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omc stringer (mechanical)

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vice11

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hi! i am from croatia and i use omc stringer mechanical sterndrive. i have 21/20 upper gear ratio and because i have a about 260 hp engine i decided to buy a new gear 21/17 ratio to increse rpm on prop...today i pick up new gear which i bought in usa and a relised that there is a difference bettwen my old and the new driveshaft!! the old driveshaft is a little longer than a new.. and now i asking you is that normally or that driveshaft is not for my stringer model??? differents is 5mm or 0.2 inch.
 
You may be the first to notice that. I was not aware of the length difference. Is the difference in length on the spline end? If so, I can't imagine 5mm causing any serious problems, but if you are concerned, you can buy a new OEM driveshaft from marineengine.com instead of aftermarket.
 
I solved the problem very easily by putting a round washers on shaft befor instaling the berings and now everything is excellent...
 
Master mechanic you may not understand what i want to achieve... earlier i had 21/20 upper gear ratio and 14/26 lower gear ratio and that means if i run engine on 2500 rpm the prop will turns about 1413 rpm because total raduction is 1.76 ...
Now with 21/17 upper and 14/26 lower gaer ratio on 2500 rpm of engine prop will turns about 1663 rpm and that is a very big difference(250 rpm).
now total raduction is 1.5

theoretically "if we ignore prop slip" prop with 21 pitch is first case goes 24.3 knots and with 1.5 ratio goes 28.7 knots.
 
That's as long as you can still reach the recommended WOT rpm's. If not, you could be putting too heavy a load on your engine and may need to re-prop with the lower ratio. Otherwise, you could do serious damage to your engine.
 
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