There is related to Gulf Coast Stacks, but this relates to an install issue so I am making a new post here under Airframe.

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

I perhaps "more

Been there too. A shop I don't normally frequent had used the wrong bolts for the attachment to the firewall of the exhaust stack holders with the result of premature wear from 1/2 to 1/8 inch and exhaust stacks rubbing the cowl flaps. Clifton Aero fabricated new, shorter exhaust stack holders, put in the correct bolts and all is well with the world again.

Thanks Pat. To the best of my knowledge I have the correct bolts and clamps. We've turned the bolts around and filed down one clamp to provide more clearance but we still had to extend one cowl flap slightly to provide adequate clearance. It seems odd that the kit would come with clamps that have this inherent issue. I guess I could look for or try to have fabricated new clamps as Clifton did for you. I wonder if they have the documentation of what they did for you and I could buy clamps from them and have them shipped to VA?

Pat - do you know if your clamps are slanted like mine or horizontal? Maybe that's an irrelevant question if you now have different clamps that were fabricated to solve the problem.

Here's another general question that I forgot to ask in my first post.... I believe (and am looking for others to correct me if I am wrong) that in flight the inside of the engine cowl is under "positive pressure" when the cowl flaps are closed. By that I mean that air is being rammed into the inlets either side of the spinner faster than it can escape around the exhaust area when the cowl flap is closed. This is why it is hard to close the cowl flap at normal cruise speed - because the "relative high pressure" air rushing out resists closing the cowl flap.... Correct?

The reason I ask the question in the previous paragraph is - I want to verify that; given a small amount of "play" in the cowl flap, in cruise the normal airflow would be pushing the cowl flap away from the exhaust flange and not towards it. With the cowl flap open or closed there is still a little (very little but it's there) play from where the pins in the push pull cable connect to the cowl flap itself. In the up or down position you can jiggle the cowl flap up and down a few millimeters. I believe with normal air-flow in flight the cowl flap (within the allowable play) will be pushed down and away from the exhaust clamps not (up) towards them.

Thanks,

- Charles

Charles,

Clifton fabricated the new 'hanging straps' not the clamps. They did position the clamps for the exhaust stack extensions differently and this did help. In the first test flight, I could feel the contact vibration in the climb so another small adjustment was made.

I would agree with your last two paragraphs.

Thanks Pat that's helpful info. I'm not having a problem with the hangars just the clamps that attach the extensions to the factory exhaust pipes. I just pulled the STC paperwork from the maintenance records and there are detailed diagrams and sure enough... they were installed correctly! So it seems to me that this is a small design defect or oversight perhaps by the original STC holder.

I just got off the phone with this company -> http://www.awi-ami.com/prodexhaust.htm who seem to make a lot of PMA exhaust components. They said that if I give them the specs we need they can probably fabricate new clamps or just supply us with new pins. Frankly if we could just move the pin position in the existing clamp by 45-deg it would solve the problem completely. Call me a fanatic but I am just not happy having the closed cowl flap not be flush with the underside of the cowl.

I am going to work on this and see what the best solution is we can come up with and I'll post back.

Pat thanks also for your confirmation of my thoughts on the "positive pressure" inside the cowl.

- Charles

Small update... The company I referenced above (AWI...) that makes aircraft components (including exhaust parts) is sending me 8 new pins (I need 4 but 8 to be safe). My IA and I plan to remove the clamps, take the pins out of the clamps and put a new pin in the clamp 45-deg further around the clamp so that when reinstalled in the existing holes the clamp flange is horizontal not 45 deg angled as seen in the pic I posted. The clamps have two holes for pin position but we will drill a new hole where we need it. The existing hole in the exhaust and extension will be reused. I'll update once we receive and install the new pins.

- Charles

Charles,
Obviously, to me, there has been a change in the design of the Gulf Coast Stacks from the original.
I installed the as manufactured by the original STC holder and they fit perfectly....so good that
I wrote a recommendation for the stacks on this site about 10 years ago.
Since then the STC holder has died and another entity has taken over.
I repeat my former claim.......why did I not install the stacks years ago instead of spending all that time
cleaning?
Steen

Can you tell me who is now making the exhaust stack extensions? I'd like to install a set for my PA-30, so that I DON"T have to keep cleaning the belly of the bird.

@ Charles... from your photo, you do not have the Kts 2 U nacelle liners.

http://www.knots2u.net/nacelle-exhaust- ... a-39-30nf/

While some say that there are no problems... most of us that do, play heck getting the Gulf Coast Stacks to work -- I did some "maintenance" and had some custom made mandrel bent SS pipes made.

All the best!
Jim