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The parts used to assemble the motor i built over the winter was from many conversations with different builders! I ended up with a sand cast block and i installed oval port sleeves. I did grind/remove the aluminum bridge that was exposed with the oval port sleeves. Also the oval sleeves were opened up slightly before being installed. I also installed a 27cc head and older 86/87 ignition system. It then ended up in a 1970’s 75hp Hustler cowling and mid.

cool..you will have to let us know how it performs

This fact is something most new to OMC 56 cubic inch engine building T guys don’t understand. They assume that if the compression reading is below 160 psi with the spec sheet SST 60 23 cc .440 pocket depth cylinder head that they can shave the head even more to a depth of 21 cc .405 pocket depth and the motor will still run on premium pump gas.

The truth is the compression gauge isn’t the correct way to measure whether or not you need better fuel. The mathematical compression ratio is....that is the ratio between the cylinder volume to the cylinder head volume. They talk about compression ratio in high performance car motors. 9 to 1, 11 to 1 etc. The higher the ratios above approximately 9 to 1 means you need premium gas. Above 11 to 1 you are generally needing race gas.

Based on the math the Spec SST 60 head at 28.5 cc with an OMC head gasket is approximately 10 to 1 compression. That’s the reason that motor can run on premium pump gas.
Shave the head down more and the compression ratio will be 12 to 1 and the motor motor will require race fuel

My point is you may think your motor is safe to run on premium gas based on the compression gauge reading, but it is not going to have a long life span .

My advice is don’t run a cylinder head smaller than the SST 60 head regardless of the compression gauge reading.

Another bit of advice is try 17 to 19 cranking degree timing instead of 21 initially and maybe try this Tim Topping suggestion....break in your fresh rebuild with the stock 120 compression head and then rejet the motor and install your higher compression head.

Thanks Paul, i will post results down the road. I’m curious to record the outcome and compare with what i have built in the past!

Two T Boat owners messaged me about their planned engine builds. One was a 1974 OMC 49 and the other was a 56 OMC Bridgeport. The 49 was a basic rebuild and the 56 questions were about modifications and the porting.

First off “I AM NOT A MOD OUTBOARD GUY!” So I cannot offer suggestions as to how large to make the ports nor what the results will be.
But I have a lot of experience with some OPC stock classes. And Ontario Canada T classes T750 and T850

Here’s some information about the OMC 56 Bridgeport powerheads.
The sleeves in the stock blocks are basically the same.
The commercial blocks can have larger decompression holes to reduce the compression to make the pull starting easier.

There can be a noticeable difference in the ports cylinder to cylinder and the sleeves can be slightly off in position in cylinder. Seeing some aluminum from the block casting in the ports is common. The deck on the block is not perfect nor is the surface of the cylinder head.

The rods with the same number stamping are usually the same length but their weights can vary from 1 to 4 grams. It is better to use 3 rods with the same part number.

The crank shafts can be 4 spline or fine spline . Other than the different spline they are the same. Some cranks are more accurate in their specifications and perform better in higher than 7000 rpm applications.

You can make a noticeable improvement in overall performance and increased life span by doing a better rebuild than the factory did when the motor was built.

The original cylinder sleeves are usually very poorly done. SST 60 sleeves and replacement CNC manufactured sleeves are more consistent sleeve to sleeve.

Many DIY T Boat guys now realize that removing the sleeves from the block is a worthwhile task. Many heat the block using their BBQ to remove and re-install the sleeves. They put the sleeve in their freezer in order to re-install them back in the block.

With the sleeves out it is much easier to slightly inlarge and remove the excess block aluminum castings near the ports. You will also notice the variances & rough wavy edges in the porting. It’s best to match the different ports so all 3 sleeves are the same and cleanup the edges. You don’t need to make the porting larger if you don’t want too. Just match each individual port size to the largest one.

If you have multiple rods to choose from. Weigh them and use 3 that are closest in weight. Surface the block and cylinder head so they’re flat and straight. If you want to increase the compression shaving the head does really improve performance.

What Don said about the inconsistency between the bridgeport sleeve openings is 100% true. Not only is the visual size different but the machined openings are so rough that its surprising they run as good as they do! There is a lot to be said about just cleaning up the port openings so each cylinder matches. Something as simple as balancing each cylinder will improve performance and have no poor effects on life span. Replacement 56ci sleeves are made to a much higher quality, the port openings and finish around the edges is significantly improved over the stock sleeves. These motors where developed to take over the 50-90hp market and they did. They are a good workhorse motor even with the poorly matched ports and cleaning them up inside can increase the enjoyment for years to come!

Stripped my Yamaha down again, the paint was really starting to fall apart. Going for powdercoat this time, in gloss black with some bits in red. Hopefully another carbon cowl in the works then maybe some custom graphics.

At powdercoating he found a crack in the swivel tube, just as well I got it done!
Crack welded up and a plate over the top of it. Need to get it all back together for Easdale