ZF63 Series
The 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.
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.
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.
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.
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.
