Torsional Coupling Replacement

[James Cowher]

I’m planning to replace the torsional coupling on one of my engines. The drive train has a lot of gear chatter at idle and I doubt these have ever been replaced before so this is going to be my winter project while the boat is on the hard.  Since the coupling is neither engine nor transmission, it seems there is limited technical information on replacement, although I see previous mention that it is “mechanics 101”.  For the novice like me, I wanted to see if anyone has any pointers or advice before I tackle the job.  One of the SBMAR staff suggested using the Centa CF-R Torsional coupling.  I have a Cummins 6BT and ZF 220A.  From what I can see, I need to disconnect the shaft from the coupling at the rear of the transmission and slide the shaft back.  Next, I will need to support the engine and transmission with jacks and/or blocks.  Then, loosen and disconnect the bolts in the metal brackets that connect the transmission to the rear engine mounts (grey mounts in photo).  This would be followed by sliding the transmission back and gaining access to the bell housing.   Once inside, disconnect the bolts holding the current coupling in place and slide it back (provided it is not rusted on the shaft).   Install new coupling, reassemble, and check alignment pre- and post-launch.  Can anyone provide any guidance or tips on this task?   Unfortunately, I don’t have a good way to support the transmission from above in the process so will need to support it from below while sliding it back.  Appreciate any feedback.  Thanks

Topic

[James Cowher]

I believe the idle speed on that engine is somewhere around 600-700.  The other engine seems to be set lower so I do hear the power pulse Clark is discussing on that engine.  

The engines are 1989 vintage but only have around 1500 hours on them.

This particular engine had a slight mis-alignment issue for a number of years due to a cracked engine mount bolt (bolt going through the stringer) that was difficult to find.  It was there when I bought the boat and took several years for me working with various mechanics to locate.  I’m guessing this created excess stress on the transmission that may have impacted the longevity of the coupling. No idea how long it was like that.  The oil samples on this side are trending decent with a slight increase in lead but nothing alarming.   

Tony, can you clarify the right angle brackets?   I’m assuming these would go over the stringers on each side  and extend under the bell housing where the wedges would be driving into place (See drawing).  Is this is an accurate understanding?  That is a lot of weight so would need some heavy gauge steel.  There are a fair number of machine shops in the area so it shouldn’t be difficult to get these made.  

Thanks for the feedbackl. 

[Tony Athens]

Easiest way to support the engine is adding a simple right angle brackets to the P&S side of the bell housing and wedge up about 1/8″ max to take the weight on the gear isolators ( remove the top nut on both 1st-(Lots of WD-40 to make it easy)  .. Safe too.. 12MM bolts fit the housing –two per side  Done right using hard wood wedges and some grease with hammer, it can be done easily without using a jack….Looking at the vintage of your adapter plate tells me you most likely have a very old Spring type coupling.. That will limit your options as to torsional coupling choices ( offset-thickness issues.

[Clark Leighs]

Do the simple things first.

You may indeed have gear chatter but before you start tearing into things check the idle speed you actually have  vs  the Cummins recommended idle speed.   

You don’t say how old the engine is but if the dash tach is analogue then find, borrow or buy a digital, laser pointer digital phototach from Ebay or Amazon for about $20..   and check the idle revs.

If they are too low then the power pulses from the engine will cause that clatter.

If low then adjust the idle speed.

If that does not work then you may in fact have to go after the gear dampener.

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