Ask Paul: The seal of approval

Q: We’re building an RV-6A using an O-360 narrow deck engine. There are six studs that hold this engine together. The center studs are wider at the base than the outer four studs. The case has an indentation where the seals fit. All six are counterbored. The size of the center studs at the base is slightly larger but the seal fits over the stud and rests into the counterbore OK. Should my gasket set have contained a slightly larger seal for these center studs or does this look OK?


questionable sealA: Thanks for your inquiry regarding the use of “O” ring seals on your O-360 narrow deck engine. After looking at the photos, it appears your crankcase is in compliance with Lycoming Service Instruction 1123D, which covers the “Installation of Dowels and Rubber O-Ring Seals at the Crankcase Thru-Stud Locations.”

While it was difficult to actually determine from your photos, the information you provided would lead me to believe your crankcase does incorporate the counterbores. I’d suggest you review a copy of the Service Instruction in order to actually confirm it. You should find this SI in your Avantext library. Also, any FAA approved repair station should have a copy of this publication if it maintains Lycoming engines. I think you’ll find after reviewing this publication the seal fit you spoke of is correct and no larger seals are required.

The RV6A is a very nice aircraft and I know you’ll enjoy owning and flying it.

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


  1. Richard says

    Paul, I am having my IO-360 A3B6 field overhauled. Prior to this I flew hundreds of hours with the backbone leaking oil. I want to do EVERYTHING possible to keep this new engine from leaking. What SB,SL,SI’s or other guidance can you reccomend?

  2. Stefan King says

    Paul, can you provide information concerning how to properly remove thru-studs on Lycoming engines? During annual I found a broken thru stud on the #3 cylinder of my IO-360-B1E engine that I need to replace. There are four threads left on the right case side of the engine at the 2 o’clock position of the #3 cylinder, the threads of the thru stud are still intact on the corresponding #2 cylinder on the left side of the case. The engine is still mounted on the airplane with both #3 and #2 opposing cylinders now removed for access to the center main bearing thru-studs. Thanks for the insight.

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