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how to measure lower unit back lash

F

fin reaper

New member
I've got a '65 Merc 900 that I need to replace the seals and the bearings in the lower unit. I've got a great older Seloc manual that give me the specs, but doesn't say how to measure the backlash.

Is this measured on the prop shaft or driveshaft - and at what distance from the center of the shaft?
 
The way to do it right is with machinist's bluing or red lead compound. To wit: Remove all trace of oil from te teeth and spray or dab bluing or red lead on the forward and pinion gears. Assemble the LU and spin the driveshaft, keeping a good drag on the propshaft as you do. Then carefully pull it all apart again (and again, and...) until you see a nice pattern.

A few tips: Eyeballing the 'fit' of the pinion and forward gear (as described in Merc manuals) will get you in the ball park; so will 'feeling' the way the gears mesh. Get that to look/ feel the best you can, then go to the bluing.

Jeff
 
What you are describing is a gear pattern check, but I'm looking for how to measure back lash - it's the measurement of rotation in inches (0.006-0.008"). But that depends how far from the center of the shaft you are. As an example, you would get a smaller reading if you measured the movement at the prop splines versus the reading at the outer tip of the prop (I know the prop is off when measuring BL, but you get the picture).

I need to know what radius from the center of the shaft it's measured at.
 
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