“4 Cylinder Thunder” – Part 1

by Chuck Petersen

The problem with retiring from competitive outboard racing and enjoying hand-build power plants for strictly recreational use is that nothing breaks.  I became used to spending winter months fixing what broke during racing season.
Today, I enjoy using motors I built for competition ten years ago at our family cabin on the Three Lakes chain in Wisconsin without worry, except for basic maintenance. This winter, I decided to sort through my parts bin and build a new 44CID Merc, just for fun.

The legal engine for class D stock outboard hydro and runabout in the early 1980s was a 40CID Mark 55 block. A 44 CID model was introduced by Mercury in 1958, but was only legal in modified class E competition at that time. I recognized early on that the 44 block would bolt up directly to the 55 H tower without modification. Parts are always an issue with outboard restoration. Just try to find newer 40 CID piston rings! This is not a problem with the 44 model that was produced well into the 1990s. From a horsepower and torque standpoint, the 44 is also a better choice.

One unique advantage to building an old style MK55 was the ability to custom machine carburetor openings to accept the larger KA model carbs. This was one of the modifications allowed under stock rules when converting a MK55 “fishing” motor to MK55H specifications.

A fairly obscure version of the 44 block configuration was the 1959 MK55A. This was a 44 CID block “detuned” by using small model AJ carbs. I had a very nice 55A block on the shelf and decided to have the carb opening machined to accept the larger carbs and blueprint a trick short block from the ground up.

In addition to the crankshaft modification, I decided on a mild port job to enhance peak hp.

One part that continues to be a challenge for Mercury restorers is the crankshaft. Rust and wear on the ends are the issues here. Fortunately, I have a few nice 1955-56 “small taper” examples left to use.

A key to long life for these motors is to use the highest quality lubricants available and prep all surfaces to maximize oil flow. All bearing surfaces should be clean and “cross hatched” with medium grit, wet-dry paper to provide a surface that will hold oil. Cutting the paper into long 1/4” strips will make this job easy on crank journals. Rod ends and even piston skirts receive this type of treatment as well.

I admit to being a “bead-blast-aholic” Block rods, pistons and anything else I can get my hands on get the blast followed by a thorough cleaning . I then take the extra step to clear coat exterior block surfaces to keep a clean look.

Rods and pistons are carefully weighed. A digital kitchen scale works great for this. Match heavy rods to light pistons to balance sets. Having 40-50 of each always helps! I have found that the older 1958-59 pistons are lighter castings and thus make the best choice. Never substitute quality for weight! Any pistons showing excess wear or scoring should be tossed. I have found over the years that low mile, 6 cylinder, mark 78 “dock busters” are an excellent source for clean pistons and rods.

Now that parts are ready, we can assemble a basic short block. Part 2 will detail this process.

I hope many of my BSLOL friends enjoyed the Mt. Dora show. My parents have continued their tradition of spending a significant time in the Dora area and always look forward to seeing family and club friends along the way. Dad did a really first-class job replacing the bottom on his 1934 GarWood Sweet 16 in 2005. This is the nicest “16” I have ever seen with 100% original wood above the waterline, original power and gauges, hardware, etc. He has the “5200” process down to a science. We have ice-out here in southern Wisconsin, so I am sure to have docks to install when they return.

Editor’s Note: Look for Part 2 of Chuck’s article in the Summer issue of The BoatHouse. We are most fortunate to have the expertise of father and son – each in different areas – who are so willing to share their knowledge. Last year, Jerry gave us a running account of his restoration on that great GarWood pictures and all. BSLOL is very appreciative of all the Petersen family has contributed to The BoatHouse.