ZF63 Series
By Chris, Hale Marine · 40+ years rebuilding Velvet Drive and ZF transmissionsThe ZF63 is the same family as the Hurth HSW630, and you'll also find them labeled as MerCruiser. Chris likes working on them. Compared to a Velvet Drive 5000, he says they're night and day: much easier to work on and less time-consuming. We've had them in 1.5:1, 1.55 (1.6):1, 1.6:1, 2.0:1, 2.1:1 and 2.5:1, in both the A (in-line) and IV (V-drive) versions.
Chris thinks ZF increased the pressure on the 630s over time, and now they're the ZF68. Basically, they really haven't changed the inside of them.
Rotation vocabulary
ZF's own paperwork rates the ZF63 for a "right hand turning" engine. That's the same physical engine Velvet Drive calls a left-hand engine: one that turns counterclockwise looking at the flywheel. Same engine, opposite naming convention between the two brands. If you're cross-shopping a ZF63 against a Velvet Drive and the rotation words don't seem to line up, that's why.
Filter and O-ring
Replace the filter and O-ring every couple of years. The O-rings get flat on the sides, and then they can suck air no matter how tight you tighten the cover down. Air gets in, the pump neutrals out, and you're dead in the water.
There's a second leak path a lot of people miss: the coupling-spline O-ring. It hardens and gets stiff with age, and fluid finds its way past it. Chris calls it one of the few things he doesn't like about German engineering.
The ZF63A has no drain plug. Don't try to use the dipstick hole. Pull the filter and suck the fluid out from there, which takes you right to the very bottom of the sump. ZF put a drain plug on the V-drive version, the 63IV. When you change the filter, make sure the cover body sits flush with the housing. If it isn't flush, most times it'll suck air and neutral the transmission out, and it'll just stop after it heats up. Buy the newer-style filter with the shorter bowl, since the case needs it for clearance.
Disassembly
To take a ZF63A apart you'll need a 17mm wobble socket, a 3-jaw puller and a clamshell puller. Chris uses the bolt that holds the spacer, which holds the whole shaft and coupling assembly together, with his puller to pull it right off. When you use the clamshell puller, grab the gear, not the bearing. If you grab the bearing you'll destroy it.
One piece of advice from Chris on any of these: never buy the first production run of anything. The earliest ZF63/HSW housings didn't have service openings built in, which makes the snap rings inside a lot harder to reach than on a later housing.
What we find inside
On the ZF 63s and 80s, pretty much that whole family, look inside the clutch drum. The clutch plates will cut grooves in the drum itself, and grease marks show you where that's headed. The input shaft is a different size for every ratio, but the sealing-ring side and pump side are always the same. On one ZF63IV where the owner overheated the clutches, Chris wondered whether it was an impact or a high-speed shift.
Pressure
Leaks
On MerCruisers everything's black, so a leak is hard to see. Touch up the housing with a little gray primer, or sprinkle baby powder around the unit, and the leak will leave a trail. Smearing silicone on the outside never works. Don't waste your time on it.
Related problem articles
Common questions
What does ZF mean by a right hand turning engine?
The same engine Velvet Drive calls a left-hand engine: one that turns counterclockwise looking at the flywheel. The two brands use opposite words for the same rotation, so check which convention a spec sheet is using before you order a transmission.
Is low neutral pressure normal on a ZF63?
No. A good ZF63A reads about 350 psi in neutral on our stand, the same as in gear. That's the opposite of a Velvet Drive 5000, where a low neutral reading is normal by design.
Sources
Public documents we used for this page. Links open other websites in a new tab.
- ZF 63 Technical Data Sheet (opens in a new tab) PDF, drinkwaard.com