Confusion abounds on difference in rocker arms

I’m confused about the rocker arm differences on the O-360-A4K parallel valve cylinders because I know about the extreme heat that the exhaust valves endure.

My question is why does the intake rocker arm #69444 have the extra oil hole that leads to the intake valve shaft to cool and lubricate it while the exhaust rocker arm #74636 doesn’t have this extra oil lube and cooling?

RICH BOWER

Spartanburg, S.C.

Believe me, you are not the only person to be confused about the two rocker part numbers you’ve mentioned. As you may know, for many years the rocker arm for the intake valve was part number 69444 and the exhaust rocker part number was 74636. Each of these was designed differently to do a specific job. The 69444 had the oil drain hole under the pushrod end of the body and primarily oiled the bushing, while the 74636 had the oil drain hole located under the valve end to allow the oil to assist in lubricating the rotator cap of the exhaust valve where the highest concentration of heat is.

On June 5, 1991, Lycoming issued Service Instruction 1454 when it decided to standardize certain rocker arm part numbers. SI1454 was issued to explain the changes when moving to the new style rocker arms.

It is important if continuing to use the 69444 and 74636 rocker arms that they be installed in their proper positions since they are not interchangeable. The 69444 always must be installed on the intake side and 74636 must be installed on the exhaust side.

If a replacement is required, part number LW18790 or 17F19353 (these must be in compliance with Lycoming Service Bulletin 477 or AD Note 87-10-06) may be used as direct replacements for either the 69444 or the 74636.

Good shop practice would dictate recording any such change in the engine logbook for future reference.

I have a Lycoming O-360 with a PS5C carb and a JPI installed in a Swick-modified Taylorcraft. On takeoff and full throttle climbs, the #3 cylinder EGT can go up to around 1,450° or more, which is 200°-300° hotter than the coolest cylinder.

When I level off to cruise and throttle back to 2,500-2,600 rpm, #3 cools off and the EGT for the other cylinders increases so that they all run about the same (variation of less than 100°), usually around 1,350°-1,380°. The carb was recently overhauled and should be in good shape.

Do you have any insights on why #3 is so much hotter during full throttle climbs but is better behaved at cruise? Also is #3 running at this temp during climbs something to worry about?

ROGER DALKE

Boulder, Colo.

I’ll start off by saying you’ve got nothing to worry about if you’ve given me the entire story regarding the readings from the gauges. As I mentioned recently in an article regarding the fuel/air mixture on carburetor engines (EGT: What’s normal, Feb. 2 issue), distribution on a carburetor engine is poor at best. Even though your installation utilizes a pressure carburetor, it’s still combining fuel and air at a central location rather than in each individual cylinder like a fuel-injected engine.

From what you’ve shared, I’d say the #3 cylinder is running the leanest of the four cylinders at full throttle. As soon as you make a throttle change, it more or less falls in line with the others. This is not unusual for an engine that uses a carburetor.

I would be interested in knowing what your cylinder head temperatures are reading. For your specific model of engine, you must not exceed 500° F maximum CHT. We would not be too concerned if it approached 450° to 480° on takeoff and climb, but would expect it to run between 375° and 400° in a cruise configuration. These numbers would be typical for summertime operation and would be less in a colder climate.

Just for peace of mind you may want to inspect the area around the #3 cylinder intake pipe flange and sump area for any possible indications of leaks, indicated by some fuel staining.

I think you’re good to go and I appreciate you taking the time to write and present your question.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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