Lower unit dissassembly 1971 Merc 800 (update with photos)

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

OK so I've had a few minutes to think about this some more .

I attach a flag to the driveshaft. I can either printout a degree wheel OR use a dial indicator to measure the rotational movement, then translate that measurement (based on the radius of the measurement) to backlash. Yes?

Because this is an angular measurement, the radius of the measurement plays a role in the calculations. Now I'm lost again. Am I supposed to use the radius of the driven gear as the radius measurement to determine the backlash?

Arggghhhhhh. I'm really making this more complicated than it should be I'm sure.

Must be raining up there too, you should be out boating. Thats why I did it in degrees for you, 2 degrees is 2 degrees no matter how big the circle. The diameter that is important would be the diameter of the pinion gear, not the driven gear....in simple terms. In complicated gear geometry like this it would be the diameter where the contact makes first. The gears are both spiral cut and beveled, so the spacing between the teeth is constant no matter where on the gear you measure.

Merc has chosen a measurement point 1 inch from the center of the drive shaft to measure their backlash.

 

[Moody Blue]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

Merc has chosen a measurement point 1 inch from the center of the drive shaft to measure their backlash.

Thanks for your patience. I tend to get overly analytical, and having never done this before has me somewhat anxious. I'd hate to make a stupid mistake and destroy what I'm trying to fix. That one statement above makes it clear on how to measure the backlash IF I understand properly.

The backlash numbers in the manuals are measured at 1" from the crankshaft center and are NOT actual clearances between the gears. So if the forward gear backlash spec is 0.006-0.008", that measurement is taken at 1" from the crank center. Oh PLEASE tell me this is right .

I do have a dial indicator so I can use that instead of the degree wheel, correct?

 

[daveswaves]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

No problem Rod,, you are correct in all your assumptions. The dial indicator is the preferable way to go.

 

[Mac uk]

Re: Lower unit dissassembly 1971 Merc 800 (update with photos)

How did you get on with your assembly & backlash problem. Ive got to do the same thing on a Merc 20hp