Bullflex Coupling Installation

[Mike and Theresa Vaccaro]

Vetus Bullflex Coupling Installation in a Stock ’88 Catalina with an M-25XP Engine

We installed a new coupling as part of a recent upgrade including new motor mounts and a new exhaust riser.  Our boat was stock, and little or no previous maintenance had been performed on the riser, mounts or coupling.  The project was initiated due to corrosion damage to two engine mounts, and a black iron riser that wasn’t long for the world (it was the original riser).  Since we were replacing the riser, we also added a silicone “hump” hose.  Since there are other good FAQs and Mainsheet articles about riser repair/replacement and swapping mounts, I only discuss the addition of the flexible coupling.

Worthy of note, a flexible coupling is not a universal joint!  Installation still requires a proper engine alignment.  The coupling is designed to damp vibration (de-coupling the drive shaft from the transmission to a certain degree at most power settings above IDLE).  A misalignment will result in premature coupling wear.

For the M-25XP engine, Vetus recommends installation of their BULLFLEX 2 coupling.  We purchased our parts directly from Vetus, and the coupling cost 330 dollars.  We also purchased a set of studs and nuts for an additional 14 dollars.  The BULLFLEX is made in Europe, and all measurements are metric, but the studs are centered on a 3.25” diameterâ€"a perfect match for the coupling on the back of the Hurth transmission on our engine.

Overall, the BULLFLEX 2 will simply slide on a stock length shaftâ€"presuming you can remove the old coupling!  We performed all of the work on our boat in the water, so removing the shaft was not an option.  Our coupling had never been removed, and the years of neglect had taken their toll.  Fortunately, the BULLFLEX throat is deep enough that the stock coupling may simply be cut adjacent to the aft face of the stock coupling.  Since the BULLFLEX uses a simple compression sleeve, no keyway machining of the prop shaft is required.

Additionally, we use a stock bronze packing gland.  Prior to modification, 3” of shaft was exposed forward of the packing nut.  This is adequate clearance for installation of the BULLFLEX.  If you have a dripless bellows, there may not be sufficient room to add the coupling.  Dimensions for the BULLFLEX can be obtained from the VETUS website (www.vetus.com). Installation manuals and product information are available for download.  Additionally, you can call (410) 712-0740 for information.

To obtain access, clean out the aft cabin and remove the cushions.  Remove the large, central access panel to expose the packing gland, drive shaft and transmission.

I recommend that you start spraying all of your fasteners that you’ll have to remove a day or two prior with some type of penetrating oilâ€"we had 16 years of neglect (i.e., rust and corrosion) to deal with.  In almost all cases, the penetrating oil did the trick One notable exception was a motor mount which was so badly corroded that it required the bracket from the block to be removed, and a drill press to remove the offending bolt and corroded aluminum and a new bushing to properly size the bolt hole.  This was the mount under the raw water pumpâ€"evidently enough saltwater had leaked and dripped on the screw and nuts to literally weld the steel bolt to the aluminum bracket.  Whether you replace your mounts or not, it’s worth an occasional spray of corrosion inhibitor on that mount to prevent trouble.  

The first step is to unbolt the coupling from the transmission.  You’ll need a 9/16” box end wrench.  A box end “ratchet” is really handy, if you have one.  You’ll need to lock the transmission in gear to unbolt the nuts.  You can do this manually at the transmissionâ€"the shift lever is on the right side of the transmission, just follow the shift cable to find it.  

If you want to try to remove the old coupling, vs. cutting it, cut the safety wire and loosen the two set screws.  Then, you’ll need to do some clever work with a hammer.  I quick straw poll amongst the local boat mechanics was that after more than a couple of years, the odds of removing a coupling successfully are slim and will likely require the use of a wheel puller or spacer bushings.  I therefore chose the Neanderthal approach and cut the shaft adjacent to the aft face of the old coupling.  

While I’m sure there are several good power tools that would make short work of this, I knew that I was less likely to make an error with an old fashioned mini-hacksaw.  Since the old coupling can be used as a guide, a nice square cut is possible by hand.  A little cutting oil goes a long way, as does a good, quality blade.  After the shaft is cut, I removed the old coupling and finished the end of the shaft with a mill bastard file to remove all cut marks.  Final burnishing of the shaft was accomplished by hand using strips of 220 grit wet/dry lubricated with Boeing T-9.  This removed all grime and corrosion from the 1” shaft.  Cleaning of the shaft was required to ensure that the new coupling would slip on properly.  In accordance with Vetus instructions, no lubricants should be used when fitting the coupling to the shaft.

Prior to sliding the new coupling on the shaft, you might want to consider repacking your gland.  An old piece of cotton fabric wrapped securely around the shaft and secured with a zip tie or hose clamp will keep the flow of water to a slow drip, allowing you to slide the nut forward and clean the old packing out.  A local craftsman showed me the best tool in the world for loosening a stop nut on a packing glandâ€"an old dull chisel and hammer!  Works like a champ and fits all size nuts…also works for tightening.  Remember, the packing nut itself should only be tightened by handâ€"the chisel trick is for the lock nut only.

Now there are essentially two options to attach the new coupling.  The first is to slip the shaft aft a bit, slip the coupling in place and then pull the shaft forward.  The 2nd is to lift the engine up off the mounts.  Since we also had to remove the exhaust riser, it was fairly easy to place a 4x4 across the companionway and will the motor with a come-a-long and some Dacron line, using the two lift points on the top of the engine.  If you chose to try option 1, use caution not to slide the shaft too far aft, as you’ll have to pull it forward into the compression sleeve on the coupling.  If you have zinc immediately forward of the strut, paint or marine growth on the shaft, it may not be possible to slip it aft.  Additionally, if you slide the shaft aft, be sure to mark it where it exits the packing nut so you’ll have a reference as you pull it forward.  The Vetus drawings will give you a good idea how far the shaft must slide up into the coupling for the compression sleeve to work properly.

Prior to alignment, attempt to center the shaft in the log/cutlass bearing.  Move the shaft left and right as well as up and down.  Use wooden blocks and wedges as required to hold it centered.  â€œCentered” is half the range of motion on each axis.  You can either measure or weigh (vertical axis only), or simply “eyeball” it.  

At the recommendation of the tech support folks at Vetus, we didn’t insert the studs in the coupling until after completing the engine alignment.  This made it easy to turn the coupling as well as the transmission to check the alignment.  Proper maintenance practice dictates a new alignment at this point.  If you were in alignment, and didn’t loosen the mounts, this should be quite simple.  Much has been written about alignment techniques, so I won’t go into that here.  Strive to align the engine within .003 inches.  

After alignment, you can insert the studs.  You’ll have to rotate the coupling and slide it aft a bit for access.  Note that the studs have a “long” and “short” side.  Check to ensure that your installation matches the drawings provided by Vetus.  A drop of red locktite should be used to secure the studs.   Use a cap nut with a “stop” nut along with a 17mm box end wrench to secure the studs.  After all four studs are in place, you can align the studs with the holes in the transmission coupling and slide it forward.  Secure the lock nuts with a 17mm wrench, torque to 45 ft/lbs.  After these nuts are torqued, you can torque the compression sleeve using a metric hex-head wrench (this required a trip to a local auto parts store!).  These nuts should also be torqued to 45 ft/lbs.

Next is an operational check of the coupling.  Start the engine and after warm-up, shift observing the coupling.  You’ll note that at IDLE there is some vibration, regardless of the type of motor mounts you use.  If your mounts are flexible (e.g., Vetus K-50’s), this vibration can be substantial.  At power settings above idle, the shaft should smooth out.

After an operational check/sea trial, and prior to filling the aft cabin back up, you should re-check all nut/bolt torque as well as adjust the packing gland (if required).

Overall, with the new coupling and mounts, we’ve almost eliminated all vibration at power settings above idle.  At idle, the engine is still isolated from the stringers, but even the flexible coupling can’t isolate the shaft from the engine motionâ€"this is a characteristic of diesel engines with relatively high output and small flywheels.

Since our installation included a simultaneous change of engine mounts, the change was very dramatic!  Don’t know what the result of just a coupling change would be, although I expect there would be some benefit in terms of reducing vibration and equipment wear.  If you’re thinking about this modification and want specific information, please drop a line to vacntess@cox.net.

Cheers,

Mike Vaccaro
“Spirit” ’88 Hull 563

[Mike Smith]

Vac -

Great write-up! Be sure to submit this to Mark for inclusion in the Projects page. Here is the link: http://c34.org/contribute/contribute-project.html

Mike

Mike and Jan Smith
S/V Breezer
www.mikejansmith.com

[Frank Stich]

Mike,

Did the new motor mounts significantly reduce vibration transmitted to the hull?

Frank

[Mike and Theresa Vaccaro]

Frank,

Absolutely!  Been a lot written about the motor mount change on this board and in the Mainsheet.  A "search" of the site and "find" on the message board will provide numerous how-to hits.  

Cheers,

Mike Vaccaro
"Spirit" '88 Hull 563