Background:
My 2003 Catalina 310 had the standard Universal M25XPB with Hurth 50 a.k.a. ZF HBW50 transmission with around 900 hours. The setup never let me down in the past 1.5 year that I've owned the boat, but I did have two concerns: 1) Plenty of posts on this forum and others reported failures of Hurth 50's around 800-1000 hours, and worse yet that rebuilds seemed to universally fail, and 2) very prominent engine vibrations and "chattering" sounds at idle.
After lots of research and planning, I actually talked myself into replacing the damper and motor mounts and upgrading to a Twin Disc transmission all at once. The more projects I do, the more I realize that if I assign a modest labor rate to my own time, it's usually both worthwhile and rewarding to replace everything that wears out "while you're in there."
After replacing all three things and re-aligning the engine, the boat has dramatically less vibration and chatter. It's still not perfect, and I suspect with a 3-cylinder never will be, but I'm very happy with the improvement. I no longer find myself fidgeting with engine speed to find a cruising rpm that doesn't cause the hatches to rattle, and idling down from cruising speeds to idle RPMs no longer produces sounds like bolts rattling around in tin cans.
I'm now very happy to listen to the motor at 1100 RPM idle speed (which I adjusted according to the Universal bulletin), whereas before I found the rattles deeply displeasing. I'm happy to know I'm running a brand-new transmission with a far superior reliability reputation to the one it replaced.
The only downside is that the Twin Disc TMC-40 transmission uses straight-cut gears, whereas was the Hurth/ZF boxes, along with the ones fit to Yanmars, all use helical cut gears. Straight-cut gears produce a higher-pitched whine sound that changes with gear speed. The folks at Beta Marine confirmed this is normal, as did other threads from folks who made the same upgrade. In the fullness of time, I plan to install some more sound proofing in the transmission tunnel area to quiet this down. While a little annoying, I'm still happier to take a slight increase in mid-frequency sound over all of the rattling and chattering that was eliminated. If you don't know what straight-cut gears sound like, they are often used in race car transmissions and rear-ends. This is a great example of the sound: Straight Cut Gears Sound Amazing. Don't worry, the transmission isn't actually that loud, but the sound and the linear relationship of pitch to engine speed is similar.
Research and parts selection process:
After much research I discovered the "chattering" sound is often liked to a failed damper plate, and found many success stories on the C350 forums of folks replacing the plate with an upgraded model and enjoying reduced vibrations. I found this post particularly helpful: ENGINE NOISE-DAMPER REPLACEMENT? - Catalina 350 International Association. This thread and others point to there being two versions of the rubber-based "R&D" brand damper plate used - a "standard" model from the factory and an "H/D" model is later upgrades. I posted a follow-up below about the differences in these models.
R&D damper plates are distributed in the US by PYI Inc, and I called and spoke to a very helpful gentleman at PYI who confirmed the "H/D" model was a much better choice for our engines. The "standard" model damper was not well suited to absorb all the torrential vibration from the Kubota 3-cylinder block of the Universal engine which, compared to other engines such as Yanmar, has a relativity lightweight flywheel. While I didn't know for sure during the planning phase if I had my original damper plate, if a prior owner replaced, if it was the upgraded model, or what it's condition was, my complaint of chatter seemed to align with needing a damper replacement.
I ordered part # RD-22-AM-4 from PYI for around $400 - this is the "H/D" (High Deflection) version of the damper, while the standard "Loop" version shipped by Universal on the engine originally would have been # RD-22-A-4. This catalog has a breakdown of the part numbers: https://www.pyiinc.com/downloads/r-d-marine/r-d-marine-catalog.pdf. See the follow-up post below for more info.
There also seems to be some debate about abandoning R&D dampers altogether and moving to a more traditional spring-based damper. Catalina Direct currently sells a spring-based damper for $1219. I've read about issues with spring-based dampers as well, so decided to stick with the much cheaper RD-22-AM-4 based on what I had read about successful service life after replacement.
Threads about vibration and chatter will almost always recommend replacing engine mounts. I had the original Universal-style mounts (see photos below) which looked fine, but I had no real way to tell their condition. I did notice that at certain engine RPMs, the prop shaft was hitting the log and causing rattling to spread throughout the boat. I could reproduce this even in neutral and pause it by simply grabbing hold of the shaft and manually absorbing some of the wiggle being produced by the engine. I figured this could probably be solved by adjusting the engine position via the engine mount adjusters so the shaft was better center in the long, but since I was already going to have to lift the engine to remove the transmission so I could access the damper plate, starting fresh with new mounts before alignment seemed logical.
The Universal mounts are expensive. The OEM Universal part # is 040510. Westerbeke sells them right now for $659 each, and Catalina Direct sells them for the same. I was tempted to move to a much cheaper aftermarket mount, but two things deterred me: 1) this thread which highly favors the Westerbeke style: Anyone Tried These Motor Mounts and 2) the mounting pattern. The Westerbeke mount holes to the boat are 5" on center, whereas all the aftermarket mounts I found were 4". I had no interest in adding the drilling of new holes and filling of old holes into the labor of the project, so I bit the bullet and ordered 4x 040510's which were mercifully only $421 each when I ordered them a few months ago.
After spending about $2,000 on the damper plate and mounts, I got busy with other things for a while. When I got back around to the project a few months later, and with a dear friend signed up to spend the day on the installation with me, I started thinking about how silly it would be to spend all those labor hours to reinstall that Hurth transmission, known to fail, when a well-respected replacement was available for about $1,200. The old transmission literally has to be fully removed in order to access the damper plate, so since I was this far in, why the hell not replace the transmission while I was at it?
The Hurth 50 - which later became the ZF 5M when ZF bought Hurth, is no longer made. The next size up replacement from ZF is the ZF 10M, which Catalina direct reports as being hard and expensive to get and recommends the ZF 12M instead: Transmission - New Replacement 12M. The problem is that the 12M is 1.46" longer and has a 0.39" bigger drop (from input shaft to output shaft) than the original Hurth 50. That 1.46" length increase would be very involved to accommodate, as there's less shaft than that exposed between the dripless seal and the flange. At the very least, new holes would have to be drilled to push the mounts forward, and the engine cover would probably have to move forward as well requiring some trim on the bulkhead to accommodate the gap, maybe the shaft shortened. Yuck. The Twin Disc TMC 40 is a far closer match, being just 0.157" longer and having 0.216" more drop. Beta Marine sells it for $1260 (TMC 40 (2:1 ratio) Transmission) and supplied this very helpful document about the considerations when converting: CONVERSION FROM ZF 5M (HURTH HBW 50) TO TMC 40.pdf. I ordered the transmission from Beta Marine, along with the horizontal control cable bracket (part# 202-09311), bracket hardware (2x #217-99292 and 2x #217-99293) and saddle clamp fitting (part# 212-99241/01) all for $1252.39 including shipping with their new user online discount.
Reading a post from Jesse's blog (Engine), and others in the 310 forums, it seemed folks liked to install spacers below the motor mounts so the engine doesn't sit up so high on the mount bolts. Perhaps this helps reduce vibration by giving the engine less leverage to flex the rubber. Since I was already at least half way up the bolts, and was going to need to raise 0.216" to accommodate the bigger transmission, I decided to make some mounts. I've worked with a local shop that can laser cut parts out of stainless steel up to 3/4" thick (which is nuts), so I designed my spacers in CAD, 3D printed to test fit, and then had the shop (Profile Laser in Portland, OR) make 2x spacers out of 1/4" 304 stainless for the back and 2x out of of 1/2" stainless for the front. Here's the drawing for my spacers as PDF: DENTON-ENGINE-SPACERS-V02-NOV-02-2025.pdf. I'm happy to provide a DXF version via email if you'd like to get some of these made. Profile Laser cut my four spacer, plus an extra part for my bowsprit project, for $250.
The spacers are very pleasing hardware:
In the interest of being over-prepared so my generous friend and I could complete the project in one day, I also ordered new hardware:
Bolts for mounting plate to transmission:
> 6x M8, standard 1.25 thread, 20mm long, class 10.9 steel, plus lock washers
Bolts for holding mounting plate to engine block:
> 6x M10, standard 1.5 thread, 25mm long, class 10.9 steel, plus lock washers
Bolts prop shaft flange to transmission:
> 4x M10, fine 1.25 thread, 30mm long, class 10.9 steel, plus lock washers and standard nuts
I used regular zinc fasteners, based on recommendations I found against stainless for this application due to galling risk. I confirmed Beta Marine uses non-stainless fasteners for such applications as well.
I bought blue LocTite for the transmission mounting bolts.
The damper plate attaches to the flywheel with special Allen-socket style screws that use a 4.5mm Allen key. Since these were so specific I ordered them from Westerbeke, 6x part # 200443 @ $3.50/ea. They come with LocTite red pre-applied, which makes sense given their important job and small size, but sure made the old ones annoying to get out.
I also ordered the replacement gasket that goes between the exhaust manifold and the mixing elbow, because it was easiest to remove the mixing elbow and disconnect the hose from the muffler than to try and remove the hose from the elbow. The gasket is part # 036425 from Westerbeke.
The project requires lifting the engine a few inches to get the new mounts in. I didn't like the idea of lifting via the boom, so I purchased 1x 8' 4x4 lumber and chopped it into three pieces. Two of the pieces I wrapped in felt to set on either side of the companion way without scratching, and the third piece to set onto as a cross-bar. I bought a very cheap chain hoist from Harbor Freight to hang from the rig. We found that while it lifted fine, the transition to lowering was sketchy and at one point dropped the engine about an inch before engaging. I returned it and would recommend a better quality chain hoist.
In addition to standard tools, I also brought the following which all turned out to be super helpful:
* An electric 3/8" drive ratchet - great for the very long threaded bolts that hold the motor mounts to the boat
* A bunch of long 3/8" drive socket extenders, allowed using the electric ratchet from the top when working on the mount books
* A set of standard and "stubby" metric ratcheting wrenches - these things always make life better
* A good set of feeler gauges for the alignment
* Torque wrenches, although there don't seem to be rigorous torque specs for any of the bolts involved
Armed with hours of research and every part and tool I could think of, we went to the boat.
In the following posts I'll show the pictures through the process. The two of us were able to complete the project in about 7 hours.
Yes, I know this post was a mile long. I hope it helps someone who takes on this project next!
My 2003 Catalina 310 had the standard Universal M25XPB with Hurth 50 a.k.a. ZF HBW50 transmission with around 900 hours. The setup never let me down in the past 1.5 year that I've owned the boat, but I did have two concerns: 1) Plenty of posts on this forum and others reported failures of Hurth 50's around 800-1000 hours, and worse yet that rebuilds seemed to universally fail, and 2) very prominent engine vibrations and "chattering" sounds at idle.
After lots of research and planning, I actually talked myself into replacing the damper and motor mounts and upgrading to a Twin Disc transmission all at once. The more projects I do, the more I realize that if I assign a modest labor rate to my own time, it's usually both worthwhile and rewarding to replace everything that wears out "while you're in there."
After replacing all three things and re-aligning the engine, the boat has dramatically less vibration and chatter. It's still not perfect, and I suspect with a 3-cylinder never will be, but I'm very happy with the improvement. I no longer find myself fidgeting with engine speed to find a cruising rpm that doesn't cause the hatches to rattle, and idling down from cruising speeds to idle RPMs no longer produces sounds like bolts rattling around in tin cans.
I'm now very happy to listen to the motor at 1100 RPM idle speed (which I adjusted according to the Universal bulletin), whereas before I found the rattles deeply displeasing. I'm happy to know I'm running a brand-new transmission with a far superior reliability reputation to the one it replaced.
The only downside is that the Twin Disc TMC-40 transmission uses straight-cut gears, whereas was the Hurth/ZF boxes, along with the ones fit to Yanmars, all use helical cut gears. Straight-cut gears produce a higher-pitched whine sound that changes with gear speed. The folks at Beta Marine confirmed this is normal, as did other threads from folks who made the same upgrade. In the fullness of time, I plan to install some more sound proofing in the transmission tunnel area to quiet this down. While a little annoying, I'm still happier to take a slight increase in mid-frequency sound over all of the rattling and chattering that was eliminated. If you don't know what straight-cut gears sound like, they are often used in race car transmissions and rear-ends. This is a great example of the sound: Straight Cut Gears Sound Amazing. Don't worry, the transmission isn't actually that loud, but the sound and the linear relationship of pitch to engine speed is similar.
Research and parts selection process:
After much research I discovered the "chattering" sound is often liked to a failed damper plate, and found many success stories on the C350 forums of folks replacing the plate with an upgraded model and enjoying reduced vibrations. I found this post particularly helpful: ENGINE NOISE-DAMPER REPLACEMENT? - Catalina 350 International Association. This thread and others point to there being two versions of the rubber-based "R&D" brand damper plate used - a "standard" model from the factory and an "H/D" model is later upgrades. I posted a follow-up below about the differences in these models.
R&D damper plates are distributed in the US by PYI Inc, and I called and spoke to a very helpful gentleman at PYI who confirmed the "H/D" model was a much better choice for our engines. The "standard" model damper was not well suited to absorb all the torrential vibration from the Kubota 3-cylinder block of the Universal engine which, compared to other engines such as Yanmar, has a relativity lightweight flywheel. While I didn't know for sure during the planning phase if I had my original damper plate, if a prior owner replaced, if it was the upgraded model, or what it's condition was, my complaint of chatter seemed to align with needing a damper replacement.
I ordered part # RD-22-AM-4 from PYI for around $400 - this is the "H/D" (High Deflection) version of the damper, while the standard "Loop" version shipped by Universal on the engine originally would have been # RD-22-A-4. This catalog has a breakdown of the part numbers: https://www.pyiinc.com/downloads/r-d-marine/r-d-marine-catalog.pdf. See the follow-up post below for more info.
There also seems to be some debate about abandoning R&D dampers altogether and moving to a more traditional spring-based damper. Catalina Direct currently sells a spring-based damper for $1219. I've read about issues with spring-based dampers as well, so decided to stick with the much cheaper RD-22-AM-4 based on what I had read about successful service life after replacement.
Threads about vibration and chatter will almost always recommend replacing engine mounts. I had the original Universal-style mounts (see photos below) which looked fine, but I had no real way to tell their condition. I did notice that at certain engine RPMs, the prop shaft was hitting the log and causing rattling to spread throughout the boat. I could reproduce this even in neutral and pause it by simply grabbing hold of the shaft and manually absorbing some of the wiggle being produced by the engine. I figured this could probably be solved by adjusting the engine position via the engine mount adjusters so the shaft was better center in the long, but since I was already going to have to lift the engine to remove the transmission so I could access the damper plate, starting fresh with new mounts before alignment seemed logical.
The Universal mounts are expensive. The OEM Universal part # is 040510. Westerbeke sells them right now for $659 each, and Catalina Direct sells them for the same. I was tempted to move to a much cheaper aftermarket mount, but two things deterred me: 1) this thread which highly favors the Westerbeke style: Anyone Tried These Motor Mounts and 2) the mounting pattern. The Westerbeke mount holes to the boat are 5" on center, whereas all the aftermarket mounts I found were 4". I had no interest in adding the drilling of new holes and filling of old holes into the labor of the project, so I bit the bullet and ordered 4x 040510's which were mercifully only $421 each when I ordered them a few months ago.
After spending about $2,000 on the damper plate and mounts, I got busy with other things for a while. When I got back around to the project a few months later, and with a dear friend signed up to spend the day on the installation with me, I started thinking about how silly it would be to spend all those labor hours to reinstall that Hurth transmission, known to fail, when a well-respected replacement was available for about $1,200. The old transmission literally has to be fully removed in order to access the damper plate, so since I was this far in, why the hell not replace the transmission while I was at it?
The Hurth 50 - which later became the ZF 5M when ZF bought Hurth, is no longer made. The next size up replacement from ZF is the ZF 10M, which Catalina direct reports as being hard and expensive to get and recommends the ZF 12M instead: Transmission - New Replacement 12M. The problem is that the 12M is 1.46" longer and has a 0.39" bigger drop (from input shaft to output shaft) than the original Hurth 50. That 1.46" length increase would be very involved to accommodate, as there's less shaft than that exposed between the dripless seal and the flange. At the very least, new holes would have to be drilled to push the mounts forward, and the engine cover would probably have to move forward as well requiring some trim on the bulkhead to accommodate the gap, maybe the shaft shortened. Yuck. The Twin Disc TMC 40 is a far closer match, being just 0.157" longer and having 0.216" more drop. Beta Marine sells it for $1260 (TMC 40 (2:1 ratio) Transmission) and supplied this very helpful document about the considerations when converting: CONVERSION FROM ZF 5M (HURTH HBW 50) TO TMC 40.pdf. I ordered the transmission from Beta Marine, along with the horizontal control cable bracket (part# 202-09311), bracket hardware (2x #217-99292 and 2x #217-99293) and saddle clamp fitting (part# 212-99241/01) all for $1252.39 including shipping with their new user online discount.
Reading a post from Jesse's blog (Engine), and others in the 310 forums, it seemed folks liked to install spacers below the motor mounts so the engine doesn't sit up so high on the mount bolts. Perhaps this helps reduce vibration by giving the engine less leverage to flex the rubber. Since I was already at least half way up the bolts, and was going to need to raise 0.216" to accommodate the bigger transmission, I decided to make some mounts. I've worked with a local shop that can laser cut parts out of stainless steel up to 3/4" thick (which is nuts), so I designed my spacers in CAD, 3D printed to test fit, and then had the shop (Profile Laser in Portland, OR) make 2x spacers out of 1/4" 304 stainless for the back and 2x out of of 1/2" stainless for the front. Here's the drawing for my spacers as PDF: DENTON-ENGINE-SPACERS-V02-NOV-02-2025.pdf. I'm happy to provide a DXF version via email if you'd like to get some of these made. Profile Laser cut my four spacer, plus an extra part for my bowsprit project, for $250.
The spacers are very pleasing hardware:
In the interest of being over-prepared so my generous friend and I could complete the project in one day, I also ordered new hardware:
Bolts for mounting plate to transmission:
> 6x M8, standard 1.25 thread, 20mm long, class 10.9 steel, plus lock washers
Bolts for holding mounting plate to engine block:
> 6x M10, standard 1.5 thread, 25mm long, class 10.9 steel, plus lock washers
Bolts prop shaft flange to transmission:
> 4x M10, fine 1.25 thread, 30mm long, class 10.9 steel, plus lock washers and standard nuts
I used regular zinc fasteners, based on recommendations I found against stainless for this application due to galling risk. I confirmed Beta Marine uses non-stainless fasteners for such applications as well.
I bought blue LocTite for the transmission mounting bolts.
The damper plate attaches to the flywheel with special Allen-socket style screws that use a 4.5mm Allen key. Since these were so specific I ordered them from Westerbeke, 6x part # 200443 @ $3.50/ea. They come with LocTite red pre-applied, which makes sense given their important job and small size, but sure made the old ones annoying to get out.
I also ordered the replacement gasket that goes between the exhaust manifold and the mixing elbow, because it was easiest to remove the mixing elbow and disconnect the hose from the muffler than to try and remove the hose from the elbow. The gasket is part # 036425 from Westerbeke.
The project requires lifting the engine a few inches to get the new mounts in. I didn't like the idea of lifting via the boom, so I purchased 1x 8' 4x4 lumber and chopped it into three pieces. Two of the pieces I wrapped in felt to set on either side of the companion way without scratching, and the third piece to set onto as a cross-bar. I bought a very cheap chain hoist from Harbor Freight to hang from the rig. We found that while it lifted fine, the transition to lowering was sketchy and at one point dropped the engine about an inch before engaging. I returned it and would recommend a better quality chain hoist.
In addition to standard tools, I also brought the following which all turned out to be super helpful:
* An electric 3/8" drive ratchet - great for the very long threaded bolts that hold the motor mounts to the boat
* A bunch of long 3/8" drive socket extenders, allowed using the electric ratchet from the top when working on the mount books
* A set of standard and "stubby" metric ratcheting wrenches - these things always make life better
* A good set of feeler gauges for the alignment
* Torque wrenches, although there don't seem to be rigorous torque specs for any of the bolts involved
Armed with hours of research and every part and tool I could think of, we went to the boat.
In the following posts I'll show the pictures through the process. The two of us were able to complete the project in about 7 hours.
Yes, I know this post was a mile long. I hope it helps someone who takes on this project next!
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