Mystery hole appeared next to cowl flap, M20J

[Lax291]

I was checking tire pressures and something caught my eye on the bottom cowling, a hole that I'm pretty sure has not been there for the past 1.5 years I've had the plane. It looks like a repair was done to that same area previously and maybe for the same reason? 

Anyone seen this before, or have an idea of what cause it?

[PT20J]

I’d disconnect the cowl flap linkage to drop the cowl flap down and have a look through the opening to see what’s rubbing on the cowling.

[Slick Nick]

Looks like something is rubbing on the cowling from inside. 

[redbaron1982]

I haven't had a hole through the cowling, but the stiffener around the cowl flaps was getting some damage. It turned out to be the muffler shroud.

Completely removing the cowling could also help identify what is rubbing. Just look for some shine metal. I would bet that is related to the exhaust system/muffler shroud.

[Lax291]

Will be doing an oil change this weekend so cowl will come off and I can investigate more. I'll post whatever I can find. 

[Paul Thomas]

1 hour ago, PT20J said:

I’d disconnect the cowl flap linkage to drop the cowl flap down and have a look through the opening to see what’s rubbing on the cowling.

I'm not sure if you are mechanically inclined and familiar with you are with our airplanes. When Skip says disconnect, the cowl flaps are held by a quick connect ball joint; no tools needed.

Edited January 3 by Paul Thomas

[Lax291]

1 hour ago, Paul Thomas said:

I'm not sure if you are mechanically inclined and familiar with you are with our airplanes. When Skip says disconnect, the cowl flaps are held by a quick connect ball joint; no tools needed.

Appreciate the point out. I have dropped the cowl flap before, but was thinking while I'm doing the oil change I'll just take everything off. Maybe when I disconnect the flap I'll do a quick look to see if I can get eyes on what's right up against the hole. Appreciate the clarification!

[takair]

Were you flying over NJ imitating a drone?  

[Lax291]

16 minutes ago, takair said:

Were you flying over NJ imitating a drone?  

This had my laughing...until I realized I did a flight from CT to NJ last weekend.  Wonder of Bruce's Custom Covers is starting to make flack jackets for the cowlings...

[N201MKTurbo]

Maybe you should consider new motor mounts and shimming your engine up properly.

[Lax291]

Just took off the bottom cowling and looks like the muffler has a lip that's sticking out and it's the thin metal line that can be seen from my original pictures. Was thinking I would just crimp this more flush with the muffler body. 

[Slick Nick]

If your muffler is THAT close to the cowl, you've got much bigger issues than just the little tab sticking out. The engine mounts are probably toast, allowing the engine to "sag" down and interfere with the cowl. Once you bend that part up, it won't be long until the whole muffler is rubbing. Post a sie view of the front of you plane, and measure the gap between the spinner and the cowl top and bottom. If the sag is noticeable, have new mounts put in.

Edited January 4 by Slick Nick

[takair]

That does seem very close. As Nick said, check the rubber mounts for sagging. Would also double check the motor mount. They do crack on occasion and the cracks can be hard to see. Also check that your exhaust risers are not broken. This should be more obvious. 

[EricJ]

It may just be contacting during startup/shutdown, when the engine shakes more than usual, or if it idles rough.    It's definitely worth fixing the little protrusion, but it is a good idea to check the motor mounts, too.    Does the spinner line up well with the cowl or is the spinner a little lower than the cowl lip?

[cbarry]

That bend on the shroud looks like someone was too lazy to take it off completely to inspect the muffler/flame tubes.

[Lax291]

On 1/4/2025 at 10:45 AM, Slick Nick said:

If your muffler is THAT close to the cowl, you've got much bigger issues than just the little tab sticking out. The engine mounts are probably toast, allowing the engine to "sag" down and interfere with the cowl. Once you bend that part up, it won't be long until the whole muffler is rubbing. Post a sie view of the front of you plane, and measure the gap between the spinner and the cowl top and bottom. If the sag is noticeable, have new mounts put in.

Here are pictures from the side. The distance on top is ~14mm and bottom is ~16mm. 

[EricJ]

9 minutes ago, Lax291 said:

Here are pictures from the side. The distance on top is ~14mm and bottom is ~16mm. 

That looks really good.   Not much sag at all.

[Lax291]

1 hour ago, EricJ said:

That looks really good.   Not much sag at all.

Thanks for looking at that. When I looked through the hole again it looked like a little more clearance. I guess I'll keep an eye on it for now. Appreciate the help as always!

[EricJ]

30 minutes ago, Lax291 said:

Thanks for looking at that. When I looked through the hole again it looked like a little more clearance. I guess I'll keep an eye on it for now. Appreciate the help as always!

What sometimes happens is that the aero forces push up on the bottom of the cowl, so the cowl may move in flight.   My cowl only lines up like yours when I first put it on, but after a flight it moves up quite a bit.   If there's not much clearance with your muffler, see if you can determine whether the cowl is moving much or if the muffler is too far down for some reason.   Fixing that bent edge will probably go a long ways toward solving the problem, though.

[Yetti]

Looks like an ongoing problem.   Hole was created.   Someone riveted a fiberglass doubler over the hole.   muffler chewed through the doubler.    Grind out around the hole and use some mat and resin to repair.     Don't mix epoxy and polyester resin.  Sand smooth and paint to match.

Getting at why the hole gets chewed open is pretty important step.

Edited Tuesday at 09:35 PM by Yetti

[Lax291]

15 hours ago, Yetti said:

Looks like an ongoing problem.   Hole was created.   Someone riveted a fiberglass doubler over the hole.   muffler chewed through the doubler.    Grind out around the hole and use some mat and resin to repair.     Don't mix epoxy and polyester resin.  Sand smooth and paint to match.

Getting at why the hole gets chewed open is pretty important step.

Not sure if coincidental, but when I looked on the backside of the cowling I could see the repair, but I noticed it was exactly where brackets was mounted for the cowl flaps.  

[Aaviationist]

Have you been using 100ul?

[Kelpro999]

1 hour ago, Aaviationist said:

Have you been using 100ul?

 So far G100UL only damages all common rubber o-rings, seals and paints used on aircraft until further testing proves otherwise.

[Aaviationist]

12 minutes ago, Kelpro999 said:

 So far G100UL only damages all common rubber o-rings, seals and paints used on aircraft until further testing proves otherwise.

That’s why I asked. Because it’s opinions like this that lead to this hole being caused by 100ul. 
 

I bet that’s what it is. You can’t prove otherwise 

Edited Wednesday at 03:40 PM by Aaviationist

[Kelpro999]

15 minutes ago, Aaviationist said:

opinions like this

 Help promote further investigation before unleashing wide spread failures resulting in loss of life and property.  
 So far there’s no claim of abrasion failures related to this fuel… yet.