7,5 HP cutting/welding driveshaft

[G DANE]

I just inherrited a 7,5 mercury blue line motor. Lower unit is missing. will a lower unit from a 9.8 HP blue line fit this one ??

 

[The Marine Doctor]

Re: 7,5 HP cutting/welding driveshaft

To answer correctly we would need some numbers to match up.<br /><br />TMD

 

[Chinewalker]

Re: 7,5 HP cutting/welding driveshaft

Hi G Dane,<br /> Likely, it will fit. There were some subtle differences along the way from the 1960s to the 1980s on that series, but year for year, things swap out. Earlier ones have a long shift rod that goes up into the belly pan. Later ones have a short rod that goes up into a coupler in the mid-section. Early ones shift opposite the later ones...<br />- Scott

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

Thank for your help. I'l go hunting a lower unit with short shift rod. Motor had plenty of compression and spark - looks nice and clean. BTW my first MERC

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

Got myselves a lower unit ( came with a 9.8 powerhead for 50$ )<br /><br />It shifts, prop is fine and turns freely, probably OK.<br /><br />BUT: Driveshaft is very rusted, at places almost to 3/4 of original size. Worst just above waterpump. Bad too where it run through water pump top. Splines are OK<br /><br />It came of a long shaft model, mine was short shaft. I guess I can weld it and correct the warp, but does anyone know exactly how much to shorten ?<br /><br />Will the shift shaft have to be shortened too ?

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

Bump - new question.

 

[Chinewalker]

Re: 7,5 HP cutting/welding driveshaft

Hi GDane,<br /> The short shaft is exactly 5-inches shorter than the long shaft model. If possible, cut out the ugliest section. Have a machinist bore holes in both sections, add a dowl pin between them, then weld them back together. You would need to completely disassemble the unit to do it...<br />- Scott

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

Thanks for your reply Scott.<br /><br />Are these shafts often so rusty, and will the water pump work, when seal area is so ugly ? I am still missing waterpump top, have to find one.<br /><br />I am a bit chocked, I only worked on Johnnyrudes and Chryslers with stainless shafts.<br /><br />I saw a price at Mercurys partsside, 450$ for that little driveshaft, they must be joking, you can have them made at a shop for less !!!

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

BTW What is 5" exactly in milimeters ? , and do I shorten shiftshaft the same ? <br />Will the water pump still work good enough when the driveshaft is badly rustet where it runs through pump lid, and above ? <br />I think cutting shaft in the middle and welding would be better than just below pump.<br />Thanks for your help. There is something I really like at that little Merc.

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

BUMP

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

One more question from a MERC Newbie :<br /><br />Does it take anything special ( tools )to disassemble lower unit. Discovered today the lower unit I bought contained more water than oil, and have been run without vent screw !! Shifts and turns though, but hard to get into reverse.<br /><br />I ran it with a drill and changed oil 3 times. All kinds of small pieces of dirt and crud came out with oil every time. I would like to take a look at the internals of that l/u before I decide wheather to repair or part out motor. Would like to wait with throwing money for a manual too.

 

[Chinewalker]

Re: 7,5 HP cutting/welding driveshaft

Hi G Dane,<br /> You have to take it apart to repair the driveshaft anyway, so go for it. Find a Merc dealer who's been around awhile and see if they'll remove the rear spanner nut for you.<br /> As for the conversion, the closest I can get you is approximately 127mm. Yes, you'd have to shorten the shift shaft, too.<br /> I suspect the water in the housing is due to the rusty shaft. I've had some success using Marine Tex epoxy to fill pitted areas for a better seal.<br />- Scott

 

[G DANE]

Re: 7,5 HP cutting/welding driveshaft

Thanks Scott, I really appreciate your help.<br /><br />I never had a Thru-hub l/u apart. I have a couple of JohnnyRude manuals and I guess there shouldnt be that much difference. I can see on your Website you are bit by the Merc flue too. I was thinking of turning the shaft down at waterpump area, and turn a little stainless shell to weld to it. Maybe I will try the JB Weld or marinetex method, if internals are useable. As far as I can see from parts list, bearings are maybee standard. I dont quite understand the ignition, both points and powerpack ?? Do the points replace the sensor coil ?