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Loose intake pipes

kaa

Well Known Member
Hi all, I need some help with a minor problem. The other day I found that one of my intake pipes was loose in the flange and was leaking (I have a Lycoming O-360A4M). I thought I'd replace the gasket and that'd be it. However, when I pulled it, cleaned everything and tried to put it back together, it turned out that the pipe is actually recessed in the flange. Not by a lot, maybe just 5 to 10 thou, but it's enough so that it is loose in the flange when everything is tightened up with a standard 71973 gasket. I think the gaskets that were there originally are thicker and more compressible, which is why it was fine. I pulled two other pipes and they are all the same way.

Now I'm pretty confused about how this happened. Is it a different part somehow? Or do they wear out this way? The engine was field overhauled 1100 hours ago, and I'm not sure about its history prior to that.

Also, any advice on what'd be the best way to proceed? I can buy new pipes and flanges are replace them all. This is the only idea I have, are there other options?

Thanks!
 
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Flanges

Check to make sure the flanges are not warped from over torqueing. Also if they have been loose for a while the pipes may have worn the flanges a few thousands.
You can put a piece of wet sandpaper, 320 or 220 grit, on a flat surface such as a drill press table and lap the flanges by sanding in a circular motion. Use WD40 or similar for lubricant.
 
Yep. Remove the retainer and surface it (remove material) until the pipe flange sits proud of the surface.

It happens because the retainer bowed and/or the gasket was allowed to crumble and allowed the pipe to "work" in the bore of the retainer, removing material. Once the tube is not captured by the retainer it goes downhill quickly.
 
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Associated but different, is that the usual cause of popping and backfiring when the throttle is closed ?

We have wondered about this for a bit but just kinda like the Merlin backfire sound as we turn final :D
 
There are actually two different style intake pipes and flanges. Some with a deep recess in the flange and others with a thinner recess. They need to be coordinated with a pipe that will work( thick or thin flange). Sounds like you got miss matched ones. The fix prescribed will work or you could get thin recessed flanges and replace your existing ones..
The backfire on throttle reduction is likely from a leaky exhaust gasket, leaky exhaust connection of very shore exhaust pipes. Air is sucked back into the exhaust through the pipe or leak, when the throttle is reduced, and it provides oxygen to the exhaust gasses in the pipe, which allows it to reignite and thus the loud pop.
Good Luck,
Mahlon
 
Interesting, the thick vs thin flanges. Do you have a dimension for the "thick" flanges handy? All the ones I've seen are a rolled flange, so the thickness is a function of pipe wall dimension. The thick flange pipes would therefore be pretty heavy wall.

That said, I discovered a loose pipe on my -8. This is an angle valve engine and the retainer is captive on the pipe. Not sure how a mismatch could happen unless the pipe was manufactured that way.
 
Thanks everyone for suggestions! Lapping the flanges was pretty easy (although I haven't had time to put it all back together yet).

Still curious about deep vs shallow flanges. Are they different part numbers?
 
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