Hi, I thought I would write my experience replacing the old HBW-100 transmission on my 2002 C36 Mk with Universal M35B, 840 hours total. I didn't find anything in the forum, maybe this helps someone else. A bit of background: I own my C36 for over 6 years, the first year I rebuild the old transmission. I had some slipping and dark atf fluid. After 4 years of use (less than 250 hours later) dark fluid returned, slipping in fwd returned. I have changes ATF fluid annually since I own the boat.

Throughout the past 5 years I checked the throttle throw and it was allways within the recommended distance. I have a three blade autoprop.

As shown in this video, I had a lot of "can of marbles" noise mostly at iddle, and lots of vibration. Whe I rebuild the transmission 5 years ago, I replaced the damper plate and the motor mounts.

https://youtu.be/IQlc8v4STcs

Similar video was published by another member here. We both seem to think that the diameter of the spline is too small, at least for the damper in use. Also the parts inside the old transmission make a lot of shacking noise.

New Transmission:
Rather than rebuilding again, I decided to go with a new transmission. I tried my best to go with a twin-disc one, and the 60 series was recommended (see posts here and in C34 forum by Mainsail regarding ZF transmission and the use of a thin layer of brass in inside parts, a weak link in these ZF transmissions apparently). But the twin-disc transmission recommended for the 35 HP engine is almost 2 inches longer than the ZF, and because of my boat configuration with engine, prop shaft, pyi shaft seal etc, I had no real way of accomodate for this additional length.

So I went with the ZF15M, NOT the 12M nor the hard to find and super expensive 10M. The 15M is almost identical in reduction rate as the HBW-100 and it is 12mm longer, and 10mm taller and rated for well abouve my 35HP. This made the job a lot easier with no changes to any other parts. The length of the prop shaft hanging between the strut and propeller is now 1.14 inches. I know this is a bit longer than the recommended 1 inch for the same size shaft diameter, so I will fix this next winter by shortening the shaft.

Needed modifications when replacing with ZF15M

-Engine needs to be raised 10mm, plenty of room in my motor mounts to do this.
-Shaft needs to be pushed aft 12mm
-Bolts use to attach transmission to bell house need to be shorter. Original ones are 25mm, new ones needed are 20mm. This is because the transmission blocks you from accommodating the longer bolt.
-minor adjustments to the gear cable bracket are needed for straight throw. Use upper holes to attach the bracket instead of lower ones.

These are the most significant modifications.

Results (ONLY 3 hours of use so far):

First, as you put the ZF-15M in, you will notice that there is no more play between the spline and the damper plate, it fits like a glove. I use a caliper to measure spline in HBW100 vs ZF15M and the difference is .13 inches smaller in the old spline!! Why are this pyi damper plate used with the hbw100?
Noise: gone, no more marble can, and overall noise is significantly reduced.
Vibration at iddle; much lower, I don't know if this is related to transmission, though before replacement, my engine was very well alligned so vibration at iddle wasnt caused by misalignment.
Gear throw: Engaging in fwd or back is much clearer now, you can feel the switch in the lever.

I dont know if these improvements will last, nor if the new ZF transmission will last, but first impressions are very good. I will update this post if anything changes. Hope this is helpful if anyone is replacing their old HBW-00 transmission.

Abe