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Tech Tip #78: Hurth Marine Transmissions: The Ins and Outs of Basic Service

Helpful tips on performing basic maintenance on your Hurth marine transmission

In this Tech Tip, one of an ongoing series that we publish for both our customers as well as the international marine audience, we present a few basic thoughts on how to maintain your Hurth marine mechanical gear. These small reduction gears are commonly found mounted on auxiliary engines. As Hurth mechanical transmissions, they have a model number prefix of HBW. The larger Hurth hydraulic marine gears have a HSW prefix; we will discuss these units in a later Foley Engine Tech Tip.

The information presented below is drawn from both our 88 years in the marine power train business as well as the Hurth Workshop Manual (call us toll free at 800-233-6539 if you would like us to send you a manual). Please call or email us with any thoughts or comments.

  1. Fluid Change Cycle. People frequently ask us how often they should change their fluid. Fluid in Hurth marine gears should be changed at the start of each season and at 250 hour intervals. Change it more often, perhaps every 100 hours, if you are using the marine transmission in a severe duty cycle such as launch duty or the placement of moorings or towing. Change it immediately if you have tangled the prop in lobster pot wrap, overheated the gear, or if the fluid is discolored.
  2. Which Fluid? The ZF/Hurth Workshop Manual indicates that automatic transmission fluid or ATF is to be used in these mechanical gears. ATF, commonly known as Dextron II or III, is far preferable to the thicker SAE 90 hydraulic gear oil. However, given that these small gears are essentially oil cooled and that they contain very little fluid, we strongly recommend that you upgrade to our Foley Hytork Fluid. Cool transmissions are happy transmissions because heat is the enemy of all marine gears.
  3. How Much Fluid? Generally speaking, the smallest Hurth gears use about a half a quart of fluid. See the chart below for exact capacities.
Model Capacity in Quarts
HBW 40 4M .32
HBW 50 (5M), HBW 100 (10M) (nee 5) .44
HBW 125 (12M), HBW 150 (15M) (nee 10) .63
HBW 150A (15MA) .59
HBW 150V (15MIV) 1.06
HBW HBW 250 (25M) (nee 20) .79
HBW 360 1.50
HBW 360A 1.60
HBW 450 1.90
HBW 630 2.1

Smelly Fluid? Why does my fluid smell and look discolored? You may have burned the fiber clutches because of heating caused by severe duty, under-speccing of the gear, or getting tangled in a line. You should immediately change the fluid and carefully monitor the performance of the gear.

Transmission Coolers. Despite your best intentions for a variety of reasons some transmissions will run hot. In addition to the problems caused by severe use and getting tangled in a line, some gears are simply under-spec’d.  The Perkins 4108 is rated at 50 HP at 4000 RPM yet many boat builders equip them with the HBW 100 which is not rated for this. One could install a more appropriate transmission such as the HBW 150 with 37% more fluid (0.63 quarts versus 0.44 quarts), and shorten the shaft to accommodate the longer HBW 150. Or, recognizing that heat is the enemy of gears you can increase the torque capacity of the gear that you now have. The first step is to upgrade to a better transmission fluid which we discussed above. The next step is to install a transmission cooler. Hurth mechanical gears don’t have a provision for the long oil coolers composed of a “tube with a bundle inside” as seen on Borg Warner and Hurth hydraulic gears. To mount a cooler on a Hurth mechanical gear, Foley Engines can supply a small aluminum box that installs on four pre-drilled holes on the outside of the transmission’s case. Raw water is run to and through the small box (about the size of three stacked decks of playing cards) and it cools the fluid inside the transmission. This Foley Cooler Kit works so well that it effectively changes the torque capacity of the Hurth HBW 100 to that of the HBW 150. It is a very cost effective way to upgrade your drive train.

Damper Plates. All Hurth marine transmissions have damper plates. They are installed on the engine’s flywheel in front of the gear. They resemble a small, spring loaded clutch that you might see on a Ford 8N farm tractor.

It is visible when you remove the transmission. To function well all the damper plate springs should be in place, tight and not loose or rattling. We strongly recommend that you change the damper plate whenever you have the transmission removed for service for several reasons. The “Always Change the Damper Plate” rule is because the plate is normally inaccessible, probably is rusted and frozen on the flywheel, is a small dollar item, and has seen as much service as the transmission which you are now servicing. Use Nevr-Sieze when you install the new damper plate so you can easily service the small metric fasteners in the future.

Shifting Problems? First check and adjust your linkage. Linkage problems are very common on Hurth marine gears when mounted to auxiliary engines. If the problem is still there, see if there is a difference in the difficulty in engaging forward versus reverse. If it is significantly more difficult to engage the marine gear in forward as opposed to reverse, this indicates that the thrust washers are worn. The gear will require service or replacement.

Hurth Transmission Filters. Small mechanical Hurth gear boxes do not have filters. One could drill and tap the case to install a remote mount filter kit (costing probably $100 or so). The transmission would last significantly longer and definitely run cooler because you would have easily tripled the oil capacity with the addition of lines and a filter holding a quart of oil. We have this idea in queue but have yet to field test it.