Ask Paul: Why are holes larger than dowel pins?

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: I recently saw your article about Narrow Deck vs. Wide Deck and believe you are just the person to ask this question. A fellow AMT has been field overhauling his Lycoming O-360A1A engine. It is a “first run engine” out of an early Mooney. It is a Narrow Deck engine. (Yes, I know it is way past the 12 year time frame).

Here is the issue: The engine has small “stepped” dowel pins in the case half main bearing saddles (where the main bearings are installed). The removed bearings (original from factory) have small holes to match the small dowel pins. The new bearings (except for the forward {long} bearing) have much larger holes for the dowel pins. I can find no information that directly addresses this situation in the O-360 series engines, although there are SBs, SLs and SIs that address bearing shift on the dowel pins in 540 engines. His “old hand experienced engine man,” who has been his guide on this work, does not seem to feel this is an issue. I do.

Can you shed some light on this and or direct me towards a piece of tech data that supports my feeling that the dowel pins need to be sized to match the bearings being used?


A: I initially thought “here’s an easy one to answer.” That thought quickly was removed as I reread the question Ken was asking and the information he provided. What caused me to jump ahead of the game was the fact that I had focused on the “stepped bearing dowel” he mentioned. Having gone through the bearing dowel transitions during my years at Lycoming and being familiar with Service Instruction 1225D dating way back to 1977 that covered “Crankcase Bearing Dowel Replacement,” I thought I could reference that publication and be done with it. Well, am I ever glad I closely reread what Ken had written because what I had in mind as an easy, quick answer would have not only been grossly incorrect, but would have probably generated some rather nasty Letters to the Editor questioning the dimwit who passed along the incorrect information.

What Ken actually is asking only pertains to some really old engines, so please pay attention when you read my answer so you don’t run off half cocked like I almost did. The engine Ken is asking about is actually an old O-360-A1A Narrow Deck that had the original “stepped dowels” which, much to my surprise, are still acceptable for use. This engine uses six each part number 67453 bearing dowels, which include the four each main bearing dowels and the two each front main bearing dowels. This particular part number (67453 dowel) may be used with main bearing part number LW-10124 or with P/N 18D23140, which are currently in stock at Lycoming. The front main bearing used with these “stepped dowels” is P/N LW-13884 or P/N 18A26093.

Ken, you mentioned the main bearings have a larger hole for the dowel and I’d be concerned about that, as you are. Please confirm the part number of the main bearings. They should be either one or the other mentioned above.

Much to my surprise, there is nothing I could find during my research that requires the “stepped dowels” to be replaced in the Narrow Deck engines of the era and Lycoming continues to support these engines by providing main bearing dowels and corresponding bearings.

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