Re: 1974 15hp Johnson, convert to 20" possible?

[Don]

This is not a tough swap to make at all…
The good news is that the driveshaft pulls right up and out of the gearcase, so it swaps easily. Be sure to get a driveshaft with an oring groove on the upper splines, don’t use the non-oring driveshafts. I think there was some sort of change to the lower splines on the early engines, so be sure those match completely as well.
Used gearcase extensions are plentiful and affordable. Be sure to get the white plastic tube/extension seal as well. These are usually just found stuck into the used extensions…
The shift rod simply unthreads up and out and can be swapped easily as well. Be sure to thread the longer shift rod all the way down/bottomed out, then back it off just slightly so the bend is facing forward for proper adjustment. The shift rod cavity tends to get plugged with dirt/sludge, there is a drain hole down there but it is usually plugged. You will want to clean this area well prior to removing the original shift rod to ensure none of the sludge goes down inside the gearcase when the original shift rod is removed. Coat the lower portion of the longer shift rod generously with grease prior to threading it in to help lubricate the shift rod oring for good sealing.
The powerhead would have to be pulled in order to change the water tube, but I’m pretty sure the make a 5" extension for the short water tube that seals with an oring that can be added to the original short water tube. This way, the powerhead does not need to be removed…
Here is the part number for the extension:
http://www.marineengine.com/newparts/pa … OMC0320025
Not cheap, but definitely worth its price to avoid pulling the powerhead…
Don