Modifying Outboard Length and Sealing Chainplates

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From Glen-L Newsletter by Dave Lewis

Instead of raising the Zip transom for a 20" shaft motor, I converted the outboard to a 15" shaft length with used parts. The 1977 Johnsons are easily converted. Here are some of the details for shortening the outboard motor shaft.

The boat dealer serviceman who sold me the used 1977 25 hp Johnson outboard told me how to do the conversion and also found the necessary parts at a local marine parts store. The replacement used parts were $100 in total (somewhat high but worth the money to me), and consisted of the 15" drive shaft, new water pump impeller and a copper water tube. Disassembly is easy on the '77 Johnsons.

Side view of lower unit after removing the 5" spacer.

First a small 1"x2" cover over the shift shaft coupling on the side of the upper housing (power head) is removed. Then the shift shaft coupling is loosened. The lower unit (gear case) actually has three vertical components inside: a shift shaft, the copper water tube, and the drive shaft.

Next, the nuts that hold the power head to the middle spacer and the gear case are removed. With the shift shaft loosened, the gear case will then pull off with some gentle prying. Once removed, the gear case has the water tube, drive shaft and shift shaft still attached to it. The prying is needed in order to pop the water tube out of its rubber seat in the power head.

To replace the drive shaft, the water pump cover is removed; the old drive shaft, and impeller will then slide out. There is a small pin that drives the impeller which needs to be saved and inserted in the new drive shaft. Then the new shaft end is greased and slipped into the gear case. The new impeller is then slipped over the drive shaft and is inserted into the pump housing. Be sure to get the fins on the impeller positioned the same as the old one. Then the pump housing cover is screwed back down.

Since the outdrive is being shortened by 5", the shift shaft and the long mounting stud extending from the gear case must also be shortened. This is not difficult but accurate measurements are necessary. Also, do not attempt to unthread the stud from the gear case since damage to the aluminum housing usually results.

It's best to cut down the mounting stud and the shift shaft while they are still in the gear case. On my outboard, I had to remove exactly 5" from both. I then re-tapped the threads onto the stud end, and ground a slot into the end of the cut down shift shaft so it would mate with the coupling in the power head. Again, this sounds difficult, but it is not.

The water tube simply pulls out of the rubber seat on the water pump cover. The shorter tube is then hand inserted. I greased the upper end of the tube with soap so it would easily slip into the power head rubber seat.

Old parts removed when making conversion from 20" outboard to 15"

Lastly, with the middle 5" spacer removed, the gear case is slid back into the power unit, being careful to align the water tube, greased drive shaft end, and shift shaft with their seats in the power head. I had to hand turn the prop a little to get the splines on the drive shaft to align with those in the power head. The shift shaft coupling is tightened after all the mounting nuts are re-installed and tightened.

Now my transom has a 2" recess in it, instead of a 3" camel's hump. Keep in mind that most late model outboards cannot be shortened or lengthened in this manner. They just don't make ‘em like they used to.

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SEALING THRU-DECK CHAINPLATES

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