mark 25 lower unit gearcase

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mark 25 lower unit gearcase

Postby beaglex2 » Thu Mar 15, 2012 2:32 am

my prop shaft would rotate 350 degrees then lock up, if i backed up and rotated forward it would catch in the same spot. thinking a gear tooth was bad i pulled it apart all gears were fine but metal fragments were stuck in forward gear . So on inspection of the fragments, one piece was large enough to get some measurements. The piece was round about .375 dia-- .190 height and it was also stepped .where does it go and does someone have one? it got ground up..
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Re: mark 25 lower unit gearcase

Postby Bob » Thu Mar 15, 2012 3:00 am

http://www.ebay.com/itm/Mercury-Mark-25 ... 9b&vxp=mtr

this is a great book and parts list.  get one and check to see if any parts are missing .  I have no idea where this fragment came from but rest assured something broke...

please post a photo and maybe we can identify it.   thanks   bob
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Re: mark 25 lower unit gearcase

Postby bobvdinghyracer » Thu Mar 15, 2012 3:34 am

sounds like the spacer ring goes between the gears 1 on each gear on prop shaft ... sounds like 1 broke ... hard part to find  icon_problem.gif
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Re: mark 25 lower unit gearcase

Postby bobvdinghyracer » Thu Mar 15, 2012 3:40 am

go here  for a picture of broken part and location for it   icon_problem.gif               
viewtopic.php?f=17&t=137&sid=5695755b1b342491e4ee2547227e5990#p634
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Re: mark 25 lower unit gearcase

Postby XR55 » Thu Mar 15, 2012 4:12 am

The part you found is a magnet that is found in some gear cases. Look for a round counter sunk area at the bottom of the housing that is the same od as that piece. This is where is was. Leave is out, to avoid having it get caught it gears again. It was a short lived idea that Mercury tried.
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Re: mark 25 lower unit gearcase

Postby beaglex2 » Thu Mar 15, 2012 4:25 am

yep you hit the nail right on the head about it being a magnet .looking at the metal it doesnt look like steel at the break. theres a recess of about a 3/8 dia x 1/4 deep,drilled spot in alignment with the driveshaft that, that would have been pressed into.dont need it do i ? thanks ken
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Re: mark 25 lower unit gearcase

Postby bobvdinghyracer » Thu Mar 15, 2012 4:27 am

never seen a magnet or a spot for 1 in any mk 20/25 /30 gearbox or any place for 1  ...
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Re: mark 25 lower unit gearcase

Postby beaglex2 » Thu Mar 15, 2012 4:32 am

And i like the fix to ;leave it out .whew
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Re: mark 25 lower unit gearcase

Postby XR55 » Thu Mar 15, 2012 11:17 am

I have only seen this on a MK20 and a MK25. Mercury tried this and found the problem of loose magnets in gear case causing more problems than helping.
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Re: mark 25 lower unit gearcase

Postby merc700hp » Fri Mar 16, 2012 3:27 am

Oh Oh my Merc 850 (1975) has the magnet, have to cut it off I guess.
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Re: mark 25 lower unit gearcase

Postby XR55 » Sun Mar 18, 2012 3:42 am

I have found three lower units locked up with these magnets jamming the gears. Why would you take a chance of doing damage if left in and trusting old cement or press fit to retain in into the case?  Mercury had a good idea but should have used a magnetic fill plug instead, to collect any metal particles.
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