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Omc cobra question

R

rustymich

Regular Contributor
Was cleaning my outdrive today and noticed the engine stated 2.5l on the data plate this out drive was rebuilt last year and though the engine is a 5.7 is there any concern here person I got the boat from again had it rebuilt and it still has a 1.5 year warranty on it..Please let me no if
I should have any concerns thanks..
 
The casing of the upper units are the same, the gears not.
Hopefully you have not got a casing from a 2,5 lower unit, then you are toasted very soon!
 
I know very little about the OMC Cobra drive.... but do know about drive ratios and the importance of the ratio being correct for a given engine.
Not to be Doom & Gloom here......, but don't under-estimate the possibility of an incorrect ratio having been installed. I sure hope not!:)
(there is very little latitude here until we get into very small and light weight hulls..... and some High Elevation applications!)

So I have to agree with with what I think haffiman37 is suggesting...... if the ratio is not correct, you will not get the performance that you could expect..... no matter how many different propellers you try!
There is no substitute for a correct over-all gear ratio.... NONE!
So you may want to check this!
 
Last edited:
copied and posted from another site:

The easiest way is to read it from the sticker.
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. You may be able to use the serial number to determine it too assuming someone didn't rebuild it and change it.

Barring that you can count teeth on the gears if it's apart or if removed, put it in gear, turn the drive shaft 10-20 turns or so, and have someone count the turns of the prop (you may need a protractor to measure fractions of a turn if you REALLY want to be accurate) and get out the calculator.

input shaft turns divided by propshaft turns = ratio (Yoke/Prop=ratio)


If it's a 1.5:1 or 2:1 drive it's easy.....3 turns of the yoke will produce 2 prop turns or 2 turns to 1 turn respectively.

If it's 1.43 to 1 you'll have to turn the input shaft more turns.... 143 to be precise. it would be 143 revs of the yoke to 100 revs of the prop etc.... This is where the protractor comes in handy...

For example, with a 1.43:1 drive the input shaft (engine) turns 1.43 revolutions for every 1 revolution of the prop......... If you measure exactly 1 turn of the prop. you would have to measure 1.43 "turns" of the yoke....... 0.43 times 360 degrees would be about 155 degrees.

so 1.43 turns would be 1 revolution + 155 degrees more
 
The only thing Im sure of is there was two owners other then me the first guy was with the coast guard the second that I got the boat from had the marina do all the repairs to this so I will have to from this find how this unit is on the prior owner says that it gets on plane very easy even without using the trim tabs checked all fluids clean and no metal filings but it did also have a complete service prior to being winterized I have all the receipts for this work it cost this guy 3400 for this outdrive rebuild and service..
 
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