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1986 Cobalt 21BR - cracked block, worth saving?

J

JAA

New member
1986 Cobalt 21BR, ~800 hours, 5.7 OMC Cobra

I love my old beat up boat. I don't want a new boat. Ive owned this boat for ~11 years.

Found a crack in my block. Below head gasket, above passenger brass drain petcock. Link: video

The boat is in good shape for an 1986. Fresh water, kept out of water on lift and on land in winter. No soft spots and hull is in decent shape. Most will look at it like its in poor shape, but Cobalt build quality is a thing, and when I get in others boats they just feel cheaper even if they are newer.

RE OMC - Im not an expert but I know my way around trying to secure parts and service for them. It is really a hassle. I have all the manuals and I do the work myself. I just purchased a rebuilt outdrive which has less than 20 hours. This outdrive was done by a long time builder who only uses OMC steel and none of the new China steel. This off season I also did the dreaded lower shift cable, which is what I was testing when I found the leak. The boat now shifts flawlessly and I was so proud of myself until I saw the leak.

Previous season I replaced exhaust manifolds and elbow risers. Ive been trying to keep up with maintenance items but it is a 1986 boat. For the shift cable work I replaced u-joint and exhaust bellows as well as water hose. Have done throttle cable and other minor efforts.

My question is what type of money to save it? And if I spent the money, what should I do? For this work I will not be doing it myself so I will be paying for the entire job. Ive been reading posts here and the general advice is (in no order):
  • Replace OMC 5.7 with rebuilt OMC 5.7
  • Replace entire OMC setup with Merc
  • other options?
  • scrap my favorite boat and buy something else
In 2024, how much do you think each of those would cost? Which would you choose?
 
the cheapest (up front) path would be to get a rebuilt long block and swap the bolt-ons...given the age, I'd probably get another intake going this way. Especially if you are happy with the Cobra drive on the boat.

Replacement with the Mercruiser setup will be very expensive and probably cost prohibitive.

Another option would be to get a turnkey engine with a warranty...

its really a matter of personal choice as only you know how much thinner your wallet can go....
 
makomark said:
the cheapest (up front) path would be to get a rebuilt long block and swap the bolt-ons...given the age, I'd probably get another intake going this way. Especially if you are happy with the Cobra drive on the boat.

Replacement with the Mercruiser setup will be very expensive and probably cost prohibitive.

Another option would be to get a turnkey engine with a warranty...

its really a matter of personal choice as only you know how much thinner your wallet can go....
Click to expand...
Thank you for the response

Speaking to a local and trusted marine mechanic he said that the 350 blocks are harder and harder to come by. In upstate used 5.7's are going for ~$4k. Full job would be $5-$6k.
 
makomark said:
They are readily available...given the current fiscal policy, the price won't come down anytime soon...
Click to expand...
Thanks

Im exploring doing the rebuild myself. I've done SBF top end work. I also have a quality machine shop I trust.

My questions:
  1. What makes an engine a marine engine? With my 86 GMC engine, what is different between it and a car engine? If I found a random 350, could I just use its bottom end?
  2. What are the differences between an OMC and Merc? Is it just bolt patterns and through-transom stuff?
Thanks again
 
On the OMC/Merc differences, regarding the engines, they are normally standard production GM long blocks that each OEM marinizes to its own specs.

As far as what makes the difference between the auto vs marine engines - block and bottom ends are similar if not identical, the cams are usually different. Many of the bolt-ons are different; usually for safety purposes: fuel pump, exhaust, distributor, starter and alternator....better circulating pumps have cast impellers vs the auto standard 'stamped' ones...

if you are gonna build one, go for max torque output at your cruising RPM...stay away from anything associated with street racing....FWIW, the standard marine grinds usually make the most sense...
 
makomark said:
On the OMC/Merc differences, regarding the engines, they are normally standard production GM long blocks that each OEM marinizes to its own specs.

As far as what makes the difference between the auto vs marine engines - block and bottom ends are similar if not identical, the cams are usually different. Many of the bolt-ons are different; usually for safety purposes: fuel pump, exhaust, distributor, starter and alternator....better circulating pumps have cast impellers vs the auto standard 'stamped' ones...

if you are gonna build one, go for max torque output at your cruising RPM...stay away from anything associated with street racing....FWIW, the standard marine grinds usually make the most sense...
Click to expand...
Thank you. Im starting my research and will report back
 
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