7.5 hp Lower Unit Problem

I just recently finished working on a 1974 Mercury 75 7.5 hp outboard. I replaced the impeller and everything seemed fine. The shifter engaged the appropriate gears when selected.
I took it out on the lake today to test it out. I putted along in a no wake zone for about 100 ft. Once I gave it some throttle I heard a pop and the motor jumped into neutral. It wouldn't go back into gear again.

I looked at the lower unit when I got it home and it appears that the prop is fine, but the prop shaft has too much back and fourth play. About 1/2". If I push in on the shaft it will engage in forward and neutral, but no reverse. Pull out on the shaft and I get only neutral. I assume something in the gearcase has let loose. Any ideas what may be the problem? I've never had a Merc gearcase open, but I do have the parts catalog.

I don't have any Merc tools and getting this thing apart may be tricky without them. Anyone have a link to the tools I will need or plans to make them?

Re: 7.5 hp Lower Unit Problem

Sounds more like you missed the shift shaft splines. Try removing the gear housing and checking that it goes in and out of gear with it removed from the engine... Unless you hit something (hard!), I have never heard of the 7.5 gearboxes 'letting go'... It's the same box they used on the 9.8 and they use them in a white-water event here each year... Rare to see anything but Mercs in that event....

There should be movement of the prop shaft... Not sure 1/2" is right though. It could just need the case cover nut tightened. See pic...

Chris............

Re: 7.5 hp Lower Unit Problem

Wayne, about the only "special" tool you need to get that gearcase apart is the gearcase cover ring tool. The 'gearcase cover' ring has (4) slots which are engaged by the cover tool to unscrew the piece. If your motor is a "Salty Dog", you're not gonna get that cover off anyway, usually it's easier to drill it (very carefully) with a small bit then split it. You can tighten the new cover ring with a pin punch if you're careful.

You could also build a workable tool out of a piece of pipe, welding (4) 'tabs' to the O.D. of the pipe, to engage the slots in the cover ring. Weld a nut on the end of the pipe to fit a wrench and there's your 'special tool'!

Check out this diagram and you'll see the cover ring is Ref. #66, Part # 86236 (which if I recall is plastic. Yours will be aluminum, plastic is the New Style):

http://www.mercruiserparts.com/Show...inbr=467&bnbr=170&bdesc=GEAR+HOUSING+ASSEMBLY

Now, regarding the sudden failure, if you'll look at Ref # 46, the Shift Cam, you can see how, if the shift cam were to break, you'd get a lot of in-out movement of the prop shaft.

So, I'm thinkin' that could be what happened, or something very strange happened to the shift shaft. Not too likely for the shift shaft to pull out with the lower unit in place, but you'll be able to shift the lower unit manually once you remove it.

Back to the disassembly, once you have the cover ring out, all that's holding the reverse gear/propshaft carrier is friction between it and the gearcase. You may need to heat up outside of the gearcase with a propane or MAPP torch a bit, this'll expand the gearcase and make the carrier easier to remove.

The propshaft will come out with the carrier, so you can use a slide hammer to "knock" it out. Or put a short piece of pipe over the propshaft, install the prop nut and use that as a "knocker".

Once you get the carrier assy out you'll see the pinion gear and Fwd gear, with the shift cam showing thru the center of the Fwd gear.

Note the pinion is held on with a nut, if you have to remove Fwd gear you'll first have to undo the pinion nut and pull the driveshaft out of the gearcase.

The best way to do this is to put a 2X4, 4X4, or other suitable chunk of wood on the lower unit, directly behind the driveshaft. Hold the driveshaft in one hand, rap the wood smartly with a hammer in the other hand and that driveshaft should pop right out.

Or, if you have a good vice, cushion the driveshaft so the vice doesn't mar it, clamp 'er down and then smack the lower unit to jar the pinion nut loose and pull the driveshaft out. Note that the upper and lower water pump bodies must be removed prior to removing the driveshaft. Nothing special to that, the fasteners which hold down the upper pump body are all that hold the lower body to the gearcase.

After that, Fwd gear comes right out and the entire area behind (where the shift cam sits) is accessible.

Reassembly is the reverse of disassembly. Be sure to use new soft goods (O-rings and seals). Spread a thin coating of Permatex No. 3 Aviation-type non-hardening gasket dressing on the outside of the reverse gear/propshaft carrier, and also on the threads of the gearcase cover ring.

Note that there is a torque value on this ring, it's gotta be pretty tight to keep the innards in place!

My book say 60 ft-lbs and that seems a bit much for a little ring but maybe so! 88 inch-lbs for the pinion nut.

Just one final thought, if the cover ring should become loose for whatever reason, the bearing carrier can slip outwards which would really mess up the shifting not to mention reverse gear! So before you tear completely into it, make sure that hasn't happened. If this has happened it'll be obvious once you remove the prop.

That should just about cover it, HTH & Have Fun!............ed

Re: 7.5 hp Lower Unit Problem

Thanks a lot, fellas. I'll try tighening the cover ring first. I did notice that the bearing carrier seemed to be farther out than the one on my parts lower unit. However, I figured if I couldn't get reverse by pushing the prop shaft in that tightening the ring wouldn't make a difference either. I'll try it anyway. Looks like I've got to go find some pipe and fire-up the welder.

Re: 7.5 hp Lower Unit Problem

I dropped the lower unit this morning and the cover ring fell right off. I tapped the bearing carrier in until there was only a little play in the prop shaft and now everything seems to work fine. Hopefully, I can boat test it Monday.

Thanks for all the help.

I've saved your instructions, ED. I have a 1968 6 hp "in a box" with its guts out. I'm sure I'll need them then.