M923 Exhaust Flapper

I decided to install an exhaust flapper on my M923 to keep rain and wandering monsters out of the exhaust pipe. I tried a paint can first, and found that it didn't sit on the exhaust pipe nicely because of the big bend at the end of the pipe. So, I got a nice-looking 4" flapper off eBay, painted it black, and tried putting it on (in the rain). Unfortunately, the slight downward angle of the end of the exhaust pipe keeps the flapper from closing over the end of the pipe. The flapper hangs straight down.

Here are a few pics. I'm sorry about the lousy quality; I took them in the rain, with an old camera that I didn't mind getting wet. The flapper isn't bolted on in the pictures; It's just sitting there to show the angle of the flapper vs. the angle of the pipe. The bracket is not lined up neatly in the pictures, but lining it up right doesn't make the flap close any better.

So, the downward angle of the pipe probably helps keep rain out anyway, but I'd still like to fully cover the end opening. So, have any of y'all addressed this kind of problem before? What do you think about cutting off the flapper counterweight to hopefully make it swing closed against the end of the pipe? Or just cutting the end of the pipe to a different angle so that the flapper can close?

A friend of mine put an additional counterweight hanging off the outside of the flapper on a horizontal pipe. It seemed to work pretty well but on this pipe I think it would be easier to just cut some of it off.

dgardner said:

A friend of mine put an additional counterweight hanging off the outside of the flapper on a horizontal pipe. It seemed to work pretty well but on this pipe I think it would be easier to just cut some of it off.

Click to expand...

Thank you! Do you mean to cut off some of the existing counterweight to change the balance, or cut off some of the pipe to change the end angle? I'm leaning towards the latter, as long as it won't cause problems like letting rain drip down into the pipe despite the flapper.

After diverman's adventure where his exhaust pipe rusted through right above the plastic air tank drain tubes and the the exhaust leak caused a sudden loss of all air pressure, I think I'd like to both keep water out of the exhaust pipe and replace the plastic drain tubes with copper in the vicinity of the exhaust pipe. I'd like to replace those cheesy drain petcocks with nicer valves while I'm at it, too.

I plan to replace my exhaust, probably will go straight pip with a flapper but I may add a commercial muffler.

Looks good, Brian. I like the curved look better for my truck, but I'll keep the straight stack idea in mind. I think that such a tall stack would need to be removed in order to get my truck into my workshop (after I dig out a truck-sized hole from all of the junk in my shop, that is). Now, if only it wasn't pouring rain, I'd walk outside and remind myself whether there's already a non-welded connection somewhere between the muffler and pipe end on my truck.

Just whack off the curved portion, and attach flapper. Just like the Air Force deuces.

I had to head outside to get something from my daily driver anyway, so I took a quick peek at the 5-ton. Yup, a nice flared fitting just after the muffler. So, it'd be easy for me to replace the curved piece with a straight one, and keep the curved piece in case I ever change my mind. Maybe I'll see if there's a muffler shop near me tomorrow that can sell me a straight piece of 4" pipe with a flare at one end.

Does a straight stack make a noticeable difference in noise inside the cab vs. a bent pipe? And, does it matter which direction I point the flapper? My first guess would be to point the hinge towards the front left corner of the truck, so the flapper would tend to deflect the exhaust towards the left rear. Pretty much like in the picture above. Note, the M923's stack is behind the cab, and the muffler is angled a bit away from vertical.

If you like the curved pipe look your best bet is to trim the stack so the end less than 90˚ (as it is now it's more than 90˚). that way the flapper has to close.

Or you could cut the stack and run it straight up. (That's how mine is).

I have to tell you that my flapper is as clanky as they get. When it opens hard the weight bangs off the stack. (gets old fast).

I'm starting to warm up to the straight stack, especially since I can try it and switch back if I change my mind. I may also put the end up higher than the stock curved pipe. With the flared joint at my muffler outlet, it'd be easy to pull the stack off to reduce height. I still need to verify that the cab and/or bows will clear my shop door, anyway.

Also, I won't know whether the flapper clanking away bugs me until I try it. Maybe I'll hate the thing?

Thanks for the input!

nf6x said:

Does a straight stack make a noticeable difference in noise inside the cab vs. a bent pipe? And, does it matter which direction I point the flapper? My first guess would be to point the hinge towards the front left corner of the truck, so the flapper would tend to deflect the exhaust towards the left rear. Pretty much like in the picture above. Note, the M923's stack is behind the cab, and the muffler is angled a bit away from vertical.

Click to expand...

I have no muffler on mine. The previous owner said it didn't really make a difference in the sound. I actually have 2 videos from today. (one on the highway and the other just coming in for landing lol) But you can get a idea against the other traffic.

Mine is hinged so it's opening away from the truck, facing right.

I'll see if I can get the videos to work for me... I'm on a cell phone for my internet connection!

I think that my tail pipe bend is only about 90 degrees, but the whole stack including the muffler is tilted a bit away from vertical. That tilt is what puts the end at the wrong angle for the flapper to close. I don't think I'd need to cut too much off to make the flapper close. An inch or two should do it, give or take some depending on what angle I make the cut at.

Maybe I should try the straight pipe first anyway, before I take a chop saw to my original bend. If the flapper clanking turns out to be too annoying, then I'll probably want to go back to the original bend, with its slight downward angle at the tip.

I could hear the flapper clanking in the first video. I think it was jammed all the way open in the second one, though.

Try this. I moved them to utube.

The Beast at home.MOV - YouTube

The Beast Highway.MOV - YouTube

That attachment links worked for me. I must have had a moment of good cell signal.

Mine does exactly the same thing. I'm going to drill a hole on the cover and put a 3/8 bolt for a counter balance. Also thought of a light duty spring as well.

cut the bottom half of the circle clamp off about half way leaving a semi circle (top) with the flapper attached. Drill through each side of the clamp and the pipe and attach with two self tapping screws. This way you can adjust the angle so the flap will close completely.

Thanks, but that won't work. The end of the stock exhaust pipe on the M923 points slightly downwards, and this particular flapper is balanced such that it wants to hang straight up and down. Moving or modifying the clamp doesn't affect that.

BTW, in case the pictures weren't clear enough: The flapper isn't stuck open because of interference from the pipe; it's stuck open because that's the natural angle it wants to hang. I just slapped it on there with the clamp a bit crooked for the picture. It was raining, and I didn't want to spend much time doing it. The flap hangs at the same angle when the clamp is lined up nicely near the end of the pipe.