91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation

Hello all,

I'm replacing the original prop on my L-Drive. I used i-boats to select the appropriate prop. Too my situation.....

When removing my old prop I noticed there was a thrust washer, or "thru-hub spacer as printed on the back of it", between the hub and the gearcase. Before installing my new prop, I decided to leave the said spacer/washer in place. After installing my new prop, I noticed a gap between my hub (barrel) and the gearcase (see pictures). I thought this looked strange. So I took off the new prop, removed the spacer/washer, then reinstalled the new prop. After reinstallation, the gap closed but the prop felt hard to turn.






My question.... Is this gap ok or should I return my new prop and bye a different one.

Thanks in advance.

Open Waters

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation

The gap is NOT OK! And you can not run without the thrust washer. The gap will bleed exhaust out over the prop blades, fouling them. Return it and try something else

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation

Wrong prop.You need the Force prop.

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation

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jerryjerry05 said:

Wrong prop.You need the Force prop.

Click to expand...

Ok, I've reordered, received, and installed the "Force" prop w/ accessories. I've installed the "thrust washer, adapter ring, and prop. Should the prop still feel slightly hard to turn by hand?

Open Waters

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation

JerryJerry, sorry for the crappy uploads.

Open Waters

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation



What do you mean hard?? It shouldn't rub anywhere.
That's still a Mercury prop.Compare the ones in the photo.
The one you have on will probably work just not as good as the factory.
What happened to your old one?
Will try get a couple more pics today.

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation

More prop shots.
This is the way a Force prop should fit into the housing.
No rubbing and no gaps.
Force props are available.The Mercury props just won't fit right.

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation



I can't tell from the photos, but if that prop has a plastic ring over the hub where it plugs into the lower unit, remove the ring. Otherwise, while the prop may be listed for Force, the hub is too big in diameter (again, where it plugs into the lower unit).

Generally, Mercs have a larger diameter gearcase (4 1/4 vs. 4 inch) and only certain props will fit the Force engines--notably from the small gearcase 90 --without modifications.

Look at this Vortex prop. It had a black plastic ring, snapped into the groove, to adapt it to large gearcase engines. I removed it so the prop would fit the Force. Unfortunately, the ring is in a safe place in my garage so I can not show it. If your prop does have this adapter, simply slip a thin blade screwdriver under it and pry it off.

Also, the flanges on some hubs--notably this Vortex-- are too long and rub on the anode inside the lower unit case. The anode must be removed to fit these props OR the webs must be machined down as I did on this prop. Also note that while Vortex props come with a Square insert and bronze front hub, I pressed in a standard rubber hub (because I gave my buddy the bronze hub and I was too cheap to buy another at 35 bucks.)

Re: 91 Force 120 L-Drive w/ Merc Gearcase - Prop and Gearcase Separation

Frank and Jerry - Thanks for all the info / advise.

Frank - Before seeing your reply, I removed the plastic adapter ring that you reference and it appears to have solved the problem. No gap and no rub.

Again, Thanks for the help.

Open Waters