Beginning a CD-12 adventure

[billw]

Looks like there has been some blow-by on the lower part of the exposed upper rings. Try pushing the rings in with a screw driver and see if they go in and spring back. If they don’t spring in and out a bit, they would be stuck….and thus can’t seal to the cylinder wall correctly……

Long live American manufacturing!

[fleetwin]

OK, I see the brown streak in the upper cylinder. The brown streak probably is due to blow by caused by a problem with the upper piston/rings. The good news is that the walls don’t appear to be scored/scuffed, but your pictures aren’t "close up enough" for these old eyes I’m afraid. Unfortunately, the CD series does NOT have intake bypass covers, so there is no way of inspecting that area without pulling the pistons out. I can’t really see much in the exhaust manifold pictures for similar reason, the good news is that you got the exhaust cover off with no problems.
That looks like a nice fresh water CD engine, I’m assuming there are no broken/castings, the engine looks to be in nice cosmetic condition as well. You might want to drain the gearcase lube to make sure it is in good shape before going much further.
So, at this point, you have a decision to make. You could try to soak those pistons/rings in engine tuner/seafoam, perhaps the brown streak is due to stuck rings, although I’m not seeing evidence of this in the exhaust manifold. Did you try to poke the rings in their grooves with a tiny screwdriver to see if they are stuck?
You mention wanting to work on a larger engine, this engine would be a great one to get experience on, like Chinewalker says! Let us know what direction you want to head in, we will help. I’m thinking this engine could be back on track with a new set of rings and gaskets, perhaps with a donor piston/rod from one of your other powerheads that you mention. The 56 powerhead you mention isn’t really compatible with the 55 spring mount engine series.
The first step in dissembling/rebuilding this engine is finding a decent manual, which I’m sure one of us can provide. Let us know how much OMC powerhead work you have done previously so we don’t bore you with basics or leave out important details. These engines are fairly simple to work on, no needle bearings or complex assembly procedures. But, like anything else, there are some simple mistakes to make that will ruin the project for you.
Don

[equad99]

in the last picture i see some pretty significant rust and scoring in the top cylinder wall that would cause the low compression readings.it looks to me that the motor was stuck from rust at one time or another, someone unstuck it, and tried to get it running.the scoring was there before it got rusty, probably from an overheat from a bad impeller,or possibly inadequate lubrication,if you magnify the picture, the rust pitting becomes apparent. with all of the issues this powerhead has, and i know this isnt what you want to hear, but i would replace the powerhead with a good one. the cylinder needs not only to be smooth, but it also needs to be round and straight. i would also imagine that the rings in that cylinder are probably stuck,and likely need to be replaced. the new round straight rings are not likely to seal in that cylinder….for the best possible result with the project,i would find a good complete powerhead,or a donor motor of the same year that has one.it would take care of the other issues also….art b

[kerry]

A quick tip for checking scratches- sharpen a #2 pencil or use a .5mm mechanical pencil and LIGHTLY run it across the scratches. Fine scratches can barely be felt, if at all. Scratches that matter will be easily felt. The pencil will not hurt anything.

If you have too many, AND not enough, you're a collector.

[chinewalker]

If you gently press the rings with a Popsicle stick, do they move?

[cron78]

Just pressed against the rings with the tine of a plastic fork (couldn’t find a popsicle stick or bamboo skewer). At first the top rings didn’t move. After a few tries, they all moved easily. Ran a sharpened pencil lead on the inside of the top cylinder and I can’t really tell the difference on the area with the brown streak vs. the rest of the cylinder wall.

Thanks to all for all the input so far!

[cron78]

Now that the rings seem free, at least on the exhaust port side of the pistons where I can check, I sprayed a little Seafoam in and let it sit a few minutes and put the covers back on. Tested compression and both cylinders gained about 10 or 15 pounds. Bottom went from 80+ up to 95, top only went up to 45 from 30.

Starting to believe there is blow-by on the top…

[chinewalker]

Tilt motor so Pistons are facing up and soak them in sea foam. Overnight. All weekend. They sound like they’re gunked up and sticking.

[kerry]

Time to ask the experts- does this motor have pins in the ring grooves to prevent the ring gaps from lining up? If not, might be the answer to the top cyl. compression/brown streak issue.

If you have too many, AND not enough, you're a collector.

[Mumbles]

Yup, these pistons have alignment pins.

[Author]

Posts