Drive Shaft Lateral Play 175HP V6

[P51D]

I have a ’91 or ’92 175 HP Mercury V-6, 2.5 liter, Serial # OD223009. I’ve had the engine for two years, and don’t know its prior history. I have both Mercury and Seloc maintenance manuals for the engine (The Merc manual is MUCH better).

I have the lower unit (L/U) off, since I have an L/U lube oil leak at the shift shaft seal. I’m also replacing the water pump and impeller as routine maintenance. After I pulled out the pump’s lower housing, I noticed about 1/16” of lateral play in the drive shaft. That is, I can look down along the axis of the shaft, into the L/U, and move the shaft side-to-side (or “stir the pot”). When I do this, I can see (and feel) the shaft bearing move laterally about 1/16”.

Is this normal? I noticed this two years ago when I changed the pump, and received one opinion along the lines of “It is not a concern”. I’m looking for more input.

With the water pump installed, the movement is apparently prevented by the pump seals and impeller acting as additional “bearings”. There is no noticeable vertical play in the drive shaft; just the lateral play. The gear oil looked pretty good, with perhaps a few milky streaks indicating some water, but not an overall milky appearance. There was a very small amount of fuzz on the magnetic drain plug; definitely no chips. And overall, the outboard is very well-behaved with no other symptoms.

My questions are:

1. Is the lateral play normal?
2. Regarding shift shaft seal replacement, any hints that are not found in the manual?

Thanks.

 

[WillyBWright]

Re: Drive Shaft Lateral Play 175HP V6

Side play is normal. The lower part of the shaft is supported by a heavy needle bearing just above the pinion gear. The bearing in the upper part just under the water pump is a tapered roller bearing. The seals don't act as bearings.

The gears can be straight-cut or spiral-cut. Straight-cut gears will always force the pinion gear away from the driven gear when rotating. On those, the cup of the tapered bearing faces down and the rollers are pushed up against it. That tapered bearing keeps the gears together. Without the rotation of the gears, the force is not present, and the shaft will have a lot of side play.

Spiral-cut gears can be wound in either direction. In one direction, the gears will be forced apart just like straight-cut gears, and the taper bearing cup will face down as well. They can also be cut the other way so that the driven gear draws the pinion gear down into it. On those, the cup will face up and the rollers will be forced down into it. Those usually have a spring-loaded pre-load pin in the top of the driveshaft. In all of these configurations, there will be side play anytime the gears are not moving.

Mercury has used all three of these types of gears. I am unaware that any have any particular superiority to the others, except that straight-cut gears are noisier. The key is the strength of the shoulder beneath the bearing of the spiral-cut gears that draw the gears together vs the strength of the threads of the spanner nut above the other kinds of gears. Personally I prefer a shoulder to threads, but they obviously both work.

You can tell which you have with the water pump base removed. If there is no spanner nut below the pump and all you see is shaft and bearing, that's the kind that has spiral-cut gears that draw the gears together. You can also push down or pull up on the shaft to see which direction the side play goes away.

 

[P51D]

Re: Drive Shaft Lateral Play 175HP V6

Thanks, Willy. Great explanation. It has the spanner nut, and pulling up eliminated the side play.