The airplane came with a few spare parts which included one Knisley tailpipe. The Annual Inspection (used for our pre-buy) found leaking/holed mufflers….and I called AWI and ordered replacements.

The mufflers arrived, and were fitted just fine…. And it should be noted that the mufflers “stub-out” (exit stub) is UN-drilled for the tailpipe-Clamp which has an integral “stud” or locating-pin…. and the installer must drill the stub to accept the tailpipe for proper orientation. (which by the way is with the lower-end scarf-cut/exit facing forward and slightly outboard.)
So with the new AWI mufflers installed…the existing Knisley tailpipes were fitted-up…the locating-stud hole drilled thru the existing hole in the tailpipe into the muffler stub-out…and all were clamped, …cowled-up and returned to service.

So-far….so-Good.

Two years later the left tailpipe was found to be thin at the stub-out (from fretting…a common matter) and the tailpipe had developed a crack…. SO the “NOS-Spare” Knisley tailpipe was installed on the left muffler. Life is good…things proceed just fine.

This years’ annual discovered the Rt tailpiple becoming at-risk so a new AWI tailpipe was ordered and kept on-hand.

Now, just last weekend during preventive maintenance on the nosewheel oleo it was discovered the Rt tailpipe was developing a crack…so the new AWI tailpipe already on-hand….did not fit-up completely to the AWI muffler because the expanded (swaged) portion was only 1” long …while the failed Knisley tailpipe had been expanded 2” long. This prevented the AWI tailpipe from fitting the AWI muffler which had been DRILLED to accept a Knisley tailpipe previously. :?
Locating Stud/Pin Holes shown are misaligned in rotation for Illustration purpose only.
Locating Stud/Pin Holes shown are misaligned in rotation for Illustration purpose only.
A call to AWI rolled to voicemail because it was a weekend.
A call to Knisley did the same.

On Monday a call to both AWI and Knisley rolled to voicemail… (it was Presidents’ Day)…. but I hadn’t noticed, of course. :?

On Tuesday AWI returned the call and explained (reminded me) that AWI ships out mufflers with the stub-out NOT drilled …so an installer can locate the tailpipe as desired. Their tailpipes are restricted to a 1” expanded-entry because of the FAA-PMA drawings FAA has approved. BUT…they agreed to get me airborne quickly by custom mfr’g (Not a PMA’d part but instead an Owner produced part) a tailpipe expanded 2” so it can fit the stub-out drilled for a Knisley tailpipe…. asking me to return the previously shipped PMA’d tailpipe.
Not my happiest day…but a fine and fair solution….and AWI agreed to pay the additional shipping.

Today, (Wed) I rec’d a call from Knisley (who reminded me of the Presidents Day Holiday) and apologized for not getting back to me on Tuesday (workload/backlog) but desiring to take care of me. I explained how I’d already ordered a replacement from AWI… but used the opportunity to discuss the problem which can arise when a mis-match of parts may exist when differing mfr’s are sourced for parts. Scott at Knisley said that their tailpipes are PMAd with a 2”-long expanded portion at the upper end. (I did not pursue this but the Knisley part did not have “PMA” engraved on it while the AWI part did. If true, it is an illustration how FAA-PMA may actually differ from region to region as to how closely PMA’d parts conform to OEM parts. The Continental/Cessna dwgs appear to show a 1” expanded pipe.)

Now, I hope this post may help my fellow aviators out there avoid such a problem…or at least be aware of the possibility so as to skip the loss of time.

I look forward to Burlington, VT Convention June 23-thru 29, 2022….where we will be discussing this and other matters pertinent. Get REGISTERED for Vermont!
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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