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In boat racing there are usually two categories...Stock and Modified. In developing the rules for the Canadian version of T class I needed different specification engine rules to allow the 56 cubic inch OMC to run in the class. When I studied the specification rules for EP/FV it was clear that for the OMC 49 and 56 motors that they were not Stock ware as the 60 hp mercury and the Yamaha 70CES were.

Stock
The governing body be it UIM, APBA or CBF provide engine specifications for each model outboard legal for the class. There could be several versions of the motor, but each version had its own spec sheet. There were 2 versions of the 60 hp mercury, the 52 and 60 cubic inch and two versions for the Yamaha 70CES.
There was one CBF version for the 49 and one version for the 56 but only the oval port version and it was not complete, no measurements only that it had to be a 2000 model powerhead that could run with a 15 inch midsection and nitro gearcase. The SST 60 had it’s own spec sheet.

Altered Stock
There were actually 4 different versions of the OMC. They had different flywheels, different cranks, different blocks, different carbs, different nitro gearcases. In APBA the 49 last spec sheet was for the 1988 Formula E only.
The Canadian racers were mostly running a mixture of the best performing parts from each version. The term was “grey area” in the rules book. If the rules don’t said you can’t do it then you can do it.
I called these motors Altered Stock because they were using original OEM parts, but from different
versions of the same motor.

Modified
Is when you physically alter an OEM or replacement stock component of the outboard. A legal
modification is one where there is clear definition or measurements as to the mod. A mod is a also when you replace a motor OEM or replacement part with a non OEM or replacement parts.

The one T850 rule I should have made different was the 160 compression rule. I should have left the mercury 60, the OMC 49 and the Yamaha 70CES alone just refer to their spec sheets. For the 56 OMC I should have had a cylinder head spec measurement for a 30.5 cc head (pocket depth 557). You ran that head on your 56 and it didn’t matter what the compression was, you had to use that head.
The initial reason for the 160 rule was based on multiple tests of the pure stock OMC Bridgeport fishing 56 motors running the SST 60 cylinder head. The compression was very consistent at being 152 to 157 compression. We had the spec sheet for the SST 60 so it was easy to use that head.
The problem arose when racers were allowed to modify the exhaust ports on the fishing 56. Raising the exhaust port lowers the compression on that motor so the motor would he below 150 with the SST 60 head. Racers then would run a smaller cc head. That made the motor stronger and is the main reason the mercury and Yamaha were no longer competitive in T850.