During the last annual inspection on my 170 I found a hole in the right muffler, which was otherwise serviceable:
This is the muffler that generates cabin heat on a '55 B-model. On the front of the shroud there's an adapter that necks down to accept the SCAT tubing that goes forward to the baffling behind the right air intake. The adapter is attached to the shroud by four sheet-metal screws that go into Tinnerman nuts clipped onto the shroud. The screws on mine had pointy ends, and were long enough to touch the muffler -- that created the hole. It had obviously happened before, and the hole had been welded shut. You can see that weld in the photograph.
My mechanic welded the hole shut again and replaced the Tinnerman nuts and sheet metal screws with real captive nuts and 8-32 screws cut to the proper length, so this problem will not happen again. I'm unable to find an illustration or part number in my IPC for the adapter or screws I'm talking about, so I don't know if the screws that were installed were what the manufacturer called for.
Happy flying!
John
John Renwick
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
Dr. Kevin West has twice made presentations at conventions on the use of CO-detectors. He said that even a battery-powered home-use unit is better than nothing.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
I know I asked for it, but I'd prefer not to turn this thread into a discussion of CO detectors, which I know may become lengthy. I welcome the discussion, but in a different thread, please. This one is about a potential maintenance issue that might affect any 170 owner.
jrenwick- The RH muffler shroud front inlet heater adapter is illustrated on Figure 55 page 95 item 34. The attaching screws are item 34 PK76XZ-8-8. The tinnerman nuts are on the shroud figure.
mod cessna wrote:jrenwick- The RH muffler shroud front inlet heater adapter is illustrated on Figure 55 page 95 item 34. The attaching screws are item 34 PK76XZ-8-8. The tinnerman nuts are on the shroud figure.
Thanks! That's a 1/2" length, which is what I think I had. Not sure if this is supposed to be a pointed screw or a blunt one, but what I had were pointed.
mod cessna wrote:jrenwick- The RH muffler shroud front inlet heater adapter is illustrated on Figure 55 page 95 item 34. The attaching screws are item 34 PK76XZ-8-8. The tinnerman nuts are on the shroud figure.
Thanks! That's a 1/2" length, which is what I think I had. Not sure if this is supposed to be a pointed screw or a blunt one, but what I had were pointed.
John
1/2" screws run close in this application. I use 3/8" long #8 "Type B" (blunt point) sheet metal screws in that location to ensure they don't chafe the muffler (also a '55 B-model with Hanlon-Wilson round mufflers). I've test-fit the scat hose adapter to the shroud (off the muffler) with these screws and you still have a few extra threads. You might also check that the "stand-offs" spot welded inside the stainless shroud aren't collapsed which would allow even the shorter screws to chafe.
Miles
Miles
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
The pointed screws are not the correct screws for use with tinnermman nuts,they are a different pitch than the blunt type "b"which are correct for use with the tinnerman nuts.I admit that the only authority that I have on that is the "Spruce" catalog info on "sheet metal screws.I also admit that I used the pointed screws for years before I changed over to the type "B"screws with tinnerman nuts & never had a problem with them.Just a FYI
Zreyn wrote:The pointed screws are not the correct screws for use with tinnermman nuts,they are a different pitch than the blunt type "b"which are correct for use with the tinnerman nuts.I admit that the only authority that I have on that is the "Spruce" catalog info on "sheet metal screws.I also admit that I used the pointed screws for years before I changed over to the type "B"screws with tinnerman nuts & never had a problem with them.Just a FYI
I'm sure you're right. Mod cessna pointed out that the IPC calls for PK76XZ-8-8, and I haven't been able to find out whether those are pointed or blunt screws. From the experience of people who've spoken up on this thread, I'd guess they're the pointy ones.
This seems like a maintenance issue to me. Miles also pointed out that the shroud part includes standoffs that are supposed to keep it 1/2"" away from the muffler, which would give it just enough clearance for a 1/2" screw. If they aren't deformed. But only just enough.
