omc cobra 4.3 l engine swap

[garrett113]

I recently bought a 1990 galaxie with a 4.3L and a omc cobra out drive. The out drive says it is ment for a 3.0 meaning that the upper gear ratio is slightly different. Correct? And is this a problem?
I feel as long as I don't hammer down the throttle I will be fine, but I could easily be wrong. Assuming that will not be a big problem let's proceed to the motor. I'm looking for a advice from those of you that have done engine swaps. Here are the couple options I'm cosidering: option one, pay out the ***** for a rebuilt marine short block to replace the cracked one. Option two, buy the equivalent automotive block then replace the freeze plugs, gaskets and use old cam.
I have just one other question for now. Is it possible/at all advised to pull motor without taking the out drive off? I have seen this question come up before and it seems most recommend you do not but I also remember seeing a post from another member quoting the service manual stating that if you can move the engine forward about 6 inches and you don't change the height of the motor mounts it can be done this way. I look forward to your feedback. P.s. I'm a first time boat owner the only thing I know about them is what I've been reading on hear and my automotive mechanical experience. So any other advice you have would also be appreciated. I have pics and serial numbers if wanted.

 

[HT32BSX115]

Re: omc cobra 4.3 l engine swap

I recently bought a 1990 galaxie with a 4.3L and a omc cobra out drive. The out drive says it is ment for a 3.0 meaning that the upper gear ratio is slightly different. Correct? And is this a problem?
.......................

Is it possible/at all advised to pull motor without taking the out drive off?

Howdy,

Welcome aboard!

1. Yes. It's a minor problem. You'll need a higher pitch prop to compensate for the lower ratio drive. It'll be somewhat less efficient but it'll probably work....

2. You can remove the engine without removing the drive but you risk NOT having correct alignment when you re-install the engine.

With a mis-alignment, you WILL eventually destroy the coupler. Then you'll be pulling the drive and engine to replace it........ after getting towed back to the marina.

Just pull the drive and engine in that order....and install it in reverse order. Get yourself an OEM manual and an alignment tool. do it right the first time and you won't have to do it again in the middle of summer, when you should be having fun at the lake etc.....



Regards,


Rick

 

[Lou C]

Re: omc cobra 4.3 l engine swap

It's so easy to pull the drive first, no reason to move the engine first.
If I remember right the 4 cyl models used a smaller lower gear housing than the ones for the V6/8s

 

[Howard Sterndrive]

Re: omc cobra 4.3 l engine swap

there are a few different 3.0 Cobra drives. The pre-1990 drive would not survive behind a V6 long regardless of how easy one is on the throttle. "400" size 13T spline propshaft lower only good to 140HP with it's uncaged needle bearings rolling on the propshaft.
The later 1990-on 3.0L drive had a 15 spline propshaft. It was basically a V6 lower unit.

As far as gear ratios - depends what upper and lowers are mixed and matched - also could be meaningless what a tag on the case says - there could be V6 ratio gears inside.
Could also be a 3.0 upper on the original 4.3 lower making a bastard ratio.

Seloc book has a good chart with all Cobra ratios - you could sit with a calculator and figure there are several different ratios you could have with various lower units attached to the two different 3.0 uppers.

 

[garrett113]

Re: omc cobra 4.3 l engine swap

Model number 986321 serial no t0716886 what can you tell me with this information. So after I get my service manual how would I physically go about finding the ratio? Thanks for the help and I will go ahead and pull the out drive then motor and do the alignment.

 

[HT32BSX115]

Re: omc cobra 4.3 l engine swap

Model number 986321 serial no t0716886 what can you tell me with this information. So after I get my service manual how would I physically go about finding the ratio? Thanks for the help and I will go ahead and pull the out drive then motor and do the alignment.

There is no reliable way to determine the ratio using case numbers. These drives are so old that they could have been rebuilt many times with different gear-sets in the last 15 odd years.

One way would be to remove the top cap and count teeth. Using the chart below, you can determine the ratio that you should have.

Another even simpler way (with the drive in FWD) is to just turn the input shaft a measured number of turns and count turns on the prop-shaft, then use a simple calculator to divide the prop-shaft turns by the input shaft turns.

So for example, with the 4.3L model, the crankshaft would turn 1.68 turns to 1 turn

Or 16.8 turns to 10 turns....... What is 0.8 turns? 0.8 x 360(degrees) = 288 degrees so if you have a protractor, you could measure it.

0.8 turns is also approx 3/4 turn.

You do NOT have to be exact though........1.68 -to- 1 is a "shade" less than 1.7:1 (17 --to--10) if you can count to 17 and your helper can count to 10 ya' got it!!




For the 5.0L model above........... 1.59:1 is almost EXACTLY 16 turns to 10 turns. (15.9-to-10)

If you do in the boat, remove the spark plugs. then the engine cannot start and it'll be very easy to turn it.

If you remove the drive, build yourself a simple wood stand. You'll probably want to replace the raw water pump if it's more than a year or 2 old, Turning the drive by hand is a little harder with the raw water pump installed.....you can also put a little dish soap in the pump to makeit easier to turn. (don't turn it backwards.....if it's "dry", you may not be able to turn it at all by hand)

 

[garrett113]

Re: omc cobra 4.3 l engine swap

Ok great! I will go ahead and give that a go, thanks a bunch Rick and everyone else.