The lower unit on my gtr cracked this past weekend right where it bolts on. There fixing it with no problem since it's still under warranty and I know this isn't the first one of the newer cast lower units to crack. They said its from the prop being unbalanced, could this be the case or is it just from the weak cast aluminum?

Combination of both, more than likely. And it's not weak cast, it's just cast, and by nature, doesn't handle that imbalance very well.

Were you running a two blade or the three blade prop?

From the ones I've seen pictures of, they are all cracking in the same place....on the bolt circle flange. It's basically a 90 degree corner where it bolts to the belt housing. It's a perfect stress riser for a fatigue failure....fatigue mechanism being vibration. Add any casting discontinuities and you have made a bad situation worse. So, ideally, a maintained three blade would help mitigate the problem.

But, as Russ mentioned....it's a casting. People can hype castings all they want about strength and durability.....but, it's simply a mechanically inferior metal product form in these applications. Sure, there are some that may last a lifetime for some individuals....but, there are too many inconsistencies in grain structure and too many "acceptable" casting discontinuities that are part of the casting process that can (and do) lead to failure.

I said this when MB came out with their cast LU's.....why do you think all the bow manufacturers went from cast risers to machined? Because the cast risers couldn't handle 5 grains of arrow weight per pound of draw weight. They were blowing up/breaking at stress risers from FATIGUE....draw, release, vibrate, repeat. Enough cycles, it fails...just like bending a paper clip back and forth...it will break.

This is where it cracked. The main reason I'm wondering is I want to know if I need to get my prop balanced.

When mine came apart it had a 360* crack all the way around the out drive on the prop side of the bolts. It also had an equally as large crack that ran the whole 180* turn of the bottom of belt housing. I would check there for cracks as well before things get to bad. Mine was because my 3 blade prop decided to be a 2 blade...check the prop as well..

It cracked exactly like the picture you just posted. The second pic is where the other crack was. Definitely check the bottom of the housing for other cracks and probably be better off getting a whole new prop.

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Well if others cracked exactly where mine has that should tell me it's not just the prop. Another thing I noticed by looking at it is some of the nuts bolting it together showed more thread than than others so I'm not sure if that had anything to do with it or not. It's currently at gator tail getting fixed so I'm just trying to figure out if the prop needs to be reworked and balanced or if it was a problem elsewhere.

It can never hurt to balance your prop I would do it anyway


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Unfortunately we all put a good bit of stress on our rigs and these things happen with cast product on MB, GTR, BOSS and PD. My brand new PD prop is at BAGGY'S getting balanced right now. I'm sure once fixed, you won't have the problem again.

What is the GT fix other then just replacement? Or is that the fix?

Well imbalance will cause a failure... If you have a balance problem something will give. With a machined lower you might not crack it , but the next thing to go with an unbalanced prop will be your case cover on the engine. Vibrations will tear things apart.
The only time I seen a CLU on a GT fail was from a prop issue.