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Fwd: [BD4] Source of HIGH CHTs on O-320 and O-360 FOUND!
This is very useful tech info from the BD4 group! Thought you guys might
find it interesting.
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Date: Fri, 02 Jul 2004 19:52:16 -0500
From: Bob Steward
As many of you know, I also participate in the Grumman email list, and this week while consulting with a well known engine shop over a problem that they'd been working on for 3 weeks a new discovery was made that suddenly dropped the CHTs 40+ degrees on engines that were running too hot.
Here is my message to the Grumman list, and you'll find that the issue is also known among Expirmental pilots with GEM, JPI and EI engine monitors.
If you have a digital CHT and have noticed spreads in temp between cylinders, this information is for you...
The names mentioned in the message are familiar to those in the Grumman owners group, but probably mean little to anyone here on the BD-4 List.
On my way back from the Chicago area, picking up a wing panel for a Tiger,
I stopped in at Bill & Carol's Precision Engine in KY to put a fresh pair
of eyes on a plane that had been bedeviling them for the last 3 weeks.
Bill had called and left a long message on my answering machine, and I KNEW
he'd been having fits with a plane that I'd previously worked on, and knew
the owner well, so I decided that a 2 hour detour would be a reasonable
thing to do.
A 74 Traveler with a fresh Precision High Compression STC O-320 was running
High CHTs on #2 and #3 (LF and RR) cylinders. They'd top 420 full rich at
cruise and break 450 at full power and #1 & #4 were in the 370-385 range,
right where you'd like them to be.
After hearing the full story this morning of all the things that had been rechecked and swapped with known good test parts on this fresh engine, I told Bill that I thought the next step in troubleshooting was to pull 1 pair of cylinders and swap them front and rear to see if the high temp followed the cylinder or stayed with the same location on the engine. Of course on the heels of a 3 week thrash with the owner living in their spare bedroom, he really didn't want to pull the cylinders. I offered to give him a hand swapping them and figured that before dark we'd be able to test fly it and have a decision on if there was a cylinder problem or some odd internal engine problem (cam?) that was causing the high CHTs.
During the course of visually inspecting the cylinders as currently installed, and recalling the 3 sets of cylinders that were installed on Hal Beauchchene's Tiger before similar high CHTs were corrected, it occurred to me that with Carol's water tight baffles, and all the rest of the work Bill had done to this engine, that it HAD to be something outside the items that are normally checked during OH, and something that Bill's 3 weeks worth of troubleshooting had not detected.
After Hal's engine gave him such fits, and it had been inspected and instrumented by LoPresti East and ECI in Texas, and ECI had provided a second set of cylinders that were just as hot as the first, we were pretty vexed about it. Eventually Hal was able to convince ECI to sell him a set of the then new Titan cylinders and take the "hot" ones back in trade. We put the 3rd set on in Hal's hangar in Elba AL, and it ran cool right out of the box, even during break in, and at that moment we both knew that the CYLINDERS THEMSELVES can cause high CHTs, even if the baffling and the rest of the engine are perfect. Not just hotter than usual, but the kind of high CHT that prevents you from even pulling the mixture back at all because to lean even the slightest amount will spike the CHTs into the 450F and rising range while in 130 knots cruise flight in January OATs.
Upon inspection of the offending cylinders on Hal's engine we noticed that SOME of the fins around the spark plug were partially obstructed with casting flashing. Not BAD, just some thin aluminum scrap poking out the sides of the fins about 1/2 way down. A tool made of a broken off hack saw blade allowed some guesstimating as to how obstructed some of the slots were.
The NEW cylinders were the latest revision of the Titan cylinders with a different alloy and a different fin pattern, so they didn't look quite the same. And we didn't really follow up on it at the time because the new cylinders cooled no matter what one did with the red knob, so Hal was happy.
Coming back to Bill's problem engine, I spotted the SAME flashing problem on the Genuine Lycoming cylinders Bill always uses (He hates ECI), as opposed to the 2 sets of ECI cylinders that we'd been through on Hal's engine. The problem was obviously worse on the #2 and #3 cylinders with much more "flash" filling the slots and limiting the air flow, though #1 and #4 were somewhat affected.
Before altering the cylinders, Lycoming Tech support was consulted again,
Bill had been picking their brain(s) for possible causes. They had never HEARD of such a thing as casting flash in between the fins restricting the air flow.
After using a set of needle files to file the flash off of the fins (6 fins, vertical around the plugs), the owner and I flew the plane at 2500' and 5000' at 2650 RPM leaned to roughness and the enriched until smooth for a 30 min test flight. We discovered a 43 degree drop in CHT on #2 from the multiple previous test flights before the fin clean up. #3 which hadn't been as vigorously filed and cleaned showed a 16 degree drop.
WOW! From 420 to 377 in one 45 minute operation!
At this very minute we are filing and smoothing the flash on ALL the cylinders to make the fins straight with no casting flash protruding and to open up the fins that had had little if any opening between them. Comparing several cylinders that Bill had lying around, some from the 70's, we were able to establish what "normal" fins should look like and see that there should be NO core shift or parting line flash blocking the 6 fins airflow.
Bill happened to have another customer's Cheetah engine in the shop and it was back for a warranty replacement of three jugs for excess valve guide wear in only a couple hundred hours. 3 of the cylinders were on the bench, and 1 which had showed acceptable wear was remaining on the plane. After examination of the 3 "bad" cylinders that Lycoming had already sent replacements for, and then the 1 "good" one that had passed the SB-388B wobble test, the exact same flash was found to correspond to each of the cylinders that were "bad". The owner reported being unable to keep them cool in a 105 knot climb at full rich based on engine monitor data.
The one "good" cylinder was clear through all the fins. We were able to eyeball a good view straight through all 6 fins to see the floor. So that supported the hypothesis, flashing = high CHT and rapidly wearing valve guides.
In another hour we'll go for a second test flight and see what the additional clean up does for us. If #3 drops to a similar temp like #2 did after the clean up, and possibly #1 and #4 go lower too, you may hear the shout all the way from Owensboro!
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!
Grumman owners rejoice, a persistent problem, oft blamed on shoddy maintenance of the baffles has been tracked down on a plane that may well be among the best instrumented and certainly one of the most obsessed over and well baffled in the fleet. And you know if Bill and I are happy with it (and you know how picky we can both be), that everyone else should be pleased with the results, too.
And the follow up message after the second test flight was even better with very tight groupings of the CHTs and the ability to lean at will without over heating the engine.
A couple of folks have already run out to their aircraft and inspected for flashing on the hot cylinders they've been battling. And guess what? They have the exact situation described earlier on the engine Bill had just completed. Both were Genuine Lycoming cylinders and 1 was on a New Tiger.
It was very difficult to get pictures down in the fins, the camera wanted to focus on the tops of the fins (damn autofocus).
I've got some cylinders here that I will use my camera that has too many buttons on it to manually focus down into the fins and get some pictures. I'll post the URL as soon as I get them up.
We did go out for that second flight after cleaning out the fins on ALL the cylinders, and the CHTs grouped very tightly and the temps at 5,000' DA and 2700 RPM leaned to roughness and then smoothed back out (on the high side of 80% power), indicating 130 mph (blunt nosed Traveler, TAS ~143) OAT 64° F.
385-390 on 3 of them, and the 4th one has a "combo" CHT to share the hole with the LASAR ignition (deactivated for testing) and we were advised by JPI that it "reads 20 degrees low", was showing 352. But its been consistently low in all the testing.
So now with the CHTs tightly grouped and able to lean at will and run high power settings, the only thing left to prove is that this cleaning out of the fins will extend the valve guide life. Check back next year for that proof, as the owner has made the suggestion that he is going to put 4-500 hours on it in 12 months, so we'll have some idea how the guides are doing next year.
In a 90 knot climb we'd see the CHT on 1 cylinder (#3 tha had been 420 in cruise and sneaking past 450 in a climb earlier) tickle the 400° number but never higher than 404. As soon as the nose was pushed over the temps went right down.
A word of caution. I'm NOT suggesting you take files or broken hack saw blades to your cylinders. The discovery phase is here, the answer to how you in the field can correct this matter remains to be seen. hopefully with the continued discussion between myself and Lycoming and the input from those that have digital gauges and have eyeballed the fins, we can build enough data to move the giant Textron corporation off dead center and get a Service bulletin issued that gives approved limits for filing to offers exchange cylinders for those with defective ones.
Don't put a big sticker on your plane that says "Experimental" just yet. G
I'll post the URL for the web site once I get the pictures and drawings completed.
Bob Steward A&P IA
BD-4 Serial #1
bd4 mailing list
Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
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"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
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.