Lower unit interchange for 73' 150 H.P. l6 1500

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gr82fish

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My unit is pretty bad but need to know what years and engine sizes will fit. What about shaft length, how do you determine that? Thanks all.
 
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Chris1956

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Lower units from 1970-85 inline 4 and 6 cylinder motors should fit. some of the 4 cylinder 850 motors had a lower gear ratio 2.3::1, and you might need a prop wiot more pitch.'

The vast majority of these were long shaft motors (20")
 
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emckelvy

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

If you measure from the top of the transom where the motor clamps sit, to the bottom center of the boat (keel) it's gonna be either approx 15" (Short Shaft) or 20" (Long Shaft).

Since there were some 1500 models that came in Shorty length, it certainly would be a good idea to confirm the length. Hopefully you have a Long 'cause those will be a lot easier to find than Shorts.

Lower units from '79-up have a redesigned gearcase that's less susceptible to blowout; the skeg nicely swept-back, and have an built-in pickup for speedometer.

You might want to stick with a 2:1 gearcase with the extra power of a 1500, some claim the 2.3:1 gearbox is a bit weaker in the pinion gear because of its smaller diameter. Never used one on that high HP motor myself so can't say.

Later-model driveshafts are stainless, a lot less trouble in the seal area than earlier mild-steel (as your 1500's unit probably has) which have that nasty tendency to rot away.

Check eBay and your local www.craigslist.org for an ample supply of parts!

HTH & G'luck......ed
 
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gr82fish

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Thanks for the info. Do the ones you listed as "will fit" all have the same drive shaft with the pin on top of the splines (or does it matter ?) and the correct shifter hookup?
Thanks again
 
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emckelvy

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Shift shafts are all the same on '66 and up as I recall. You won't have any issues there.

Re: the preload pin, only those lower units with tapered bearings will have the preload pin. You'll notice a disturbing amount of play in those when you move the driveshaft up and down. No problem, this is taken up when the pin presses against the crankshaft. That's why the pin must be there and be free to move, or else the bearing won't have any preload and will probably fail (not to mention what it's doing to the pinion gear!).

For those lower units which have "straight" ball bearings, they won't have a preload pin 'cause they don't need one.

Seen 'em both ways so it's kinda hit-or miss. Don't think it'll make any diff since either type will fit your motor..........ed
 
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gr82fish

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

I found a l/u off a late 60's motor. Are they weaker than the newer ones and is the skeg larger on the later models?
 
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emckelvy

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Not necessarily weaker, is it off a 6 or 4?

A late-60's lower unit will have the mild steel driveshaft and corrosion concerns mentioned previously.

Skegs kinda vary on these, some have quite deep skegs while others are somewhat shorter. Of course there's also the ones that have been Crunched then cut off short to even the skeg out!

It might be a good idea to ask the seller to figure out the gear ratio (or check it yourself if it's local).

All you need to do is turn the driveshaft (2) revolutions.

If the propshaft turns less than 1 turn: gear ratio is 2.3:1

If the propshaft turns exactly 1 turn: gear ratio is 2:1

If the propshaft turns more than 1 turn: gear ratio is 1.78:1

Your 1500 could certainly spin the "overdrive" 1.78:1 gears but you'd likely have to go to a prop with 2" less pitch than your existing.

On the other hand, 2.3:1 gears would require a prop with pitch 2" greater to be roughly equal to the prop you're running now with 2:1 gears.

BTW the later-style gearcase with swept-back skeg and improved gearcase design was used thru the end-runs of all the 66-Cube Fours and 99-Cube Sixes.

HTH........ed
 
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gr82fish

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Well, can't seem to find a decent priced lower unit complete so I took mine apart and seems the shift cam is bad, along with someone broke the prop shaft bearing retainer trying to take it apart. The gears look great. Anyone know what engine years and sizes the parts would be the same as I found a couple I could use for parts. Found a 85 hp older "850" unit and a 89' 90 hp unit. Thanks for the help
 
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emckelvy

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Besides the shift cam, check the plunger the cam pushes on for wear. Also check and make sure the spring-loaded 'shift dog' clutch is free to move back and forth on the propshaft.

The parts from the 850 would work; the 90HP, if it's an Inline Three, has a different gearcase design and wouldn't bolt up to your motor but appears to use the same shift cam. Not much else would interchange though.

If the 90 happens to be an Inline Six, yes.

BTW the shift cam is part #76039-4, about $20, and the plunger is #30625, around $26............ed

p.s. the big gearcase retaining nut has a torque of around 150 ft-lbs so it has to be real tight or the carrier won't fully seat in the gearcase. Be sure it does or the lower unit may not shift correctly.
 
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gr82fish

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Any chance parts from an I/o lower unit will fit? Thanks again
 
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gr82fish

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Maybe a dumb question, but would a mariner lower unit fit as long as it is an inline 6. Don't know if the color is the only difference or not.
 
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gr82fish

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Thanks, will a 1980 80 hp 4 cylinder have a 2:1 ratio?
 
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gr82fish

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

Thanks for helping Ed. What size prop would I need? It's a 1500 motor on 18 foot bass boat i'd be putting it on. Also is the lower unit strong enough for a 150 hp motor?
 
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emckelvy

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Re: Lower unit interchange for 73' 150 H.P. l6 1500

A 2.3:1 gearbox will need a prop with approx 2" more pitch than what you would run with the 2:1 that originally came with your motor.

As far as what'll work with your boat, it all depends on the weight and hull design. With that gearbox on a bass boat, I wouldn't start with less than 21" pitch. Just watch the rev's when you first take it out.

Here's a prop-slip calculator which you can use to plug in actual values or check your theoretical boat speed with various sizes prop:

http://www.go-fast.com/Prop_Slip_Calculator.htm

As far as durability goes, just hearsay that the pinion is weaker on these. I'd imagine the pinion would have less teeth, thus smaller diameter. The fwd & reverse gears would have to have more teeth. Whether this is gonna make it weak in the Real World in a Non-Racing application, anybody's guess.

I've never personally heard of anyone blowing up the gears of an Inline Six on a pleasure boat, but maybe someone here has.

You could probably hedge your bets by running a synthetic gear lube, this should help with any extra stresses on the lower end.

HTH & let us know how she runs!...............ed
 
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