"Bruce A. Frank" wrote in message
...
This is very useful tech info from the BD4 group! Thought you guys might
find it interesting.

[...]
So there you go: Check your cylinders on the 6 vertical fins between the
spark plug and the valve cover. There should be 0.060" - 0.090" clearance on
both sides of the fins. Look down between the fins about 1.5" for the
parting line of the casting molds and you'll see the flashing of which I'm
speaking. In the diagonal corners 90 degrees to the fins we found openings
ranging from NONE to 1/8" holes that appeared to have been drilled post
casting to a distorted "Y" at the outboard corner and a 5/8" long slot at
the inboard corner.

This explains why some planes in the fleet DON'T have CHT problems, and
some DO, and some do only on 1 or 2 cylinders. It may be that being blessed
with 4 good cylinders lets you brag to your buddies about your low CHTs, and
they might never be able to achieve those readings if their cylinders have
the flashing at the mold parting line!



Thanks for the info. Several years ago I overhauled an IO-360 Continental
engine and found one jug looking a little fried. Since the other cylinders
appeared normal for the hours in service, I went to the logs and found where
the A&P and owner complained about that cylinder's constantly sticking
exhaust valve, and indeed the guide was worn out of limits and the studs
were so corroded they wouldn't back out. We broke them. So we had to
drill them out and do some machining and detail work with helicoils and an
oversize stud. The intake valve guide was worn more than usual too, and
there was abundant coking around and under the springs in the rocker boxes,
especially on the exhaust valve guide. When you see coking like this,
something is wrong. It's hot as hell. We had to use carburetor cleaner (the
nasty kind) to get the springs and keepers out, and the exhaust valve was of
course stuck in the guide and had to be "helped" out. Everywhere we looked
around the exhaust port of that cylinder was evidence of too much heat.

So I took the damn thing home and began to contemplate it and soon found
gobs of flashing between the fins around the exhaust port and the spark
plugs. You could easily see the cooling airflow had been obstructed. And
so I took a small keymaker's file and removed the flashing, fin by fin.
That cylinder is now happy. A Continental IO-360 is a hot-running engine
anyway. It turns up to 2800 RPM in most applications, like in the
Skymaster. You can tell Continental and Cessna were concerned about cooling
because of all the elaborate baffling, a heck of a lot more than on the
daddy O-300.

Anytime I work on a cylinder, or install new cylinders, that's the first
thing I look for --- to see that all the fins are there and that there's no
crap between them to keep them from doing their job.

I just installed a brand new Millenium which had a broken exhaust port fin,
probably damaged in shipping. Luckily it is no. 6 on an O-300, stuck out
there in front of the cowl where you can see it. It WILL be watched.
Unluckily, on these cylinders, you don't have a bayonet mount for a temp
sensor; you have to monitor CHT with an under-the-plug sensor.