changing upper drive gears from 1:65 to 1:98 or 1:50

[marvinjansen]

I am considering changing my drive upper on a Mercruiser 165hp inline 250cid 6 cyl on a 19' Aristocraft. I may be able to get a drive off of a MC 140 that would probable be a different ratio, a 1:98 versus a 1:65 on mine now. Or maybe a drive off a V8 with a 1:50 ratio. what I want to know is if this will make a big difference or if the prop can be changed to compensate. Anyone ever make this change and can tell me what to expect?

 

[GLENN M]

Re: changing upper drive gears from 1:65 to 1:98 or 1:50

to get it right will be a lot of trial and error,and possibly wont get it.you will need odd ball props.there is a reason they mate ratios to motors.so you can get max wot rpm designed for each application.easy to screw your motor,i dont think anyone would recommend that route best getting right drive correct ratio.

 

[Bondo]

Re: changing upper drive gears from 1:65 to 1:98 or 1:50

Anyone ever make this change and can tell me what to expect?

Ayuh,... Which prop ya runnin' Now,..??

A 1.98:1 will require a much Bigger prop....
If you're at 21" now, you're already near the Top of the normally available selections....

A 1.50:1 will require a much, Much smaller pitched prop....
If you're runnin' a 17" now,...
You're screwed, 'cause they don't make props small enough....

 

[ziggy]

Re: changing upper drive gears from 1:65 to 1:98 or 1:50

personally, i would want to run what mercruiser specifies. a 1.65:1 ratio. anything other than that i would assume leaves something somewhere to be desired.
try a prop slip caculator. http://www.csgnetwork.com/marinepropcalc.html
assuming your boat performs like mine. i get 42mph @ 4300rpm w/ a 19p aluminum prop. slip is .10xxxxx something something. .10. leave the pitch blank and use them numbers and see what ya get. looks to me like somewhere between a 17-23p depending on 1.5 or 1.98:1. might be doable. but it ain't right. 1.65 is. i would try hard to stay within the oem spec. imho...

google prop slip calculator. you'll come up with other variations of that link. + some explanations on prop slip.

considering your drive shaft is rusty and your driven gear yoke is likely rusty. i'm with bubba. your heading towards a new upper price wise.

 

[marvinjansen]

Re: changing upper drive gears from 1:65 to 1:98 or 1:50

I like the Sei at $675. Can get a used upper off a 140hp for $200 but the gearing will be different. could always use my old 165 gear set, still cheaper than buying parts or an Sei.

 

[marvinjansen]

Re: changing upper drive gears from 1:65 to 1:98 or 1:50

On the used drive from a 140, the Welch plug on the starboard side behind the MC 120-140 sticker is out. Easy to buy and replace, but wonder why it is out? the boat was salvaged after the owner didn't get the water out before winter and cracked the block, so the drive was working ok.

 

[ziggy]

Re: changing upper drive gears from 1:65 to 1:98 or 1:50

could always use my old 165 gear set, still cheaper than buying parts or an Sei.

you'll still have to R&R two races, top and bottom of the driven gear shaft. + you will still have to shim it...
i was talking to the marine tech (at my local dealer) the other day & he ask me if i knew why you had to shim. i didn't of course. but anyways, he stated it's because they (mercriuiser) can't produce the housings exactly the same every time, hence the need to shim gears and brgs to get them just right for ea. case....

anyways, it's not just the expense of the parts to me, it's the having the right tools to do the job also. these tools are special mercuriser only tools that are quite expensive. http://www.iboats.com/Drive-Gear-Sh...7940989--**********.555814785--view_id.271232 (and them ain't even real mercruiser tools, they're copies) + even the necessary hand tools are expensive and not in your average tool box. like having a dial indicator type in. lbs. torque wrench capable of reading rolling torque of 6-10 in. lbs. i'm in need of one of them right now. i'm changing my upper brg. and race on the driven gear shaft on mine. that torque wrench is one of my obsticals. that and a press capable of pressing the brgs. off the driven gear shaft.
while i don't know if it will be cost effective in my case. i'm having the dealer R&R my brg and race + check the rolling torque of the driven gear shaft. i'm hoping to come in around 150. even that is 1/5 of the way there towards a new sei w/ warranty. what i'll have when i'm done will still be 35 years old.. time will tell. good luck in your decisions. just remember, when your boat breaks, you can't just get out and walk home.. or maybe worse..



don't know why the welsh plug is out of the 140 drive. other than if the po let the boat go to heck, it makes sense that he let the drive go too.