Force 90 drive shaft bearing

[showman3003]

Can anyone confirm that the upper bearing, just under the water pump, of a 90 hp force lower unit drive shaft is a Timken M84548 cone/M84510 cup? I'm rebuilding my lower unit following a water pump impeller shred and the upper drive shaft bearing was terribly sloppy. It's either really worn out or the wrong bearing. I really don't want to buy an oem bearing just to ensure this is the correct one. I can get the same thing from a bearing supply for tons less money if this is the correct one. By the way, I wouldn't have torn it down this far if I hadn't taken out the shift pivot screw. The manual I was using said to take it out to drain the lower unit and then, two pages later suggested looking at the bottom of the "bullet" for another screw. Thanks a bunch, huh? Oh, this is a '91 single exhaust 90 hp engine. Any help would be appreciated.

 

[Frank Acampora]

Re: Force 90 drive shaft bearing

IF: If it is a single piece drive shaft and if it does not have a Mercury designed water pump, and if it has a "crush ring" on top of the bearing, then two things should be noted.

1. The gears load the drive shaft downward while running so the cup faces up and is shimmed underneath it for proper backlash of the pinion/drive gears.

Since the lower bearing is a caged needle bearing, and since there is clearance between the pinion and gearcase, It is possible to pull up on the driveshaft a bit. With the water pump removed, this will result is looseness of the cone in the cup and the shaft will wiggle due to the clearance. This looseness is not to be misinterpreted as worn bearings. It is normal.

The test is to push down on the driveshaft to remove all clearance and then feel for roughness of the bearing as you turn the shaft.

2. If the bearing is bad, then the cone and cup are standard items. I don't remember the number of the cone, but the cup is Timken U I G O L Or U 160 L. Difficult to make out the numbers/letters. The cone has a special ring around the outside which bears against the cup and holds it tight to the shims. Last time I bought a cone, it was 45 bucks just for that part.

If it has a Mercury designed water pump and a two piece drive shaft, the bearing cone and cup face the other way and are different numbers. M84510 For the cup and M 84548 for the cone. Again, this cost me about 50 bucks for the pair.

In the Mercury lower unit, the gears load the shaft upward and the shims are also under the cone but there is a threaded ring which holds down the cone onto the shims. AGAIN: The lower bearing is free rollers so the shaft is free to drop a bit into the drive gears until the engine is running. Again, this is not to be misconstrued as wear. It is normal. The test is to pull up on the drive shaft stub and feel for roughness in the bearings.

In either case, It is a little more work, but you can remove the pinion and shaft and eyeball the tapered cones and races. If they are smooth with no pitting, spalling, or rust, then they are good and fully serviceable.

Do not needlessly replace bearings because they appear to be "loose."

 

[showman3003]

Re: Force 90 drive shaft bearing

Thanks for the detailed reply. Mine is the two part shafted Mercury pump version. So, no crush ring. The bearings are rough but they are the Timken M84548/M84510 combo. Thank you for confirming those numbers. This has been an unfortunate adventure. I would suggest everyone caulk over the shift pivot screw however, I guarantee you if you ever take it out and don't miraculously manage to get it back in before something moves, you'll never touch it again. Again, thanks for the great reply.

 

[showman3003]

Re: Force 90 drive shaft bearing

Thanks for the detailed reply. Mine is the two part shafted Mercury pump version. So, no crush ring. The bearings are rough but they are the Timken M84548/M84510 combo. Thank you for confirming those numbers. This has been an unfortunate adventure. I would suggest everyone caulk over the shift pivot screw however, I guarantee you if you ever take it out and don't miraculously manage to get it back in before something moves, you'll never touch it again. Again, thanks for the great reply.

 

[showman3003]

Re: Force 90 drive shaft bearing

Sorry for the duplicate reply. I want to clarify a point of Frank's reply. If I put the bearing cone small end up on the drive shaft, so the cup is above it, the only thing holding the shaft up when the drive is in neutral will be the water pump impeller. The cone drops below the ledge the shims are on. The upper narrow face of the cup or outer race will sit on the shims and the cone will drop some when out of gear. Gear engagement raises the cone and with some end play the cone will rest against the race under power. The bearing I am replacing was the other way around and the retainer was probably 1/8"+ above the bottom or wide end of the cone. I think someone had read the Seloc manual which actually illustrates the shaft as you have stated but, next shows the race going in first facing up. Really confusing. I just want to be sure I am correct in understanding on the Mercury pump version that the cone need to face up with the cup above it so the shaft sort of floats when not engaged. Does the same shimming apply either way?

 

[mtlurz]

Re: Force 90 drive shaft bearing

Great info here guys, I've got the same project going on as well, is your upper bearing "pressed" on or should it slide freely on the driveshaft?

 

[Frank Acampora]

Re: Force 90 drive shaft bearing

The bearing is a light press fit. To install a new one, heat the bearing on a 100 watt light bulb for about 15 minutes and cool the shaft. Quickly slip the bearing on it should rattle right down to the correct position--Ahhh! Be certain it is in the proper orientation--Taper facing correctly.

And since the impeller is slotted for a woodruff key, it really will not hold up the shaft. Only the friction of the seals will keep it from dropping.

 

[mtlurz]

Re: Force 90 drive shaft bearing

Thanks Frank, first time tackling a lower rebuild. Did exactly what you suggested, buttoned everything back up and took it out Sunday. Ran awesome all day! I did however forget to put my drain plug in the boat and had a scary few moments...closest iv'e ever come to sinking a boat!