This may not be what you had in mind, but while you are waiting for an answer I use eBay pictures to help. I put in the 33hp head #311170 and a few came up. Pictures are pretty good. One looks like they actually stamped 170 near the spark plug holes in the combustion chamber? I have a 1965 model. It’s missing the motor cover, starter, etc. I know the pistons are different (part number) & of course the head doesn’t have compression relief valves. Never got a chance to measure port height or anything?

https://www.ebay.com/itm/NOS-OMC-EVINRU … 1438.l2649

https://www.ebay.com/itm/P45-New-OMC-Jo … 1438.l2649

Dan in TN

I never understood the 33, or the 28. I always thought they might just be marketing schemes from some suit instead of really different.

Low priced motors to fill the gap between the 18/20 and 40hp motors. Made simpler to keep costs down. But while holding onto the legendary OMC quality.

A little history – – – when the Big Twin came out in 1951, Evinrude had been working on a 25 horse power apposed four. The Johnson twin was lighter and much cheaper to build. It also put out 26 prop shaft horse power where the Evinrude 4 tested 26 horses at the powerhead, The prop shaft power was 24. After fine tuning the prototype the production Big Twin averaged 27 horse power at 4000 RPM on the dynamometer. It hit 25 HP at 3750 and dropped below 25 at 4450 RPM. At the supreme test of being put in the hands of the public some failures were noted. As more aluminum was added to various places in the outboard, you will note an increase in weight over the years.

In1955 big changes were made to quiet the noise the Big Twin produced and to isolate the noise from the boat. This resulted in an increase in weight and loss of power. The tight fitting sound dampening motor cover and carburetor intake muffler did not allow enough air into the motor. Service bulletins were issued to open the quarter inch hole in the front lower motor cover to two inches and to remove the rubber elbow and the acoustical liner from the intake air silencer. These changes brought up the horse power up to 25. They pushed it to 30 the following year by higher compression pistons and heads, needle bearings on the wrist pins, and revised port timing.

Meanwhile they were working on a bigger model. The 35.7 cubic inch Big twin was bord out 3/16" (from 2-7/8" to 3-1/16") to 40.5 cubic inches. The prototype was judged to be hard to pull over by hand and compression relief valves were added to the production model. So if you have a weak battery that is having trouble cranking your 35, you can reduce cranking effort 30% by pulling the starter rope out about six inches. A well tuned motor should start in just a few revolutions. This Big Twin was bored out another 1/8" in 1960 to 3-3/16" which brought the piston displacement to 43.9 cubic inches. It had a long life as a 40 horse power outboard. This left a big gap between it and the the 18 horse model. A slightly modified 30 horse became the 28 horse Ski Twin. Then as the 18 horse was "souped up" to 20 and 25 HP, a slightly lower compression 35 took it’s place at 33 horse power. I hope this explanation helps clear the air . . . 😎