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Repower omc cobra

raguardian

New member
my 5.7 omc cracked and i repla

my 5.7 omc cracked and i replaced it with a chevy 350 four bolt main from a 1986 corvet all was well and it runs great but i lost the out drive afet 3 days in the river i just hade the out drive rebuilt prior to the new engine do i need to have it rebulit for a higher hp engine. i lost the upper gears it spun them right off. should i just go to a different type out drive i curently have a cobra outdrive omc
 
"5.7 and 350 is the same engin

"5.7 and 350 is the same engine, no need to rebuild. Why did the upper gear strip? Did You only fill the lower gear case with oil?"
 
"What Morten said - the Cobra

"What Morten said - the Cobra is a VERY tricky outdrive to fill correctly. One air bubble can cause the upper gears to run without oil.
A 350 or 5.7 L engine doesn't make any more power in a boat, because the rpm is limited to about 4800, so "Corvette" parts are of ne benefit. Plus, a 1986 Corvette had a pretty low tech engine - virtually identical power to stock marine engines of the same era."
 
Didn't the marine motors c

Didn't the marine motors come with stronger cranks and rod's to handle the added stresses placed on them by running full out somtimes for the better part of a day.
 
"Hy Stat good point,what is th

"Hy Stat good point,what is the best way check or get the air bubbles out. I also have a cobra and want to change the oil. Thanks"
 
don't need a stronger cran

don't need a stronger crank to make 260HP out of a 350. Cast crank fine. Most marine motors are just slight variations of truck engines with brass core plugs and other minor differences like maybe a head gasket and linkage brackets etc. OMC's entire year's supply of 4.3 and 5.7 engines from GM would be run off in about 2 days in GM's St. Catherines plant each year. There's nothing special done....2 bolt mains even. just cheap engines is what Merc/OMC were after in the 80's.
I always completely remove the top cover on the upper when filling a Cobra and I measure the oil going in so I know I have the specified quantity in there.
 
"Middle plug fill procedure fr

"Middle plug fill procedure from Dave Brown at Brown's Marina:
OMC Cobra 1986-1993: There is only one reservoir (the upper and lower gearcase share oil). There are, however, three screws. The drain screw is located in the *bullet* at the bottom, and the vent screws are located midway above the anti-ventilation plate and at the top center in the form of a dipstick. Pull the top and bottom screw to drain, but refilling requires that you install the bottom screw first, then fill the entire drive from the middle screw only until you get the proper reading on the top dipstick.

** FAILURE TO FOLLOW THIS FILL PROCEDURE WILL MOST CERTAINLY RESULT IN UPPER GEAR FAILURE. **

If the unit is filled incorrectly, an air pocket will form inside the drive such that when it *burps* up at a later date, will result in the upper oil level dropping thus destroying the top gears. Trust me on this as I have made quite a bit of money on people who were unaware of this procedure (they now leave their boat in my care ). Again, new gaskets are advisable on the screws.
"
 
"Is there a special technique

"Is there a special technique to remove the drain/fill screws when conventional methods so far, will not work? I'd hate to strip those things. "
 
"Ayuh,.....

I think I'v


"Ayuh,.....

I think I've owned an Impact Driver All My Life........
1 of My 1st tools......
Probably the 1 I've replaced the most often too,.....
Amazing how they'll just grow Legs,+ walk away,....
Almost as Bad as Screwdrivers,+ Vise-grips.....

Jap bikes back in the 60s,+ 70s had phillips headed screws holding'em together,...
Steel screws,+ Aluminum cases.....

If you didn't have an Impact Driver,.... You weren't fixin' Nothin'......"
 
"Having restored vintage motor

"Having restored vintage motorcycles and engines like the Henderson 4, Harley VL Flatheads, 45 flatheads, knuckleheads and panhead, I have a collection of these tools, MAYBE EVEN 1 of your vise-grips
! However, I did try to get those things out with an impact driver and all they did was laugh at me. MY LACK of large engine experience kills me in this arena."
 
"Ayuh,....

And the aluminum


"Ayuh,....

And the aluminum just Sucks all the heat away anyways......

Drilling clean thru the center might relieve it enough to Impact it out......"
 
"Did you whack it with a hamme

"Did you whack it with a hammer? The nylon washer on the drain screw compresses, allowing the screw to move on impact. This dislodges any corrosion and often allows the screw to be turned.

Impacts work in most cases, but in severe cases they simply break the head off."
 
"Amazing what I DON'T know

"Amazing what I DON'T know...I never thought about that one either Rick...IF it warms enough for me to hang outside for a while, I will go try all of these things. This is the reason I still have not finished the heads on the Crusaders...That wind blows across the water and cuts through me quickly. Once my fingers get numb, I keep dropping all the small stuff!"
 
I have a 91 OMC Cobra with the

I have a 91 OMC Cobra with the 3.0 motor. After changing oil this year I have noticed that the oil pressure gauge is very low and the engine seems to be running hot. It has even missed a few times. However the temperature gauge is ok. What can I look for?
 
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