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Old 10-11-2018, 07:17 AM
Senior Member
Join Date: Jul 2003
Location: NENC
Posts: 817

Just to "finalize" my post, I'll report on my experience with replacing the belt tensioner assembly. My motor is a 2010 Mercury 150 Optimax (serial number starting with 1B7...). It had 895 hours on it when the tensioner assembly gave up the ghost (it had enough "slop" in it that the belt rode up on the pulley, and started rubbing on the underside of the flywheel). This can be catastrophic, as it starts to wear down the spot on the motor case where the tensioner arm mounts, or can result in the stud that screws into the motor case breaking off. I decided to replace everything; the stud, washers, bushing, nut, belt, spring, tensioner arm, and pulley. Part numbers and quantity needed:

(2) washers, part number 4002320
(1) Screw, PN 4010835
(1) nut, PN 826292
(1) Pulley assembly, PN 8M6005030
(1) Spring, PN 855798001
(1) Bushing, PN 8568941
(1) Stud, PN 8M0034457
(1) Belt, PN 828507Q4

I probably didn't need to replace everything, but since I was at it, and I assumed everything was worn out, I went ahead and got all new parts.

The stud came with the end that screws into the motor with blue loctite already applied. I applied a little extra for reasons I'll explain.

Removing the old assembly is simple enough if you can remove the stud. You don't have to pull the flywheel, but if you can, removing the stud, and replacing it with the new one is easier. If you leave the flywheel on, like I did, you can't install the stud, and then put the tensioner arm and bushing on the stud, as the tensioner arm contacts the flywheel. Probably the better way is to remove the flywheel, but that wasn't in the cards for me.

I put the spring into its location, with the "ear" of the spring locked into the groove on the case where it belongs. Then I put the stud through the bushing and tensioner arm, and the bottom washer (after applying some grease to all of it) and slid it onto the mount on the case, where I could screw in the stud. Unfortunately, the portion of the stud that sticks through above the tensioner arm is all threaded, so you risk bunging up the threads if you use vice grips or pliers to tighten it into the case. You can use two nuts screwed onto the stud and locked together to give yourself something to tighten the stud in. I was able to use pliers on the threads to torque it down satisfactorily without damaging the threads (this is why I used the extra loctite, just in case).

Before you put the nut on top, you then position the tensioner arm to where to top "ear" of the spring slides into the groove on the tensioner arm. There's a lobe on the tensioner arm that, if you tighten the tensioner arm too much, it contacts one of the mounts for the alternator. So, you have to "load" the tension on the spring by partially tightening down the nut on the stud (to keep the tensioner arm from lifting up and allowing the spring's "ear" to come out of the groove), and then pushing the tensioner arm, loading the spring, until this lobe gets past the mount of the alternator, then you can tighten the nut.

Then you just need to put the new pulley and screw it on, and put on the new belt. You do need to loose all four bolts of the starter in order to get the belt past the gear on the starter and the flywheel (you might figure that out when you're trying to remove the belt, but in my case, the old belt wore down enough that it slid out without loosening the starter).

I then squirted some more grease using the nipple on the tensioner arm, and voila, it was all done, and my problem (at least this particular one) was solved.

I assume others will one day be search THT for information on how to do this job, so I thought I'd go ahead and post my experience. It's not a difficult job, and probably only took about 1.5 hours total. Sorry for no photos.

Given that mine failed around 900 hours, I'd advise anyone with an Optimax with that many hours on it to check their tensioner assembly, and make sure it doesn't have too much play. These things do wear out, and given that they can do serious damage to your motor case, or result in a broken stud in your motor case, it's nothing to mess with.
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