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Author Topic: FP 40 loses compression after adding an additional head gasket  (Read 971 times)

Offline Allen Eshleman

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FP 40 loses compression after adding an additional head gasket
« on: May 14, 2017, 01:22:56 PM »
I just added a second head gasket to an FP 40.  It seemed to have compression after the work was finished.  When I went out to fly, it didn't have compression.  I need to check some things out.  Any thoughts as I do so would be helpful.   When I took the head off, the piston liner came out also.  I didn't notice which side slot was out toward the exhaust.  In putting it back together I eyeballed the the slots and put the one out that seemed the same size as the outside port toward the muffler. I guess that's what you call a port.  Just learning.
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Offline Tim Wescott

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Re: FP 40 loses compression after adding an additional head gasket
« Reply #1 on: May 14, 2017, 01:51:22 PM »
Possibilities:

  • You messed something up with the head gasket itself.  Look for leaks by dribbling some fuel on the outside of the cylinder/head joint and slowly turning the engine through -- if you see bubbles you have a leak.
  • You got the liner in backwards or cockeyed.  If you hold the cylinder liner crankshaft side down, the top of the exhaust port should be higher than the tops of the bypass or boost ports, because the exhaust opens first (a very few engines have them opening at the same time).  There may be witness marks on the liner and cylinder block -- if there's a groove on the step at the top of the liner, that should line up with a groove or scratch-mark in the cylinder.
  • You screwed up something else unrelated to the gasket or liner -- did you have the back off the engine?
  • It's something I'm not smart enough to think up.

Think about these possibilities, and report back.
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Offline Steve Helmick

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Re: FP 40 loses compression after adding an additional head gasket
« Reply #2 on: May 14, 2017, 03:46:33 PM »
On an OS FP (and I expect most OS engines), there is (or should be) a very tiny split pin pressed into the crankcase, and a matching notch in the flange of the cylinder. Pretty solid alignment if you notice it, but some don't. Did you get the pin into the notch, or just smash it all together? It doesn't sound like the OP noticed the pin/notch. That's not good. D>K Steve
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Offline Mark Scarborough

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Re: FP 40 loses compression after adding an additional head gasket
« Reply #3 on: May 14, 2017, 09:23:52 PM »
as Steve noted, typicaly the FP will have a small locating pin to align the liner in the case. I know one NW flier had a 35 FP that suffered from really inconsistant runs, I recall him saying it finally came to light the problem was in that pin not being in the liner lip,
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