Advice on restoring a 7.4L Mercruiser Bravo

JayDT

Recruit
Joined
Sep 19, 2013
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4
About 2 months ago I picked up a 27ft IMP from an auction. It was a "buy it as it comes" auction. When I picked the boat up it looked just the the pictures and info said (typical damage and wear and tear that was described) but once I got it home and cleaned it out I realized that someone had stolen the carb, coil, and a few other smaller components off the top of the motor. Everything else was still there.
The problem is that during the 5 months from when it was stored at the storage center until the time I got my hands on the vessel it had sat open to the elements and rain water had poured into the engine via the exposed intake. When I first got the boat the engine would not turn. I pulled the plugs and all looked good except cylinders 5 and 6. Both of them were full of water. I drained the water and used a breaker bar to un-seize the engine by hand. The engine is now able to turn over fine but the problem that I now have now is how to get the rust deposits out of the intake and cylinders and make sure they are clean so I don't score the cylinder walls.
The engine in this boat was well maintained and has very low hours on it. The issue is that the rest of the boat needs around $6k in repair work to get her back on the water and another 2-3k to upgrade the cabin. The budget on this project is $11k to get everything done to it. We factored in a buffer of $2k for any engine work that might be needed (obviously a new carb and components have to come out of that now) but there is no budget to replace the motor. I would like to salvage this project and get her on the water. Any suggestions as to how I could safely clean out the rust?
 

500dollar744ti

Senior Chief Petty Officer
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Jul 23, 2012
Messages
691
Re: Advice on restoring a 7.4L Mercruiser Bravo

If you have managed to get the engine freed up, you've already dislodged most of the rust that would cause damage. At this point, you might as well get it running, if it makes good compression and good oil pressure, it will be ok.

If it makes you feel any better, the fact that you got it turning makes me think there wasn't enough rust to be a long term issue anyway. I've freed up car engines like this and used them for many reliable years after.
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,093
Re: Advice on restoring a 7.4L Mercruiser Bravo

short of pulling the heads for a visual ,theres no way of knowing exactly how much rust there is.
 

Bondo

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Re: Advice on restoring a 7.4L Mercruiser Bravo

If you have managed to get the engine freed up, you've already dislodged most of the rust that would cause damage. At this point, you might as well get it running, if it makes good compression and good oil pressure, it will be ok.

If it makes you feel any better, the fact that you got it turning makes me think there wasn't enough rust to be a long term issue anyway. I've freed up car engines like this and used them for many reliable years after.

Ayuh,.... I agree, light 'er off, 'n blow it out the exhaust,...
 

wrench 3

Commander
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Aug 12, 2012
Messages
2,108
Re: Advice on restoring a 7.4L Mercruiser Bravo

Oil up the cylinders through the spark plug holes and spin the engine over a couple of times with the plugs out, before you fire it up. This will help prevent more damage.
The worst I would expect, would be excessive blow by from the crankcase vents and maybe oil consumption. Other than that, it should work OK.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Advice on restoring a 7.4L Mercruiser Bravo

..The worst I would expect, would be excessive blow by from the crankcase vents and maybe oil consumption. Other than that, it should work OK.

Or there's some crud up around the top of the cylinder bores and it smashes the rings to bits... :noidea:

Short of pulling the heads off, ya ain't gunna know.... A borescope might help.....
 

wrench 3

Commander
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Aug 12, 2012
Messages
2,108
Re: Advice on restoring a 7.4L Mercruiser Bravo

Like 500 and Bondo said, if the engine is turning over OK, then any major "crud" is all ready gone. You may have a rough surface that will not seal up properly, but the odds are in your favor.
 
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