I have the Gulf Coast Stacks and am happy for that as they help keep the underside of the wings clean(er). The stacks my bird has were installed by the previous owner a year or so before I took ownership of the plane.
Each "stack" or extension has 4 cuts or slots in the end (leading edge really) of the pipe that sleeves over the outside of the existing exhaust pipe and then there is a flanged clamp that goes around the outside to tighten it on. The clamp has a pin that is riveted in and the extension has a hole the pin goes through. During instal a hole is drilled in the existing stock pipe and thus the pin from the clamp goes through the extension and through the original exhaust pipe inside and the clamp is then tightened down to secure everything snugly. The pin serves to ensure the extension does not slip off if the clamp were not tightened as it should be and also prevents the extension from rotating on the stock pipe which is important because the Gulf Coast Stack extensions are angled downward and if they were to rotate 180 degrees then the would be directing hot exhaust up at the bottom of the wing (obviously a bad thing).
The four cuts or slots in the front end of each exhaust extension (referenced in the para above) result in the leading edge of each exhaust extension having in effect four "tabs" that overlap the outside of each stock exhaust pipe. The hole the clamp pin goes through goes through one of these four tabs. All this info is a build up for the issue...
The way the tips are installed, ends up having the clamps angled at about 45-deg. The problem is (I have now discovered) that when the cowl flaps are closed, the lower edge of the angled clamp / flange contacts the inside of the cowl flap and in the course of normal engine vibration a small hole appeared in my cowl flap as seen in the pic below.
- Hole (now drilled out) in cowl flap created by contact with exhaust extension flange)
As a result of this, my IA and I turned the clamp bolts around so that the shorter end (the bolt head instead of the bolt end and nut) is on the bottom side. This provides more clearance. My IA also filed down the end of the clamp to prevent contact and he also extended the adjustable rods slightly so that when the cowl flap is fully closed it is no longer perfectly flush with the bottom of the cowl but it provides more clearance. I had wanted to aesthetically fix the cowl flap by drilling a counter sunk hole on the bottom and installing a flush rivet and then painting the flush head but there was not enough metal on the inside to do that given the erosion. My IA drilled out the hole to sufficient size to prevent cracks emanating from the hole.
I plan to put some zinc chromate primer in the small hole, then put PRC sealant in it and paint the outside (all for aesthetics which may be unnecessary as it really is small. An aluminum plate patch seems like overkill. If anyone has better ideas I am all ears....
An ideal scenario would have been to rotate the clamps so that the flanges are horizontal instead of 45-deg. Unfortunately that is not an easy fix because to do so means that the position of the pin built into the clamp would then have to be rotated to a new position and new holes drilled in the exhaust pipe but the bigger issue is that the pin would then be aligned with one of the aforementioned cuts / slots in the extension and not one of the tabs and would defeat the purpose. I could also remove the pins, try to find new pins and rivet them into the clamp in a new position. Seems like what my IA and I did is the better approach overall.
I wanted to post this because wonder if others with Gulf Coast Stacks have run into this and if not you might want to check the clearance on the inside of your cowl flaps when they are closed. I never noticed because on the ground mine are always kept open in the hangar.
Thanks,
- Charles
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