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Mercury 500 thunderbolt drive shaft... plug?

M

MaxMalmgren

Member
Hi,

Last fall I bought a new boat with a mercury 500 thunderbolt and attempted to remove the lower unit and stored it in another location to prevent thieves (they usually don't go for incomplete, old engines).

Now I want to put it together again. The thing is, there is one piece I can't remember where it goes, and it doesn't seem logical to me where it seems to fit. The piece is cylindrical, and made of hard plastic or some similar material, with a somewhat thick metal washer that fits snugly in one side. The other side of this cylindrical piece is splined, and fits on the lower drive shaft end. I just can't find it mentioned anywhere, and I can't find any other place where it possibly fits. It seems weird to me to basically plug the drive shaft splines, aren't they supposed to fit directly into the upper drive shaft or engine? Can someone help me? Please see the attached images as well.

Also sorry for the extremely large pictures!
rZpCIru.jpg

aLBRuaT.jpg
 
OK, first off, I don't think that stainless washer is supposed to be there. It looks like it might be one of the washers used with the lower unit stainless mounting nuts. I think perhaps the 2 parts just happened to come together in the parts bucket. So see if that washer can be easily removed from the plastic piece.

I'm assuming that your 50hp is somewhere in the late-70's to 80's+ and that there is no O-ring groove cut into the driveshaft splines where they engage the crankshaft.

If that's so, then this plastic piece is one part of a 2-piece seal assembly that presses against the end of the driveshaft to keep water out of the driveshaft/crankshaft splines when the L/U is bolted up.

Most motors I've seen lately are missing one or both parts, 'cause the P.O. didn't have any idea where they went. Or one piece stuck to the crankshaft and then fell out of the motor before the L/U was installed.

I know very well, because (2) motors I picked up last year, a 50hp-Four and 50hp-Triple, were both missing the seal parts!!! Makes it really hard to get the L/U out when the splines are partly stuck.

The P/N for the "seat" assy is 78654A1 and it's stocked at this site:

http://www.marineengine.com/newparts/part_details.php?pnum=MER78654A+1

You used to be able to get the rubber seal separately but the newer version comes with both the plastic splined piece that you have, which slips over the driveshaft first, and the rubber seal which installs on top of the plastic piece.

The kit comes with instructions for which way the plastic piece is installed on the driveshaft; it varies with the hp/model.

Be sure to look up the center section with a strong light to check that any stray parts (like the rubber seal) aren't stuck to the end of the crankshaft. If you find yours there, you can retrieve it and reuse if it's still in good condition.

Spread a thin coating of Mercury anti-corrosion grease or an equivalent on the driveshaft splines then install the plastic piece. It'll scrub some of the grease down, just take the excess and spread it on the splines again. Then install the rubber "bellows" seal and grease up the splines above the rubber. Clean off any grease that ends up on top of the driveshaft; you don't want it there, 'cause when the driveshaft enters the crankshaft snout, excess grease can cause a "hydraulic lock" which will force the crankshaft upwards and do bad things to the crank bearings.

HTH & let us know how the reassembly goes...........ed
 
Thank you, you saved me a lot of trouble!

The stainless washer was indeed from somewhere else, I don't know how it got stuck in there when we disassembled the lower unit last fall, but it was definitely confusing!

I found the rubber part of the seal stuck inside the shaft housing, which was really lucky since it must have been stuck there all through winter and spring, and didn't get lost when we started fiddling with it again.

Anyway, I followed your instructions and except for a few hours spent wondering why we couldn't put the mercury control box into neutral after assembly (reverse cam lock thingy had come loose and went in the wrong way, of course) all went very well!

The motor starts very well, only a second or so with the choke engaged. Thank you for your time, I guess it saved my motor from getting water up the driveshaft splines and getting completely pitted!
 
Can you tell me what the name of the part it where the driveshaft sides up into at the end of the crank. Mine is wore out or broken out. No spline to turn the driveshaft. I just bought the boat, had lower unit rebuilt but no water coming out the hole and will not turn the prop shaft to engage the lower unit. Any help is greatly appreciated. Mine is a 1979 Merc v6 150hp
 
Unfortunately the splines on the end of the crankshaft are broached into the metal of the inside diameter of the end of the crank itself. So if the splines are indeed rusted or stripped-out, there's no replacing them without disassembling the powerhead and replacing the entire crankshaft.

I would pull the lower unit and shine a strong light up into the mid-section, and see what you can of the end of the crankshaft. If you can see splines in there still, that's not your problem.

With the lower unit off you can connect a supply of water to the copper water tube sticking down, and see if you get circulation thru the powerhead. There could be other reasons why you're not getting water out the engine.

Remove the spark plugs before you try this: Put the controls in Reverse and attempt to turn the prop counter-clockwise. You should feel quite a resistance in Reverse and if the shift is working properly, it's gonna also spin the driveshaft in a clockwise direction (normal direction of rotation).

If the splines are shot, you may be able to listen carefully at the bottom of the powerhead and hear the driveshaft rattling around inside the crank. Otherwise, if splines are engaged you're gonna be turning the flywheel in a clockwise direction (viewed from the top).

HTH & let us know what you find..........ed

p.s. might also be a good time to start a new thread with the problem in the title, it'll get you more responses.
 
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