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Exhaust exit contacting belly - best way to fix?

Mach-4

Member
I seem to have found a problem that I believe is caused by the slip joint in Van's crossover exhaust binding, there is no play in it. I was clued into this by a vibration under my left foot at certain RPMs and especially when leaning aggressively inflight, essentially anytime the engine might stumble or vibrate enough that the exhaust would move and contact the belly, only about a 1/2” clearance.

My plan is to simply apply some mouse milk to the area. Is the correct method to apply the mouse milk to the exhaust just to dribble it on top and let gravity to the rest?
 

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Yep..and more

Jeff,
You may need to loosen the supports, then apply "mouse milk" or any good penetrating oil and try to wiggle the joint free. Most folks give the joints a shot of MM at annual time for good measure. Since they are stainless steel, rust is minimal but carbon deposits build up. Also, look and see how much clearance you have below the pipe before they hit the cowling..may need some adjustment on the supports to lower them. Also, if your engine mounts are worn and you have excessive sag, the pipes will get closer to the firewall. On a side note, where in Pa are you? I'm from Pgh and and also see you "rent" the KC135's..I manage KC10 "Big Sexy"Depot work .
 
You should apply a little mouse milk on those joints any time you have your cowl off. You might need to adjust your pipe hangers a little or tighten them. I’ve found cracks on some exhaust support hardware a couple times over the years.
 
If the joints have been neglected for a long time you may have to disassemble the system, clean all the joints and reassemble with antisieze.
 
Does your spinner line up well with your cowl nose? Are your motor mount rubber bushings loose or old and sagged? Just some other things to look at.
 
Fabric heat shield.

I was having trouble with one of my Vetterman pipes causing heat damage on the lower half of the engine cowl of my 6A. I bought a fabric heat shield (from Pep Boys if my memory is right) that clamps on with hose clamps. Something similar might help protect your exhaust pipe and reduce heat transfer although you doubtless need to deal with the interference.

You have to be careful with those hose clamps though. One of my clamps ended up touching the #4 cylinder pipe and rubbed a hole in it. I use mouse milk every year but needed to spray WD40 into the slip joint to get it loose so I could ship the damaged pipe to Clint at Vetterman. Even with the WD40 I had to loosen the pipes on all 4 cylinders and rotate the pipe in the joint slightly to separate it. John
 
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Thanks for all of the advice, just a quick update. I was able to apply mouse milk and get some movement back in the slip joints. I readjusted the exhaust hangar and now have about 3/4" on both exhaust pipes in their "resting" position. The spinner and cowling have the same space all the way around so seems like that's not a suspect. Time to fly the airplane and see if the floor vibration goes away.
 
Another update

I put about 5 hours on the airplane after sliding the exhaust hangar down the engine mount to gain some additional clearance for the exhaust to fuselage underside. Floor vibration is completely gone! Mouse milk will be a regular part of my routine now when the cowling is off as many have suggested and I’ll also keep and eye to make sure that the Adel clamp on the engine mount doesn’t creep back upwards. Now I just have to figure out how to repair the small paint damage underneath, this area is usually covered in some amount of oil and grime.
 
Exhaust close to the firewall/fuse joint will transfer heat to the center "tunnel" on a 7. It contributed to temperature rise of my fuel. 1/2" clear was good for mine, and I added a little 1/16' fiberfrax with a piece of aluminum tape over it.
 
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