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Tornadosailing 06-08-2019 15:06

Yanmar SD-50 prop shaft shimming
This is an FYI for anyone unlucky enough to have to do this job. Mine always had fore and aft play in it, and recently started taking on water. I knew it had been apart before and obviously reassembled incorrectly. I bought a new shaft, bearings, shim sets, and seals. Hauled the boat and thought it would be a pretty straight forward procedure. And it would have been if the service manual had the correct info in it.

To calculate the shim thickness, you need the depth measurement of the lower unit, from the face of the outer flange to the back of the bearing seat (Measurement D, figure 4.2.2 in the manual). Then you need the measurement from the back of the gear with the rear bearing removed to the face of the forward bearing cup (M3, figure 4.6.1). According to the manual, the standard depth for D is 49mm, and the standard depth for M3 is 108.25mm, and therein lies the rub. You obviously can't put a 108.25mm shaft (plus the rear bearing) into a 49mm hole. To top things off, the formula for calculating the shim thickness is just as wrong.

Yanmar was absolutely no help, so I worked the problem backward and came up with the correct standard measurement for the case. These are all the standards, so you'll just have to measure what you've got and adjust the shims to make the math work. Everything is of course in millimeters.

Forward shim S4=.75
Bearing retainer bearing seat J4=3
Rear bearing shim S2=.75
NSK 30207J rear bearing=18.25

Add them all up and we get the standard case depth (D) of 131mm

The correct formula for calculating the forward shim (figure 4.6.2) is S4=(D-22-M3)


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