Tips for DIY Transmission Replacement

[4arch]

I’ve decided to replace my Hurth transmission and damper plate (yes, I know these transmissions can be rebuilt, but for various reasons I don’t want to go there so let’s try not to discuss rebuilds in this thread as there are several existing threads already doing so). As far as I can tell, it has nearly 4,000 hours, which I understand is almost unheard of for a Hurth. Although it’s not in dire straits, it’s showing some age related symptoms and I don’t want it to fail next season when I plan to do some longer distance motoring. I was hoping to hire the project out, but I’m having a hard time finding a local mechanic to even return my phone calls or e-mails, so I’m going to have to climb the learning curve and take this one on myself. To install a new unit of the same model looks like a fairly straightforward process of unbolting and rebolting although I’m sure as a novice it will take a lot more time than I expect or want it to.

A couple questions I've been wondering about...

Should I haul the boat to do this? I have the Volvo-Penta shaft seal typically used on Beneteaus. It’s not leaking now and was replaced fairly recently. I assume the shaft can be slid aft but I worry about this creating a leaking situation.

Will I need to re-align the engine/shaft? Although the new transmission will be an exact replacement, I assume there are some differences in manufacturing tolerances but am not sure if this is enough to require realignment. If I do have to realign, any tips?

Any other tips from those who've been down this road would be greatly appreciated!

[Sailmonkey]

I've swapped out my own hurth box 3 times before I changed to a twin disc. It's a cakewalk in a big beneteau compared to my installation on the dungeon.
Sliding the shaft back shouldn't create any problem if you remember to support it with some wood blocks in to keep it centered in the seal. You don't want it to lay off on one side for a long time though.
It's a good idea to check the alignment before you bolt he shaft back up as the flexible mounts do sag over time.


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[model 10]

There will be a chance the trans input shaft will be stuck in the damper plate. The trans splines become worn and catch on the damper hub splines. Wedges of some sort will help forcing things apart.

[sammyo]

Do you have zincs on your shaft? One of those can prevent a shaft from sliding back.

[Arthur Garfield]

My question would be if the gear ratio will change and of so would you need to replace/resize the prop ?

[Uncle Bob]

Originally Posted by 4arch

I’ve decided to replace my Hurth transmission and damper plate (yes, I know these transmissions can be rebuilt, but for various reasons I don’t want to go there so let’s try not to discuss rebuilds in this thread as there are several existing threads already doing so). As far as I can tell, it has nearly 4,000 hours, which I understand is almost unheard of for a Hurth. Although it’s not in dire straits, it’s showing some age related symptoms and I don’t want it to fail next season when I plan to do some longer distance motoring. I was hoping to hire the project out, but I’m having a hard time finding a local mechanic to even return my phone calls or e-mails, so I’m going to have to climb the learning curve and take this one on myself. To install a new unit of the same model looks like a fairly straightforward process of unbolting and rebolting although I’m sure as a novice it will take a lot more time than I expect or want it to.

A couple questions I've been wondering about...

Should I haul the boat to do this? I have the Volvo-Penta shaft seal typically used on Beneteaus. It’s not leaking now and was replaced fairly recently. I assume the shaft can be slid aft but I worry about this creating a leaking situation.

Will I need to re-align the engine/shaft? Although the new transmission will be an exact replacement, I assume there are some differences in manufacturing tolerances but am not sure if this is enough to require realignment. If I do have to realign, any tips?

Any other tips from those who've been down this road would be greatly appreciated!

Hi been there done that replaced my hurth with a PRM, direct swap except that the prm had the linkage on the other side and is a few millimetres longer. A very easy swap, it just means the prop is a little further back (millimetres). Didn't take long at all and no real problems.
I am lucky that my coupling is a polyflex but I did an alignment check without it in place and was near perfect.
Ratios were nearly exact so no prop problems.
And NO MORE SLIPPING transmittion .

[4arch]

Originally Posted by Arthur Garfield

My question would be if the gear ratio will change and of so would you need to replace/resize the prop ?

I know that going back with another Hurth the gear ratio will be exactly the same and I assume the dimensions will be an exact drop-in replacement. I understand that Twin Disc may be a very close replacement in addition to PRM while a Velvet Drive/Borg Warner won't be a drop in replacement at all.