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Author Topic: Replacing Hurth Gearbox with ZF12M  (Read 2322 times)

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Lexatola

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Replacing Hurth Gearbox with ZF12M
« on: August 08, 2021, 01:16:23 PM »

We are replacing the original Hurth gearbox on our '86 C-34 with the larger and more robust ZF12M.  The old Hurth was struggling to get into gear and on the mechanical survey showed water, sodium and unmeasurable viscosity due to contamination.  So biting the bullet.  Does the motor "have" to be shimmed (lifted) and does the prop shaft "have" to be shortened?  Just wanting to gather all the facts before proceeding.  Can the aft section of pan be cut back to give more clearance or is the issue purely around alignment?  Curious to hear from some folks who have done it. Oh yes - how many hours would you say it should take to remove, replace, reconnect, and align? 

Thanks so much!

Alex
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Sea Glass #126
1986 MK1 Std Rig
Kingston, WA

Ron Hill

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Re: Replacing Hurth Gearbox with ZF12M
« Reply #1 on: August 08, 2021, 01:47:20 PM »

Alex : You didn't mention which engine you have, but if it's a M25XP then the replacement for the Hurth 50 is a Hurth ZF10 (old Hurth 100).

You shouldn't have to "lift" the M25XP engine, but the ZF10 is about 1 " longer and you should shorten your shaft.  This maybe a problem if you use the keyway shaft to transmission attachment.

If that is a problem change to flexible coupling that doesn't need a keyway!!

How long this is going to take? depends on which engine you have?  I removed the engine because the bell housing holds the INSIDE transmission attachment bolts on the XP engine!!  I'd also recommend changing out the damper plate when you change out the transmission. 

A few thoughts
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Ron, Apache #788

Lexatola

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Re: Replacing Hurth Gearbox with ZF12M
« Reply #2 on: August 08, 2021, 02:15:17 PM »

Hi Ron - My '86 has the M25 engine and currently the ZF10 costs something like $3500 so most folks seem to recommend going with the ZF12M which is under $2K and is said to be a more robust unit.  Agreed on the damper plate for sure!

Best,

Alex
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Sea Glass #126
1986 MK1 Std Rig
Kingston, WA

Bill Shreeves

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Re: Replacing Hurth Gearbox with ZF12M
« Reply #3 on: August 24, 2021, 08:32:42 PM »

Thought I'd share a few on my experience when I replaced my HB5 back in 2017 including damper plate and everything back to and including the prop.

I went through many of the same considerations and, after much research, I was very fortunate to find a spectacular deal on a ZF10 which was way cheaper than a 15 making my decision very easy.  That said, before i found my deal, I was reluctant to accommodate the additional offset on the 15 but, at the time, a new 10 was about $400 to $500 more than the 15 but, increasingly tough to find.   if memory serves, I think the 15 was about 10mm greater offset?

After reading your post I was curious and did a quick search and found a couple sites overseas with the ZF10 with the correct gear ratio for much cheaper than CatalinaDirect.  Not sure how adventurous you are but, if you needed warranty work, Fla is ZF's U.S. main location and they were very, very  helpful to me when I contacted them about a problem with my replacement ZF10.

Have you considered / looked in to having the current one rebuilt?  Assuming you have an average number of hours / season on it, it had a pretty darn good run.  At the time I was researching my replacement, a shop within driving distance wanted nearly the same cost as a new 15 which I considered as well.  You may find a more competitive $
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The only thing that always works on an old boat is the owner...

Bill Shreeves
s/v "Begnnings" 1987 Shoal Draft #333
M25XPB, Worton Creek, MD
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