TOPIC: Merc 500 Lower Unit Rebuild / Shimming

Hi

Following on from Dave Bernard's advice regarding bearing sizes, I am now starting to rebuild the lower unit. Today I have been going through the process of checking the shimming. Although using new bearings and a new drive shaft I have taken the original shimming as a start point as I believe the shims take account of variations in the machining of the castings as there is unlikely to be a variation in the machined components.

Having gone through the assembly procedure about six times, I'm now getting the hang of it.

The problem I have now is that I cannot get it completely out of forward gear. Forward works fine and the dog clutch ratchets as it should and in the same way it did before I stripped it down. I can also select neutral easily by hand using the cam lock. However, it is extreme hard, and takes some force to select reverse. having managed to get the selector to the reverse notch, I'm getting a lot of lash and the dog clutch will not ratchet.

I started by using the original shim behind the cone of the forward gear taper roller bearing. I then disassembled and reassembled adding shims under the drive shaft roller bearing until I was happy that there was the prescribed 0.003" to 0.005" lash between the forward gear and the drive pinion. I then added the original shims in front of the reverse gear thrust washer.

With everything assembled and the cover screw finger tight I'm getting about 1/16" end float on the propeller shaft if I depress it against the clutch spring. I have not disturbed the clutch assembly and it remains as it was when I stripped the unit down.

I'm struggling to visualise where the problem is. I can only think the ratchet is not travelling back far enough to fully disengaged from the forward gear, either that or the reverse gear is too far to the rear and consequently I should remove the shim behind the bearing cone?

I could the reduce the number of shims under the drive shaft roller bearing, as I’m conscious I could run into problems getting the water pump body low enough in the casing to align the drain screw.

Lastly as a sanity check I have attached a couple of pictures, taken from a video, showing the orientation of the shift cam, is this the right way around?

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