Removing M35AC Engine and Replacing Transmission

[Dave Spencer]

After 13 years of reading and contributing to the C34 forum, I've noted that most C34s are fitted with either M25, M25XP, M25XPB or M35B Universal engines.  The engine in my 1994 Mk 1.5 is an M35AC. There weren't too many of those built so some of the things I've seen on this forum about engines don't fully apply to my boat... but I'm usually able to figure it out.  The following is my description of how I replaced my failed Hurth HBW 100 transmission with a new one.  I'm sure there are things I could have done differently but I'm very happy with this whole process.  This is a long post broken up into several pieces to allow pictures and files to be included at the most appropriate place.  Your boat may be different.

Signs of trouble
For several years, I had a slight rattle from the engine at idle.  After reading posts on this and other forums, I suspected that this was likely due to loose / worn springs in the damper plate and would require some attention at some point.  Late in the summer of 2023, I noticed a new rattling sound from the engine / transmission at cruising speed (2200+ rpm).  I initially thought (incorrectly it turns out) the damper plate was badly worn, possibly at the spline and quickly moved its replacement to the top of my to do list.

Transmission failure
In mid-September 2023, were motoring home from a nearby Georgian Bay anchorage at a sedate pace and our speed dropped from 5.5 kts to 4.0, then 2.5 kts within about 15 minutes.  I suspected transmission slippage and went below and verified that the shift lever was fully engaged.  There was a distinct smell of ATF and my temperature gun recorded the transmission casing at 239F which is far too hot.  We killed the engine and sailed in light wind to within about 100 metres of our marina's entrance and then restarted and gingerly motored back to our slip.  In a pinch, I could have motored to our slip in reverse which seemed to be functioning normally.  Our sailing was done for the summer but fortunately, it happened only a couple of weeks before we hauled out for the season.  I elected to attack this problem once we were on the hard.  The engine has 2555 hrs on it. 

[Dave Spencer]

Removing the transmission (and the engine)
I was hoping to simply unbolt the transmission and raise the rear of the engine to allow for clearance over the propeller shaft flange as described in a post from a C350 owner. http://www.catalina350.com/phpBB3/viewtopic.php?f=11&t=685. Alas, when I examined my M35AC engine, I saw that the transmission was bolted directly to the bellhousing from the inside meaning I would have to pull the engine to remove the transmission.

I scanned the C34 site and my back issues of Mainsheet for tips on removing the engine.  It seemed like the greatest challenge was going to be removing the lower nut on the exhaust flange.  I bought a 6-point 9/16 combination wrench as recommended and soaked all three exhaust flange nuts with PB Blaster for 2 days.  I removed the alternator and all electrical connections from the engine (alternator, oil pressure switch, thermostat, starter, glow plugs and fuel pump).  I labeled each cable and took lots of pictures.  I also protected the terminals on high current cables with a short scrap of hose.   I shut off the fuel at the tank and disconnected the fuel delivery and return lines.  I drained as much coolant as I could into a suitable receptacle and disposed of it at our marina's oil and coolant dump.  I then disconnected the 7/8" coolant hoses at the water pump and the exhaust manifold and removed the heat exchanger.  The three shift cables (transmission shifter, throttle and engine shut down) were easy to remove.  I measured and recorded the height of the motor mounts as a starting point for the new mounts that would have to be 5.5mm higher and removed the top nuts using a 15/16" combination wrench.  The exhaust flange was the last item to remove from the engine.  I feared the worst but all three exhaust flange nuts were easy to remove with my 6 point 9/16" wrench including the notorious lower nut that has caused some C34 owners grief.  It was tricky to pull the flange off the engine since the lower stud is quite long and the other end of the exhaust elbow was still connected to the hump hose and the muffler, but I succeeded after wrestling with it for a few minutes. 

The engine is now ready to lift.  Although most sailboats have suitable tackle to hoist an engine already aboard, an inexpensive 500kg Canadian Tire chainfall turned out to be an excellent investment.  I secured the boom with two halyards, one at the end and one mid-boom and then added a 2x4 post by the companionway to provide additional, but likely unnecessary support.  I hoisted from the boom instead of simply placing a beam across the companionway because I wanted a longer pendulum to swing the engine from its compartment to the galley.

I protected the galley sole with particleboard and, using dimensions from the M35A brochure, I built a temporary cradle to support the engine by the front mounts once removed from the engine compartment.  This cradle needs to be a surprisingly tall to keep the engine from resting on the oil pan. Because the cradle is so tall, the cross brace is essential to prevent the engine from tipping.  (see pic) 

I lifted the engine in two stages.  I first swung the engine out and then lowered in so the bellhousing mounting flanges were resting on the forward engine mounts.  (see pic) I then shifted the chainfall forward along the boom allowing me to swing the engine into the galley.  I likely could have done it in one step if I had a second person working with me but I was on my own.  Removing the bellhousing from the engine and then the transmission from the bellhousing was easy as was removal of the damper plate. I kept track of the hardware in appropriately labeled zip lock bags and, where necessary, made a note on where the different length bolts go.

I took the transmission to Anthony Keats Marine in Wheatley, Ontario who are our local authorized Westerbeke and ZF dealer (among other things).  They have been in business since 1949 and enjoy a good reputation.

[Dave Spencer]

Transmission damage report
After about a week, Anthony Keats Marine called me back with a report on my gearbox.  In addition to the notorious thrust washer being badly worn, the bearing surface on the main gear the thrust washer acts on was worn enough that a new gear was required.  The gear is fabulously expensive (C$855) and even if I was willing to pay for it, there were none in North America and uncertain stock in Europe.  They didn't know when or even if one would be available.  I appreciated their frank assessment and, although they gave me options, we agreed that it wasn't worth fixing.  They returned my failed HBW100 to me and I had some fun taking it apart to see how it worked and how it failed.  The clutch surfaces looked to be in good shape to my untrained eye but the thrust bearing was badly worn.  (see pic)  There was no evidence of the "fairy dusting" of bronze that Mainesail noted in his comments on the Hurth / ZF transmissions on another forum.  https://www.cruisersforum.com/forums/f114/hurth-zf-m15a-transmission-failures-8248-6.html#post530495
I change the ATF every time I change the engine oil... approx every 100 hours. 

[Dave Spencer]

Researching alternate transmissions to replace the original Hurth HBW 100 (ZF 10M)
There are no transmissions that are an effortless drop-in replacement for the Hurth HBW100 except for the limited availability and high priced ZF10M.  All others will require some or all of following changes / considerations:
•   The engine must be raised between 5.5 and 16.75mm to align the output flange with the propeller shaft
•   Consider the extra (or reduced) length of the transmission and its impact on the propeller location with respect to the cutlass bearing and the strut.
•   The shift lever is on the port side vs starboard on ZF / Hurth transmissions.  This will require relocation of the shifter cable.
•   The shift lever works opposite that on ZF / Hurth transmissions.  This means the shift lever on the non-ZF transmissions must be rotated 180 deg so that it is pointing down.  In some cases, part of the port engine mount my need to be ground away to provide adequate clearance to accommodate the shift lever throw.
•   The gear ratio is higher on all alternate transmissions than on the original Hurth HBW100.  This means the engine will have to operate at a higher rpm to turn the propeller at the same rpm as the Hurth or the propeller will need to be re-pitched.

I considered the following transmissions and summarized my findings in a table in the attached .pdf file:
ZF10M (new and rebuild current Hurth HBW 100)
ZF12M
New Age PRM 60
New Age PRM 80
New Age PRM 90
Twin Disc Technodrive TMC 40P
Twin Disc Technodrive TMC 60P

I selected the Twin Disc TMC40P since it was rated to be more than adequate for my 30hp M35AC engine, it was physically smaller than my old Hurth HBW 100 and I would only need to raise the engine by 5.5mm to align with the prop shaft.  It was delivered to me promptly by Marine Parts Supply in Nanaimo, British Columbia.  The shift lever on the transmission is easy to rotate approximately 180 degrees to allow our current shift cable to operate correctly at the binnacle (push ahead for forward gear and pull back for reverse gear.)  A custom bracket will be required to hold the shift cable.  Full credit to "Steve" on a Catalina 30 forum for posting a design that was apparently developed by Catalina to accommodate Twin Disc and PRM transmissions. https://groups.io/g/Catalina30/message/103541 Pdf file attached.  Print it full scale on 8 1/2 x 11 paper and that's it.   You can have a shop make one or build it yourself if you have the skills and the tools.  After mocking it up with cardboard, I built mine from 1/4" aluminium bar 1 1/2" wide and reused the cable clamp from the Hurth cable bracket.

I also replaced the damper plate with a new aftermarket plate from Anthony Keats in Wheatley, Ontario.  The old one wasn't too bad.  The spline was only very lightly worn but two of the six springs were slightly loose potentially causing a rattle at idle.  The new damper plate seemed like a good investment since it would be significant effort to pull the engine again to replace the damper plate should it fail in the near future.  Anthony Keats also provided new exhaust gaskets – Universal p/n 298600.

I had no reason to suspect my OEM engine mounts were bad but they were almost 30 years old and, since the engine was already out, it seemed prudent to replace them with Vetus K-75 mounts recommended as a suitable replacement for M35 engines in other posts on this forum and by the sizing formulae provided by Vetus.

Installation of TMC 40P
Mounting the new Twin Disc TMC 40P transmission to the bellhousing and reinstalling the engine back into its cave was straight forward.  The old gasket was tough to scrape off the exhaust flange.  It took a brass brush on a power drill to clean up the flange.  Although I have an M35AC, after reading Patches write up about his installation on an M25XP, (later changed to a Beta?) https://c34.org/bbs/index.php/topic,11578.msg93563.html#msg93563  I was slightly concerned about the need to cut away a small portion of engine bed rail to allow clearance for the throw of the inverted transmission shift lever.  My early measurements indicated there would be no problem but I brought tools along to cut the mounting rail if necessary. It turned out that there is lots of room for the inverted shift lever on my boat as shown in the pictures. 
There's still lots of work left to do to recommission the engine before spring launch but I'm not concerned about it. Restoring the wiring, reinstalling the heat exchanger and renewing coolant hoses including those to the water heater should be fairly simple (famous last words!) Left / right alignment of the engine / transmission with the propeller shaft will likely be the trickiest bit but it is certainly doable, especially with two people working on it. 

[Dave Spencer]

Summary / What I Would do Differently
The design of the Twin Disc (and PRM) transmission with a cone clutch appears to be simpler and more robust than the Hurth / ZF transmission with a clutch plate package.  Here are a couple of good videos by D. H. Crouch Marine in the UK showing the inner working of the Hurth / ZF and a New Age PRM transmission:
Hurth: https://www.youtube.com/watch?v=tooPyV-cNAc&t=510s
PRM (similar design as Twin Disc). https://www.youtube.com/watch?v=Mcrrdq6W_iU

I have no regrets about any aspect of this job.  Since I was working on this alone, I worked methodically and carefully which gave me the opportunity to weigh each step and consider if there was a better way to proceed.  It was very satisfying to be able to complete this job on my own.   I found only a few incomplete online resources for this job scattered across various forums. Hopefully, this writeup can serve as a resource for anyone facing a similar job.

[Jim Hardesty]

Dave,
Thank you for the excellent write-up. 
Transmission failure is a worry.  From what I understand there's little or no signs of upcoming failure and no practical way to check the condition of the transmission. 
Jim

[Dave Spencer]

I agree Jim. I took good care of it and changed the oil regularly. The oil was always red and not discoloured when I changed it. My wife and I both listen carefully for any changes in sound that may be a warning. About 4 engine hours before the failure, I detected a new slight rattle at high torque (higher than my normal cruising speed). The transmission oil still looked good after I heard this new rattle. I wasn't even sure it was the transmission but likely it was. I'm glad it happened near the end of the season.

[Ron Hill]

Dave : To add to your comments about changing the transmission oil.  I've always advocated that in the fall layup that owners fill their transmissions to the top to prevent condensation and to keep everything inside the case WET with oil.  Then in the spring - suck out all of the oil and refill to the correct level.

A few thoughts