Tower .46 Muffler Problem
02-20-2005 | 03:07 PM
#1
Tower .46 Muffler Problem
I have a Tower .46 engine and have a problem with the long muffler screw. I wanted to find out if anyone else is having the problem detailed below.
I am a fastener engineer and in my opinion, the design is faulty in that the two recessed shoulders (on the front and rear castings) the center tube fits over are too small and provide minimal support. The long screw then takes all the stress in both a stretching and rocking motion due to movement from the long muffler. The screw shank fatigues just under the head, then breaks off and the muffler is lost.
I have had my first muffler tube and rear casting lost due to the screw breaking under the head.
Tower Hobbies replaced the muffler under warranty. I used permanent thread lock on the new muffler threads, both the screw and the lock nut, and installed the new muffler. After 5 flights, the rear of the muffler was turnng, but not because the threads loosened, but because the muffler screw is stretching from fatigue. I worked very hard to remove the locknuts and rear muffler casting. The shank under the head was discolored and showed evidence of stretching: It will fail again.
My solution is to use an oversized "case hardened" screw (the stock screw does not appear to be case hardened), or hardened "All-Thread" with a locknut on each end, and JB Weld the three muffler pieces together to make the unit solid.
Is there anyone else having this problem?
Regards,
I am a fastener engineer and in my opinion, the design is faulty in that the two recessed shoulders (on the front and rear castings) the center tube fits over are too small and provide minimal support. The long screw then takes all the stress in both a stretching and rocking motion due to movement from the long muffler. The screw shank fatigues just under the head, then breaks off and the muffler is lost.
I have had my first muffler tube and rear casting lost due to the screw breaking under the head.
Tower Hobbies replaced the muffler under warranty. I used permanent thread lock on the new muffler threads, both the screw and the lock nut, and installed the new muffler. After 5 flights, the rear of the muffler was turnng, but not because the threads loosened, but because the muffler screw is stretching from fatigue. I worked very hard to remove the locknuts and rear muffler casting. The shank under the head was discolored and showed evidence of stretching: It will fail again.
My solution is to use an oversized "case hardened" screw (the stock screw does not appear to be case hardened), or hardened "All-Thread" with a locknut on each end, and JB Weld the three muffler pieces together to make the unit solid.
Is there anyone else having this problem?
Regards,
02-20-2005 | 03:36 PM
#2
My Feedback: (21)
RE: Tower .46 Muffler Problem
Over tightening is the cause of the breakage....my technique is to Lock-tite
the assy. when it is new (for maximum effectiveness) and use two pop-rivets
to keep the parts from rotating. I fought with problem for awhile when the Tower
engines came out....this has been the most effective cure.
FBD.
the assy. when it is new (for maximum effectiveness) and use two pop-rivets
to keep the parts from rotating. I fought with problem for awhile when the Tower
engines came out....this has been the most effective cure.
FBD.
02-20-2005 | 03:37 PM
#3
My Feedback: (16)
RE: Tower .46 Muffler Problem
There's also the problem with the difference in the coefficient of expasion between aluminum and steel. The muffler will get tight and stretch the screw when hot and then be loose when cold?
I lost some too. GMS has had about three different designs of the screw because of this problem but none of those has solved the problem.
The first one I had was of the 2nd design. I lost the back end like you did.
I put a piece of threaded rod, I'm thinking 4mm, but 4-40 should work, all the way through the muffler with double nuts jammed on both ends. I never had anymore trouble. Self locking nuts do not work because of the heat. A single nit will not stay tight against just the aluminum. I never had anymore trouble with the muffler and that was years ago.
The second version did not have a head on the screw. The front piece was threaded and the screw would pull the threads out of the front piece.
Enjoy,
Jim
I lost some too. GMS has had about three different designs of the screw because of this problem but none of those has solved the problem.
The first one I had was of the 2nd design. I lost the back end like you did.
I put a piece of threaded rod, I'm thinking 4mm, but 4-40 should work, all the way through the muffler with double nuts jammed on both ends. I never had anymore trouble. Self locking nuts do not work because of the heat. A single nit will not stay tight against just the aluminum. I never had anymore trouble with the muffler and that was years ago.
The second version did not have a head on the screw. The front piece was threaded and the screw would pull the threads out of the front piece.
Enjoy,
Jim
02-20-2005 | 04:03 PM
#5
RE: Tower .46 Muffler Problem
Dave,
My problem is not due to overtightening and I did "lock" the threads on the new muffler screw. The screw is stretching under the head from the muffler movement. The combination of the HOT exhaust hitting it at this point and the leverage from the long muffler working it is the problem. You can see with a magnifying glass the shank discoloring and stretching about 1/4" long just under the head. Your rivet idea is a good solution. It locks the three pieces together and transfers the load from the screw to the muffler castings. Was there enough material in the two castings "ledges" to drill through and have a good uniform hole to upset the blind rivet?
Regards,
My problem is not due to overtightening and I did "lock" the threads on the new muffler screw. The screw is stretching under the head from the muffler movement. The combination of the HOT exhaust hitting it at this point and the leverage from the long muffler working it is the problem. You can see with a magnifying glass the shank discoloring and stretching about 1/4" long just under the head. Your rivet idea is a good solution. It locks the three pieces together and transfers the load from the screw to the muffler castings. Was there enough material in the two castings "ledges" to drill through and have a good uniform hole to upset the blind rivet?
Regards,
02-20-2005 | 06:31 PM
#6
My Feedback: (21)
RE: Tower .46 Muffler Problem
ORIGINAL: Red Baron Mike
Dave,
The screw is stretching under the head from the muffler movement. The combination
of the HOT exhaust hitting it at this point and the leverage from the long muffler
working it is the problem. Was there enough material in the two castings "ledges"
to drill through and have a good uniform hole to upset the blind rivet?
Regards,
Dave,
The screw is stretching under the head from the muffler movement. The combination
of the HOT exhaust hitting it at this point and the leverage from the long muffler
working it is the problem. Was there enough material in the two castings "ledges"
to drill through and have a good uniform hole to upset the blind rivet?
Regards,
when the things are overtightened. Use a short, fat pop rivet right at the seams of
the center and the end pieces. By preventing the parts from turning, and just
snugging up the threads....the stripping, stretching, and breaking problems are
eliminated.
The "ledge" does not adversely effect the pop-rivet.
Dave.
02-20-2005 | 07:26 PM
#7
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From: warwick,
RI
RE: Tower .46 Muffler Problem
I had the same problem with a tower hobbies 75. They gave me a new muffler ended up come in part within a few flights. What I did was disassembled muffler assembly and clean it very good. Then mixed up some thirty minute epoxy and reassembled the muffler the way I wanted it. It's been over six months flying with no more issues of it coming apart.
