Progress Report – Merc 200 Project

by Chuck Petersen

Working on an engine rebuild is much like a wood boat restoration. Step one includes disassembly and overall assessment of work to be done, parts needed, etc. Step two involves cleaning, machining and restoring individual components. Finally, step three, reassembling these finished components into a working system. One tip I learned during my first boat project was to carefully label hardware, trim pieces, floor boards, etc., as these pieces might lay in storage for years before bing attended to. I use lots of zip lock bags and old business cards to keep track of components and their fasteners. In addition to labeling parts, make notes on details like mounting location, wiring patterns or hose connections. I find that by keeping complete systems in tact such as fuel systems and electrical systems and then restoring them one at a time prevents the “Hey, where does this part go?” syndrome.

I am mid-way through step two on the 1969 Merc 200. The good news is that the crankshaft cleaned up nicely with no rust pitting or wear grooves. Since this is the hardest to find and the most expensive part, I am relieved. Motors with roller bearings in the rod end like the Merc run the risk of rust pitting the crank bearing surface if left too long in storage. Many low-mileage 50’s and 60’s motors have met a violent demise when run without inspection of these surfaces.

In addition to new needle bearings, the crank has been “dressed” using strips of emory cloth and a lathe. Making a crossing motion while the crank is turning in the lathe will create a honed surface similar to a freshly bored cylinder wall. This type of surface will hold oil better during operation.

The bad news is that after 30 years of operation, the bore is oversize and losing compression. New +.0015 pistons and rings are on order. I always have a professional do both the machine work and measurements of things like bore dimensions and port heights. Special tools and an experienced hand are worth a little time and money down the road. Fortunately, I have access to a glass bead-blasting cabinet. By removing all the old paint, dirt, etc., small cracks or other imperfections are easy to spot. I also really like the clean look of blasted and clear-coated aluminum. After the bore is enlarged, a chamfer of port edges is critical to prevent rings from hanging up. This is done by hand. Then the block returns to the machinist for final finish hone. Rods have also been blasted, honed, checked for straightness, and magna fluxed for cracks. After all of this, it would be silly to use old bearings and rubber seals. So with these on order, the short block assembly is on hold for now. Now is a good time to clean up components like water-jacket covers , port covers and engine cowlings. I hate paint work, but some is inevitable. More details to come! See you in Minocqua or Red Wing.