mercruiser 470 outdrive compatibility

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4pslater

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Hello all, thanks for the wealth of info on the site. I have a 78 merc470. serial # on drive is 5188062 Got it out this year and found prop was hitting trim tab/would not rotate. Long story short my lower unit froze and pushed the prop shaft support out, also took the threads out of the case making it non-rebuildable. I have since found a real nice used unit off of an 84 boat. I believe it is the same unit with a slightly different ratio.
1st question- What is my original ratio and will I notice a difference if my new unit is 1.98? The ratio on my original unit is not stamped in the case and a search yields different results. If the unit will work I plan on playing with different props starting with a 21 (17.5 foot boat, rally175 by cruisers) until I find the best setup.
#2-- The yoke on the upper shaft(engine to outdrive) was longer on the new unit than the old. I swapped the yoke end of the shafts to keep the same length on my new unit. Will this work or is this a sign that I bought the wrong part?
My new (used) unit looks really nice with nice original paint. Seller recommended installing and using as is with no new seals or water pump. He claimed that the squeaky noise when turning the shaft was a sign of a good tight pump, that sound like bs? One last thing, the new unit says 3.0 on it and the 470 is a 3.7, does that matter? Thanks in advance for any and all info.
 
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stonyloam

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Re: mercruiser 470 outdrive compatibility

It is BS! You got a 470, it would be most unwise to not change the impeller!
 
Bondo

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Re: mercruiser 470 outdrive compatibility

4pslater said:
Hello all, thanks for the wealth of info on the site. I have a 78 merc470. serial # on drive is 5188062 Got it out this year and found prop was hitting trim tab/would not rotate. Long story short my lower unit froze and pushed the prop shaft support out, also took the threads out of the case making it non-rebuildable. I have since found a real nice used unit off of an 84 boat. I believe it is the same unit with a slightly different ratio.
1st question- What is my original ratio and will I notice a difference if my new unit is 1.98? The ratio on my original unit is not stamped in the case and a search yields different results. If the unit will work I plan on playing with different props starting with a 21 (17.5 foot boat, rally175 by cruisers) until I find the best setup.
#2-- The yoke on the upper shaft(engine to outdrive) was longer on the new unit than the old. I swapped the yoke end of the shafts to keep the same length on my new unit. Will this work or is this a sign that I bought the wrong part?
My new (used) unit looks really nice with nice original paint. Seller recommended installing and using as is with no new seals or water pump. He claimed that the squeaky noise when turning the shaft was a sign of a good tight pump, that sound like bs? One last thing, the new unit says 3.0 on it and the 470 is a 3.7, does that matter? Thanks in advance for any and all info.
Click to expand...

Ayuh,.... Welcome Aboard,.... The 3.0l, 'n 470 use different gear ratios,...

If ya swap Just the lower unit, 'n Not the Upper, you'll be just Fine, as the gear ratio is determined by the Upper unit,...

Also, yer 470 should be usin' the Long input shaft,...
Only the older GM In-line motors used the shorter shaft,...
 
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4pslater

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Re: mercruiser 470 outdrive compatibility

stonyloam , i'll replace the pump.
Bond-o- I decided to purchase the whole outdrive instead of just the lower unit because when i drained the oil out of mine there was none, just water. It sat with water in it for two years and at some point the water froze, busting the lower. When i went to look at this new outdrive i had plans on just buying the lower because the seller offered to split it. However, when i saw how nice and clean it looked(lots better than mine) i figured it may be better to leave it assembled and replace the whole thing. Do you still think I should re-use my upper to maintain the same ratio? My plan was to change oil in the new unit, run it a day or two, change oil again and look for water. After that I think i am going to start servicing my outdrive oil in the fall instead of the spring to prevent this from happening again. thanks.
 
Bondo

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Re: mercruiser 470 outdrive compatibility

4pslater said:
stonyloam , i'll replace the pump.
Bond-o- I decided to purchase the whole outdrive instead of just the lower unit because when i drained the oil out of mine there was none, just water. It sat with water in it for two years and at some point the water froze, busting the lower. When i went to look at this new outdrive i had plans on just buying the lower because the seller offered to split it. However, when i saw how nice and clean it looked(lots better than mine) i figured it may be better to leave it assembled and replace the whole thing. Do you still think I should re-use my upper to maintain the same ratio? My plan was to change oil in the new unit, run it a day or two, change oil again and look for water. After that I think i am going to start servicing my outdrive oil in the fall instead of the spring to prevent this from happening again. thanks.
Click to expand...

Ayuh,... Just curious, what'd ya pay for it,..??

The gear ratio is gonna be Waayyy low, 'n ya may not be able to buy a prop big enough to help,...
 
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4pslater

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Re: mercruiser 470 outdrive compatibility

ok, bondo has me worried about ruining the performance of my boat now. (thanks for your input though) Anyone ever run 1.98 gears behind the 3.7motor on a similar boat? If you were me would you run my upper that had water in it on my new lower? Don't mean to pester or run this into the ground, just want to end up with a good setup. thanks
 
Bondo

Bondo

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Re: mercruiser 470 outdrive compatibility

Ayuh,.... What prop pitch have ya been runnin',..??

Generally speakin', the window of available pitch is 15" to 23",...

It depends on where ya started, as to whether you can get to where ya gotta be,...
 
nola mike

nola mike

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Re: mercruiser 470 outdrive compatibility

Ive been running a 1.98 behind my 470 for 3 seasons, went from a 23p to a 25p prop. Just switched back to a 1.84 this season. Can't say I've noticed too much difference.
 
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4pslater

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Re: mercruiser 470 outdrive compatibility

i started with a 21p prop, banged it up and went to a 19p. I couldn't tell a whole lot of difference between the two. So maybe i'll just try the 1.98 gears with a 23p and see what i think.
Nola mike---when you went from the 1.98 back to the 1.84 did you leave the 25p prop on and couldn't tell a difference?
 
Bondo

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Re: mercruiser 470 outdrive compatibility

4pslater said:
i started with a 21p prop, banged it up and went to a 19p. I couldn't tell a whole lot of difference between the two. So maybe i'll just try the 1.98 gears with a 23p and see what i think.
Nola mike---when you went from the 1.98 back to the 1.84 did you leave the 25p prop on and couldn't tell a difference?
Click to expand...

The difference is keepin' yer motor in it's Sweet Spot, at Wot,...
 
nola mike

nola mike

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Re: mercruiser 470 outdrive compatibility

No, you need to make sure that you're hitting the proper rpm range at wot. A 23 on the 1.84 was equivalent to a 25 on the 1.98. In both cases I'm right at 45-4600 rpm at wot. I'd be bogging with the 25 on the 1.84
 
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4pslater

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Re: mercruiser 470 outdrive compatibility

ok thanks for the info guys, i'll try the 1.98 gears with a 25 and if it's not right i'll try and sell it and look for an upper with 1.84's or have mine rebuilt.
 
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4pslater

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Re: mercruiser 470 outdrive compatibility

Not sure if i should have started a new thread for this but....This evening is the first chance i've had to install my new outdrive. Installed new gimbal bearing and grease seal behind it. My old bearing was toast after about 1.5 seasons. Since it had such a short life span i decided to order the alignment bar to check bearing to coupler alignment. After pressing in my new bearing(fully seated and all) the alignment was nowhere close. Couldn't even tap the alignment bar into the coupler with a mallet. After adjusting the front mount, the best alignment i can obtain is with the front mount at it's lowest position and a bit lower may even be better(unobtainable though). As is i can at least slide the bar in and out by hand with a little effort. A spare yoke shaft goes in and out like butter. Is this cause for concern? Inspecting everything i see no rot in the transom, the rear engine mounts look pretty good(slight cracking), and if the front mounting point was rotted(it's not) it seems like it would cause the opposite. Any insight appreciated, Thanks in advance.
 
Eric 3.7LX Alpha 1

Eric 3.7LX Alpha 1

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Re: mercruiser 470 outdrive compatibility

Did you set the gimbal bearing in by tapping the alignment tool up, down, left and right?

The clearances for the yoke are not as tight as the alignment tool. You really can't use a spare yoke to set alignment... as the yoke may say it's right on, but the alignment tool says different.
 
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4pslater

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Re: mercruiser 470 outdrive compatibility

Yes, after pressing in the bearing it was way off. Giving the tool some pretty good bops with the palm of my hand in all directions helped a lot. Best alignment is still all the way down on the front mount. Also checked coupler by rotating engine 90deg at a time to see if the splined center was actually wobbling. It looks good though.
 
achris

achris

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Re: mercruiser 470 outdrive compatibility

If the rear mounts collapse then a 'good' alignment might be found by lowering the front of the engine. Although this may appear as a good alignment, the tool most likely hasn't gone all the way in. Check you rear mounts, they're only made of rubber and will 'sag' with time.

Chris......
 
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