Force 125 hp lower unit

[tjello327]

Hello forum,
I have a 1986 Force 125 hp engine. My lower unit would not go into forward all the way which ended up being the forward/reverse dog and gears. I re-milled the pockets square and re-assembled the lower unit just as it came apart. I noticed about .030" end play in the prop shaft. The book says not to be more than .010". I am in the process of making a new thrust bearing washer (hardened and ground tool steel) to take up the end play. I am shooting for zero end play, and not to tight either.
As I started to put the pinion shaft back in, with a new crush ring, the shaft has quite a bit of end play too. This dont seem right to me.
I can see the bearing cage is moving which is creating the end play. There is a ring on the bearing cage that is loose from the cage. This ring is where the crush ring is pushing on. With this ring not being part of the cage, the crush ring really isnt loading the bearing. It is accually pushing onto the bearing rollers! This cant be good can it? How is this set up supposed to work?
Thanks!

 

[Frank Acampora]

Re: Force 125 hp lower unit

Zero end play on the prop shaft is not good. It will tend to load the bearings and wear the thrust collar (machined into the shaft) against the reverse gear. You need between .006 and .010. Actually, .030 won't hurt, although it will be a little sloppy especially going into reverse.

Look again and look closely at what is engineered into the drive shaft, bearing, and pinion. Due to the curve of the gear teeth, while running, load pulls the pinion down into the gears. The top tapered roller bearing LOCATES the pinion; that is why it is shimmed UNDER the outer race--to have the correct depth of engagement of the pinion and gear teeth. The lower bearing in the gearcase is a plain caged needle bearing and only locates the shaft and pinion laterally.

Since different shim packs are used for each lower unit, the upper bearing outer race will be at a different depth for each lower unit. That is why a crush ring is used. The crush ring holds the outer race down in the bore and against the shims. The solid ring on the roller part (cone) of the bearing actually presses down on the outer race and the crush ring presses on it. When properly assembled, it contacts nothing in the moving parts of the bearing so contributes little to no friction to the assembly. Until fully assembled with a new crush ring and the water pump bolts are tightened, the shaft will have a LOT of play. after assembly there will still be a small amount of play but this will NEVER be noticed while running since the load always pulls down on the pinion and shaft.

By the way: although that top drive shaft bearing is actually a Mercury part used in some Merc engines, it is in reality a standard bearing used in some food processing machines. It costs about 45 bucks--cheap considering that the factory will only sell you a shaft and bearing assembly.

 

[tjello327]

Re: Force 125 hp lower unit

Great help Frank! That makes total sense to me now. I didnt think about the gear tooth helix pulling the gear down into the outer race. I thought the drive shaft would be dancing around being loose like it is.
As for the thrust bearing goes, I will make the new thrust bearing washer with .005" end play. For some reason my prop came off when I had to bring my boat back in reverse. I thought too much end play may have caused the prop to rub the housing and then unscrew itself??? Still not sure why this happened. I never check the set-up of the washers and spacers before I took this boat out. I did notice the spacer behind the prop looks like it is worn where it shoulders onto the prop shaft. There is no square shoulder in the spacer or the shaft. It is worn to a taper. Is this normal?
Thanks again for the help!

 

[Frank Acampora]

Re: Force 125 hp lower unit

The taper is normal. It came from the factory that way.

The only way the prop could have come off in reverse is if the prop nut was not a nyloc nut, loosened and screwed off with use. The prop is splined to the shaft and can only slide on it--not rotate. The nut is 15/16 outside and since the shaft is 7/8, even without a washer, the prop can not slide over the nut--so: the nut was gone.

 

[tjello327]

Re: Force 125 hp lower unit

Yeah, that sounds right to me. The splines prevent the prop from unscrewing. The guy I bought this boat from said that he thinks a washer was missing behind the nut. It was a lock nut but I am not sure if is was good or not. I noticed the plastic hub thing that goes on after the prop was not drawn up tight. I brought this to the prior owners attention and that is when he said he thinks a washer is missing and it should not matter.
So you think if I put it back together the way it was, but with a new washer and nylock it should be right?
Thanks.