GM over the counter 'N11 Mufflers'

[David K.]

Greetings; I have GM, over the counter N11 mufflers on my car (65' roadster). These units were already installed when I purchased my car last month. The GM logo and part numbers are stamped on underside of units. Unfortunately, the passenger side muffler is beginning to blister at the lower end on the underside (typical area). I assume this is rust/rot coming out from the inside. Is there any way to retard or slow down the process? Thank you; David

[Timothy B.]

Re: GM over the counter 'N11 Mufflers'

David, The only thing to do is wire the heat riser open on the passenger exhaust manifold. This will allow the hot exhaust to evaporate the condensation that's present in the mufflers. You could also drill a small hole to allow the water to drip at the lowest point.

[David K.]

Re: GM over the counter 'N11 Mufflers'

Thanks Tim;

I'll probably drill the bottom rear end of the mufflers. Otherwise, I will drive the car long enough to always reach proper operating temperatures. Regards & thanks; David K

[Joe L.]

Re: GM over the counter 'N11 Mufflers'

Greetings; I have GM, over the counter N11 mufflers on my car (65' roadster). These units were already installed when I purchased my car last month. The GM logo and part numbers are stamped on underside of units. Unfortunately, the passenger side muffler is beginning to blister at the lower end on the underside (typical area). I assume this is rust/rot coming out from the inside. Is there any way to retard or slow down the process? Thank you; David

David------


I'm afraid there is little you can do NOW. Drilling the hole as mentioned by others and eliminating the heat riser would have worked well IF they had been done at the time the replacement GM muffler/pipe assemblies had been installed on the car. By now, the corrosion has progressed to an advanced stage based on your description. Once this sort of corrosion begins it will proceed inexorably. The next stage may be separation of the pipe at the muffler. There is no way I know of to repair this. If you were to open up this muffler I think you would find extensive corrosion damage.

[Wayne M.]

Re: GM over the counter 'N11 Mufflers'

N11 pipes had some different materials, side-to-side. IMO, that's why the price was higher for the RH pipe assy. LH 3845735 is described as w/exhaust pipe, at $67.20, in the '67 P&A 30 catalog; whereas the RH 3845736, $69.25, says w/exhaust pipe (aluminized w/st. steel end plates). Do the judges check ?

Sounds like the shell eventually goes. (touch wood; 32 years and counting, in my case).



c

[Joe L.]

Re: GM over the counter 'N11 Mufflers'

N11 pipes had some different materials, side-to-side. IMO, that's why the price was higher for the RH pipe assy. LH 3845735 is described as w/exhaust pipe, at $67.20, in the '67 P&A 30 catalog; whereas the RH 3845736, $69.25, says w/exhaust pipe (aluminized w/st. steel end plates). Do the judges check ?

Sounds like the shell eventually goes. (touch wood; 32 years and counting, in my case).



c

Wayne-----


The different construction from side-to-side was also true of the standard mufflers. I think the difference was that the right side muffler had a stainless steel end-plate. The remainder of both mufflers were aluminized steel. However, on the right side I always found if I could get 10 years from a mufler, I was lucky.

[Terry M.]

Re: GM over the counter 'N11 Mufflers'

Wayne-----


The different construction from side-to-side was also true of the standard mufflers. I think the difference was that the right side muffler had a stainless steel end-plate. The remainder of both mufflers were aluminized steel. However, on the right side I always found if I could get 10 years from a mufler, I was lucky.

Joe,

There is stainless steel and then there is stainless steel. Without knowing the alloy one can not be sure of the qualities of the material. We could both probably go into that a lot farther, but I know you are not fooled that ALL stainless steel is forever or that ALL stainless is stronger than ALL steel. As I know you know -- it all depends on a whole lot of things, but the first is knowing the material composition, second is the conditions it will live under. I know you love stainless steel, so I know you are aware of all this, but many reading may not.

[Jeff C.]

Re: GM over the counter 'N11 Mufflers'

David, The only thing to do is wire the heat riser open on the passenger exhaust manifold. This will allow the hot exhaust to evaporate the condensation that's present in the mufflers. You could also drill a small hole to allow the water to drip at the lowest point.

I just put a NOS set of the N11 pipes on my '67. Good tips. Seems like wiring open the heat riser is less painful that drilling holes in the mufflers - or should I do both? Wouldn't the holes create an exhaust leak sound? Hate to mess up the new pipes! Thank you - Jeff

[Peter L.]

Re: GM over the counter 'N11 Mufflers'

Shows it's tough getting old even for mufflers, but I'm sure most of us have experienced the muffler problem(s) numerous times over the years. Pete

[Ken R.]

Re: GM over the counter 'N11 Mufflers'

In the 2 years my new 65 was under GM warranty as a daily driver; GM installed 2 replacement sets of mufflers for me. The weld to the front of the muffler would rust out each time on both mufflers. So they were not expected to last very long IMHO.