switching out long shaft OMC stringer outdrive with short shaft on 400 series 120HP

[budmanboat]

I have a mid 70s stringer electric shift 400 series 120 HP on a 28 ft pontoon boat. The engine is a 4 cyl.GMC. Due to age and recent issues with current outdrive I recently purchased a complete outdrive with intermediate housing but the one i just bought is a short shaft and my original is a long shaft. the overall difference
in length is 4" from top of outdrive to bottom of skeg. I believe the intermediate housing with 12 bolt holes will marry up to engine but i am concerned about overall
length of outdrive with respect to performance with 4" less outdrive in water. The outdrive and intermediate housingI just bought looks to be in great shape and not used in salt water . I have 12 days left to return to ebay seller if the consensus from my fellow boaters says return or keep. Either way I will be out the shipping
which is $300 total. The clock is ticking and I need help on this one. Thanks,Budmanboat

 

[VE8EV]

What engine was in front of the drive you bought? Each engine/drive combo had a different ratio in the upper gearcase so you need to be at least close or just won't work.

As far as the length goes, usually the idea is that the prop is just below the surface when you're on plane. If your boat was set up properly from the beginning you can't replace a long shaft drive with a short one.

If the problems with your existing drive are just with the lower gearcase then those are identical across all models 1973-77.

 

[budmanboat]

Thanks for the prompt reply. I called Seller today an he said the engine ahead of out drive I purchased was a 1970 Chevy 4 cyl 2.5. inline.I think the one in my boat now is a Chevy II or maybe a Chevy Nova inline. It is 120 HP. He said he would take the unit back if the short shaft would not work for me. This weekend, I will measure to see if cavitation plate on lower unit is below the transom engine well. The main reason I sent looking for another outdrive was two fold. 1) while underway 2 weeks ago, heard metal on metal on outdrive and had lost forward propulsion. Actually, no forward or reverse . Got towed in, trailer boat and had local mechanic come look at it. He had me turn key while he turned prop. He diagnosed problem as the coupler was damaged and splines on shaft probably worn down. A coupler is about $ 80 bucks and shaft another $90 or so. While we had boat out, noticed the upper gearcase housing was sheared where the 3 -9/16 bolts go through to bolt to quadrant gear. Even a used one is about $ 350. I was able to buy the "new"/used outdrive and intermediate housing for $ 550 plus 150 shipping. It is in very good condition and never used in salt water. Seller told me that if it will not work for my application,he would take the unit back but I would have to pay for return shipping which I agreed to do. We have not removed old outdrive from my boat as all mechanics are busy during summer season. Do you have any thoughts about why I lost forward -reverse on existing boat or do you concur with local mechanic given what I have described.

 

[southkogs]

Catching up a little here, but you bought a sterndrive from a 1970 model, but your drive is somewhere mid 70s? 77 was when they changed shift systems, so I would make sure you've got the same shift system.

Assuming you do, that's an electric shift which makes it nice in that you can swap the drives in about 15 minutes. You can know if it'll work real quick. That's a good price on a working drive, so if it'll work that's great.

Not sure what the "coupler" is that the tech is referencing. My first thought is your ball gears. Those can get roughed up if you're not careful. The shaft in the bottom of the lower gear case (at the impeller) is notorious for stripping. So, assuming that's the shaft he's talking about that's pretty typical, and it would account for you losing propulsion in forward or reverse.

 

[Howard Sterndrive]

The 4" shorter drive will not work. I have put a short shaft drive on a hi profile boat once to test and tried to see if the boat would plane. It just ventilated and revved like mad.