1975 Merc Thunderbolt 90hp LOWER END

[vampirefish]

Hello all! I'm new to this forum and am hoping to get some help <br /><br />My mom and step-dad just gave me their 1968 16' Tahiti ski boat with a 1975 Mercury Thunderbolt 90hp!! It's a NICE boat and FAST (not right now ) -- the engine needs work. My step-dad says it needs a new lower end. He says it's a 'short shaft'. Can someone please tell me what years I can swap out with this engine? For instance, I found a Merc lower end for sale but it's from a 1970 65hp thunderbolt 20" shaft. THANKS IN ADVANCE!!

 

[WillyBWright]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Welcome to iboats.<br /><br />Short shaft? Ouch! Longshaft versions are common as dirt. If your driveshaft is still in good condition, that's all you need to convert.<br /><br />A lot of different models used the same gearcase, but they had different gear ratios. Sometimes you can compensate for a different ratio with a different propeller. All are inline 4- or 6-cylinder models. Models include 650, 800, 850, 1000, 1150, 1250, 1350, 1500. Also 75Hp 4-cylinder and 140Hp 6-cylinder models. Model years range from the early 1960s thru most of the 80s. You're more likely to find a shortshaft from the 1960s.

 

[vampirefish]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Thanks WillyBWright. Shortshaft OUCH, is right...I'm beginning to understand why my parents are giving it to me - they couldn't find a lower end replacement. But I'm always up for a challenge.<br /><br />I do have a couple questions/comments:<br />1)So in your reply you state "If your driveshaft is still in good condition, that's all you need to convert" ...do you mean convert to a long shaft? I'm thinking I shouldn't do this because of the type of boat (the boat sets very low in the water - like a racing/ski boat).<br /><br />2)Any suggestions on where to find a lower end replacement for this? I'm not in any big hurry. I do know (from this website) that a short shaft is 15". OUCH - these are rare, aren't they?<br /><br />thanks again!

 

[WillyBWright]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Try eBay for a gearcase. They pop up now and then. Reconditioned units are available thru dealers. Some dealers keep junked motors around. You never know if you might find one from a blown up motor.<br /><br />If your shortshaft driveshaft is good, you can replace a longshaft driveshaft on any of the gearcases I mentioned. The shaft is the only difference between gearcases when it comes to fitting the motor. But it also has to be the right driveshaft. There are driveshafts with a spring loaded pin at the top and others without. You can't swap pinned and unpinned driveshafts.

 

[vampirefish]

Re: 1975 Merc Thunderbolt 90hp LOWER END

I am planning on picking up the boat in a couple weeks and this way I'll be able to see the 'real damage' to the lower end. Hopefully it just needs to be rebuilt rather than 'replaced'. My step-dad said it could be replaced or fixed.<br /><br />Thanks so much for the info, I totally appreciate it!!!

 

[WillyBWright]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Good luck and feel free to drop by anytime. A manual would be a wise investment. I believe the old factory manual is still available thru a dealer. Aftermarket manuals are available right here on iboats. <br /><br /> http://www.iboats.com/mall/index.cg...manual&**********=124047299&*******=271999935

 

[vampirefish]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Hey WillyBWright - I may have a chance of getting a hold of a Merc outboard gearcase (1981 90 hp inline 6 - long shaft) that's not spring-loaded. Should these gears work with mine? Also, I'm still not sure about what type of shaft (spring loaded or not) my engine has...do you know how I could find this out (without taking it apart)?<br /><br />Thanks!

 

[vampirefish]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Here's another thing...Assuming my driveshaft is OK...<br /><br />You wrote earlier "There are driveshafts with a spring loaded pin at the top and others without. You can't swap pinned and unpinned driveshafts."<br /><br />If I only use the gears and not the shaft (still use mine, springloaded or not) is this possible?

 

[WillyBWright]

Re: 1975 Merc Thunderbolt 90hp LOWER END

You never know about the driveshaft until you remove the gearcase. Back then I'd expect to see a pin, but it might not be an original gearcase.<br /><br />The gears determine what kind of driveshaft you have. It's in the way the teeth are cut. It's also how the upper driveshaft bearing sits. Some go in taper-up and some go down-down.

 

[vampirefish]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Hey, thanks so much - looks like I'll wait on this buy to make exact sure of what I really need

 

[Clams Canino]

Re: 1975 Merc Thunderbolt 90hp LOWER END

As long as your short *driveshaft* is OK, you can swap it into a matching gearcase of the same species. ALSO, (for a fee) a machine shop can convert a long shaft to a short shaft.<br /><br />Nothing is impossible.<br /><br />-W

 

[vampirefish]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Hey, I just found out (since I now have the boat in my possesion) that the engine is a 1965 Merc 900E (short shaft). Does anyone know which years engine I can swap out the lower end on this?

 

[Dick S]

Re: 1975 Merc Thunderbolt 90hp LOWER END

Hi vampirefish what parts are bad? I may have <br />something. E-mail me<br />doldiron@yahoo.com

 

[papasage]

Re: 1975 Merc Thunderbolt 90hp LOWER END

maby you can mount the power head on a long foot and build a bracket to rais the motor. i have a 85 hp 1974 but long foot motor. it has a spring loaded pin in the top i think .

 

[vampirefish]

Re: 1975 Merc Thunderbolt 90hp LOWER END

I need a rebuilt lower end. My engine is actually a 1965 90hp inline 6 shortshaft.