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OMC Cobra bent propshaft questions.

Scrid

Member
Hello everyone!
I haven't posted here in a while because I have been pretty busy with my job. Occasionally I do get a chance to take the boat out though. :eek:
I have a 1992 Sunbird with the Cobra 4.3 I/O.
I noticed the other day that my cavitation plate on the port side was bent up. No other immediate damage noticed.
I figured the only way that could have happened was if a log got caught between the prop and the cavitation plate.
The last time I had it out with my brother we were cruising pretty slow probably about 5 mpg on the Missouri river and we both heard a "thump" from the rear.
It definitely must have been when it happened however I didn't see any logs in the vicinity of the boat at the time.
Well today while planning my attack of straightening the bent cavitation plate, I decided to check the prop and prop shaft. I can visually see that the prop shaft is off center by looking at the threaded prop nut and spinning the prop by hand.
The bearings feel ok. If I stop the shaft on the "high" spot that seems the most off center,...and check the prop blade opposite of it I can tell I have a blade bent back about a half inch.
So now I know for sure something got caught in there.
The shaft looks like it almost has about an 1/8" runout on the very end. Nothing is rubbing anywhere though.
I realize that running a bent prop shaft can cause vibration, which I have noticed, and early seal and bearing failure.
I am fully capable of replacing the shaft but I am wondering if I do so will I have to do any re-shimming of the drive gears in the back?
Also, there isn't much boating season left here in Omaha, NE and I was wondering if it might be OK to run it until winterization. Then pull the lower unit and do it over the winter season.
Here is a couple pictures.
Thanks for any input.
Chris.

IMG_20140820_131752_935.jpg

IMG_20140820_131759_756.jpg
 
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Pull the prop shaft and take it to a good machinist.
Depending on where and how severe it is bent, he may be able to straighten it.

As for replacing the prop shaft and not needing to go through the shimming procedure... I can't help you, unless these are similar to the early Merc A drive prop shafts... of which basically float, and require no change to any shim values when replacing the shaft only.




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Kim, I do very littel (if any) with the OMC Cobra drives. Does the Cobra "drive" gear (aka pinion gear) need to be removed in order to pull the prop shaft out?



OMC-800-stern-drive-parts-and-drawing.jpg
 
Thanks everyone. I'm not sure I want to mess with it. Seems due to the cost of the propshaft and other gaskets, bearings, seals, etc... it's time ahead to just replace the lower unit as an assembly.
Looking at the parts diagram though I can't figure out just what gaskets I need to seal the lower to the upper.
Don't they list that as a separate kit?
 
Ok, since theres still some season left and I still plan to order a new lower unit,... today I pulled the prop off, drained all the gearcase oil.
Made up a quick pressure test hose and gave it 15lbs pressure. Sprayed soapy water around the prop seal and carrier bearing housing.
Saw soapy bubbles from the rear carrier o-ring. The prop seal was not leaking at all.
Borrowed my brothers dial indiacator and found the high spot on the prop shaft. Marked it with a felt marker. Rotated the mark to the top side and left the dial indicator installed.
The dial indiacator said I was .008 out of round.
Slipped a small bottle jack in between the prop shaft splines and the bottom of the cavitation plate.
Took about 8 times to pressurize the shaft but I got it down to .003 which I believe is what the book spec is.
Then I pulled the carrier assembly out. Checked the seal. needle bearings and removed the o-ring from the back of the carrier.
Removed my reverse gear and inspected, all the dogs look great as well as the teeth.
Molly lubed all the bearings back up and the prop seal.
Cleaned the o-ring groove and o-ring. Applied new sealer to the groove and put the o-ring back on. Added more sealer to the outside and installed the carrier after cleaning the housing surfaces.
Loc-tited the retaining bolts and reinstalled the prop. I should have left the prop off but tomorrow I'm going to re pressure test it and if all is well add new gearcase fluid.
 
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make sure you are aware of the weird fill procedure on those Cobras.
SEI is making Cobra lowers now too (not sure if all ratios). Looks to me like they basically bought the Sierra stuff and chopped the price about half and doubled the warranty.
 
make sure you are aware of the weird fill procedure on those Cobras.
SEI is making Cobra lowers now too (not sure if all ratios). Looks to me like they basically bought the Sierra stuff and chopped the price about half and doubled the warranty.

I have the OEM service manual so filling shouldn't be an issue.
Thanks.
 
Yesterday I slapped together a pressure tester and ran it up to 15 lbs.
It held there for over an hour and didn't lose any pressure.
IMG_20140902_180354_354.jpg
Then I modified it for a vacuum test.
Once again ran it up to 15 inches of vacuum and it held without leaking down for 48 minutes.
IMG_20140902_185930_851.jpg
I figured thats good enough?
So I refilled the case with oil.
 
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