Beginning a CD-12 adventure

[cron78]

I didn’t mean to draft a dissertation, but I’m guessing one or more of you has dealt with the issues I am asking about below.

I have a CD-12 (’55 5.5 Johnson) with compression at 80 in one and 30 or 35 in the other. I received what I believe is a ’56 head to use as a replacement and I have a few questions. This is really for a learning experience as much as anything. There are a few minor differences that I see. The ’56 has a hose from the top of the cylinder/crankcase to the bottom while the ’55 does not. The gasket between the intake manifold and the leaf plate has different part numbers (but looks the same in the parts diagrams). And the arms on the intake manifold are different where the wing screw attaches the cover to the head. I will rebuild the card and the mag assembly on whichever head I decide to use. Will also replace the impeller. BTW, the ’55 was $60 and the ’56 head was free from a generous AOMCI member. I’ll try to figure out how to attach a couple photos. Now for questions…

1. The ’56 head has compression of 75 in one and between 80 and 85 in the other (my gauge reads in increments of 5 pounds). Am I asking for trouble if I have that much difference? I’m not interested in racing, just a running motor. Should I avoid using this head? A rebuild of one of the heads would not likely be something I could handle with my current very limited outboard mechanic abilities. My 14 year old is helping and his abilities aren’t much behind mine.

2. Do I need to replace or clean or do anything else to any of the parts on the leaf plate?

3. It looks like I do need to clean up the head. Pretty dirty. What is the best way to do that?

4. One arm on the ’56 intake manifold is broken off, so I will have to use the IM from the ’55. Other than the difference where the cover attaches, is anything else different between the ’55 and ’56 intake manifolds? Can I just use a new gasket and mount he 55 IM on the ’56 head?

5. My ’55 calls for gasket #303454 while the ’56 calls for #304116. Uh-oh. Will either of these gaskets work between a ’55 IM and a ’56 leaf plate?

5. There is a roller on the ’56 follower but not on the ’55 follower, so I’m guessing that there may be differences between the ’55 and ’56 mag cams but they are the same part number. What’s the deal with this difference? Could I still use either mag assembly or do I need to use the ’56?

Thanks for any input you good folks are willing to contribute.

Cron

[aquasonic]

In short, it sounds as if you have one of the 1954 CD-12’s with the wing-screw type cowl mounts on the intake manifold. There are so many differences between that and a CD-13, that I don’t think you want to go down that road.

Besides cowl mounts, the starter bosses may be different configuration. The carburetor is set up a little differently, and I think the base gasket is different.

[cron78]

Thanks, Aquasonic.

Yep, I compared a lot of part numbers between the heads but failed to look at an obvious one. The bottom gasket is different between them, so it looks like the ’56 head does me almost no good.

I have a CD-11 in addition to the CD-12 and the ’56 red head. The CD-11 only has one working piston, the other doesn’t move when the crankshaft rotates. Maybe I’ll have to tear down both green heads and try to build one good one from them. I’m afraid of that option, though, for two reasons. I am thinking that the gaskets and other replacement parts will cost more than possibly buying a good green head and I will be on a 100% learning curve.

Is making one decent head out of two donors something that a newby should attempt?

[aquasonic]

Cron, is it possible that your CD-12 powerhead is good, but just has a blown head gasket? That would be a great solution to the problem.

There are others on this forum with much more experience and knowledge than I have. If you are patient, I’m sure that one of the experts can guide you.

[1946zephyr]

What I would do, is pull the exhaust cover to have a look at the pistons. Like aquasonic stated, you may possibly have a bad head gasket. You can pull the head and get more of a visual of the cylinders. If push comes to shove, you can use the internals from the later powerhead such as pistons crank etc. I’m not sure if that later powerhead will will work, as a complete unit. I think I read somewhere that they will work, but don’t quote me on that. However, the internals will. If you have the wing nut fasteners on the shroud, then that is a ’54 or early ’55. They were changed in ’55, to where they slipped off, like the ’56 – ’58 did.

[chinewalker]

I’d rebuild a powerhead out of the best of the ’54-55 parts and go from there. For a newbie, these powerheads don’t get much simpler. Great ones to learn on!

[cron78]

OK, pulled the cylinder head…first time ever. I don’t know what I’m looking for, but it looks pretty good to me in there. head cover came off easily and went back on easily. Same compression before R&R and after.

Photo link below. Anyone see anything unusual? Top cylinder has 35 psi, bottom has over 80 psi.

https://www.flickr.com/gp/92111218@N04/7331FL

[fleetwin]

OK, well not real close up pics, so hard to judge. BUT, is it the lower cylinder that reads low? If so, your pics make it look like there is a gouge in the cylinder head on the lower cylinder. If my eyes are playing games with me and everything is OK, then have a closer look at the cylinder walls. Any aluminum scuffs or gouges? If not, maybe the rings are just stuck. I’m guessing this is a fresh water motor, so go ahead and "try" to remove the exhaust cover screws to have a better look at the rings/pistons through the exhaust ports. Don’t force the screws if they are stuck, you will probably break them off which is a mess not worth making.
You could probably sneak a peek at the lower piston through the exhaust ports without removing the exhaust cover by just peaking up inside the manifold while rotating the crank.
Let us know what you see, or if my suspicions are correct about the gouged head, post some real close up pics of the pistons/cylinder walls if nothing else is obvious.

[fleetwin]

OK, just reread your post, you say the top cylinder has low compression, so my eyes must be playing games with me. Did you pull the head gasket off the block? Any signs of leakage or gouged block/head surfaces that might cause a leak? Your pictures don’t seem to show signs of water in the cylinders. Sometimes those head gaskets like to fail right between the two cylinders, which wouldn’t always create a water leak. BUT, a leak between the two cylinders would cause low compression in both cylinders.
What is the story on this engine, did you just get it? If so, have you attempted to run it?

[cron78]

Fleetwin,

This engine’s story is that it was purchased from an older AOMCI member that had it sitting in his garage with probably a hundred others for years. He didn’t know if it ran or what the compression was. My intent was to learn how to work on a 1950s OMC before I tackle a ’58 SSH 35 that I want to get running again. This little CD is my learn the basics test motor, but it is in decent shape so I hope to get it running.

I checked compression (which wasn’t good in the top cylinder), rebuilt the carb but had a leak in the bottom of the carb which I believe is due to a crack in the bottom of the filter yoke assembly and/or a leaky gasket. I now have another donor carb to pull a replacment part and a new gasket for the fuel filter bowl so I guess I could fix that issue and try again. Maybe I need to put it back together and try to start it again. The mag assembly looks pretty good and it has spark to both plugs.

I didn’t remove the head gasket from the cylinder head. It is still on there in the photos. I removed the exhaust port cover…screws were easy to remove…lots of carbon build-up in there. I also wiped the inside of the cylinders with a soft cloth and ran my fingers on the cylinder walls. Even though a dark streak makes it look like there is a gouge in the top cylinder, I certainly don’t feel one. It feels smooth to me. Looking at the pistons, the rings just look like stripes on the piston. Do they look correct?

More photos in the attached link.

https://www.flickr.com/photos/92111218@ … 2928443000

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