Prop size differences

Boat-ryder

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So I recently picked up a used lower unit to replace my old one. It was suppose to be identical and seems to be as I mounted it but the prop isn't going on right. My engine is a 1990 force 120hp and the lower came off a 1991 force 90hp. The prop splines up but it seems to grab where the prop and foot come together. Did they change prop sizes between these years or between the HP's? Or maybe I'm doing something wrong? Thanks for any help
 

jerryjerry05

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You missing the thrust washer on the new prop?
​It's probably stuck on the old prop.

​It goes: thrust washer, prop and then the splined washer and then another washer and the lock nut.
 
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Boat-ryder

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I had thrust washer in. I was looking again today and it seems the prop is the same size as the housing. On the edge of prop there is a small drop down. With just enough of an edge to prick your finger, this used to slide under foot housing but it is now hitting flush. I found an old prop that came with the boat but doesn't have a hub in it and it has about and 1/8th cut in it. This prop seems to fit the lower unit perfectly. I'm still just baffled at how they could be different? I realize this must be hard to understand since its hard to explain. Will post pic's shortly
 

Nordin

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The prop must be able to "slide" into the gearhousing end a little bit if you have a dual exhaust gearcase.

If not, exhaust will leak out and it will create air/exhaust behind the prop and it will cavitate.
 

Boat-ryder

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The prop must be able to "slide" into the gearhousing end a little bit if you have a dual exhaust gearcase.

If not, exhaust will leak out and it will create air/exhaust behind the prop and it will cavitate.

Yea that makes sense. My prop fit correctly on the old foot but not this one. The other prop I have without a hub fits perfect without the thrust washer. I guess there are different size thrust washers?
 

SkiDad

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there shouldn't be any differences in the 2 piece units to my knowledge.
 

Boat-ryder

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there shouldn't be any differences in the 2 piece units to my knowledge.

You or anyone else I talk to. Can't seem to post the pics from my phone but will do it Monday. Guna try and find a loaner prop so I can test this lower unit before I spend money on a new prop. Seems I can't catch a break this month. Now that boats been sitting I've got mice chewing up wiring.
 

SkiDad

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are both 2 piece unit you have "dual exhaust" units ?

force lowers.png
 

pnwboat

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Remove the prop and measure across the circular opening where the outer diameter of the prop slide in to. Kind of hard since the prop shaft sticks out and is in the way, but do the best you can. The older Force single exhaust lower units have a 4" diameter opening. The newer 1995ish and later dual exhaust lower units have a 4 1/4" diameter opening and require a different prop that is slightly larger in diameter. Maybe your "original" lower unit was not really "original" and was a later model unit???
 

Boat-ryder

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Remove the prop and measure across the circular opening where the outer diameter of the prop slide in to. Kind of hard since the prop shaft sticks out and is in the way, but do the best you can. The older Force single exhaust lower units have a 4" diameter opening. The newer 1995ish and later dual exhaust lower units have a 4 1/4" diameter opening and require a different prop that is slightly larger in diameter. Maybe your "original" lower unit was not really "original" and was a later model unit???

This is the only thing I can think of, will measure tonight.


Skidad- Yes both units are dual exhaust and are identical other than the prop housing I guess. finally can post a pic. In the picture with the prop on the shaft it is hitting flush where on my old unit it would go inside about a 1/2 inch. The black prop is one that came with boat in one of the compartments. it fits the new lower but not with my thrust washer. So I guess I need a prop the same size as black one and a new thrust washer. anyone know how many different size washers there are?
 

Boat-ryder

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SkiDad

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that 1st prop is not designed to fit a force/chrysler lower unit - the flange is too thick. You need a prop that has a 3-3/4" flange like the 2nd prop

this must have been rigged up some way to work on old engine but I'm not sure how.
 

Boat-ryder

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that 1st prop is not designed to fit a force/chrysler lower unit - the flange is too thick. You need a prop that has a 3-3/4" flange like the 2nd prop

this must have been rigged up some way to work on old engine but I'm not sure how.

The old one measured 4in. The black one is 3 7/8 I believe. What size thrust washer do I need?
 

Boat-ryder

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Ok this is driving me nuts. just to clarify SKIDAD you are correct when saying I need a 3 3/4 inch prop like the black one in picture above. But I still need a thrust washer for this size prop. different online places show different things but im posting a picture from one that only shows 2 different size washers. One of them is the one I have already I believe, and im hoping someone can tell me if the other one is the correct one I need now with new prop size. they are asking $65 so this is something I want to be sure of before I buy.
 

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Boat-ryder

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the bottom left shows to be for a 90-150hp duel exhaust. the one just above it says its for a 90-120hp duel exhaust. I don't know what factor determines which one it has and it doesn't say. The one I have looks like the 90-120hp but it doesn't fit the smaller prop so I assume I need the other one but like I said I want to be sure before I blow $65 bucks. Any help is greatly appreciated
 

Boat-ryder

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the black prop you have takes a hub kit (which i asume you have ??) - you could get one of these kits and it will have the proper thrust washer.

http://www.iboats.com/XHS-II-Hub-Ki...1614721--**********.564954988--view_id.798320

no I don't have the kit, this prop came with the boat as is in a compartment. I didn't know these kits came with the washer. Thank you, is the kit you linked the one I need or was that just an example? if so how I can I find the right one. the prop is says on side 13 1/4X 17 and I have a 15 spline shaft.
 
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