Velvet Drive Glossary: Parts and Terms Explained

A to Z

AS number

An older style of model number, used before Velvet Drive settled on the 10-17/10-18 numbering. AS1-71C, for example, is the same transmission as 10-17-000-001. See reading a Velvet Drive ID tag.

Assembly number

The part number stamped on the tag that identifies the exact transmission: 10-17 for a 71C in-line, 10-18 for a 72C in-line, 10-04/10-05 for the V-drive versions (some older 10-04 numbers are 70C in-line units), 10-06 for a 73C, 10-13/10-14 for a CR-2. See reading a Velvet Drive ID tag.

Bushing

The bearing and sealing method used in transmissions built before September 1978, replaced afterward by sealing rings. A bushing-style case and planetary won't hold a solid positive neutral, no matter how good the rebuild. See Velvet Drive history.

C and CR

C marks a unit built at the factory for a clockwise-turning input; CR marks one built for counterclockwise, viewed from behind the transmission looking forward. It's stamped into the assembly number as an odd or even digit. See reading a Velvet Drive ID tag.

Clutch pack

The stack of steel and friction plates, squeezed together by hydraulic pressure, that actually engages forward or reverse drive. A 71C forward pack uses 4 steel and 5 friction plates; a 72C uses 6 and 7. See paper vs. bronze clutches.

Coupling

The flanged fitting that bolts the transmission's output shaft to the propeller shaft. Getting the two faces aligned within the factory tolerance is one of the most important parts of any install. See don't paint the coupling.

CR-2

A counter-rotating, drop-center gear design that lets twin-screw boats run two same-rotation engines while still turning the props opposite ways. 10-13 is built on 71-series internals, 10-14 on 72-series. See the CR-2 series page.

Damper plate

A flexible plate, usually with rubber blocks or springs, that sits between the engine's flywheel and the transmission input shaft and absorbs torsional vibration. A knock at idle that fades out by around 1,000 rpm is usually the damper or the engine, not the transmission. See rotation and driveline notes.

Direct drive

A 1:1 transmission with no reduction gearing: the output shaft turns at the same speed as the input. See identifying your transmission and ratio.

Dry weight

The transmission's weight with no fluid in it, the number the factory publishes on its spec sheets. See Velvet Drive specs.

Duty class

The factory's rating for how hard a transmission is used: Pleasure (full throttle about 5% of the time), Light Duty Commercial and Heavy Duty Commercial, each with its own hours-per-year and throttle-percentage limits. See how much horsepower a 71C or 72C can handle.

Neutral safety switch

A switch that keeps the engine from starting unless the shift lever is sitting in the neutral detent. See maintenance by model.

Planetary

A gear set with a sun gear and a carrier. Every 71C/72C has a reverse planetary, direct drives included, and a reduction unit adds a second planetary that sets the ratio. See the collapsible spacer.

Pressure plate

The plate in a clutch pack that takes the push from the hydraulic piston and clamps the plates together. The reverse pressure plate on a 71C/72C uses 11 springs, part number 71-97, which Chris calls the reverse piston return springs. See paper vs. bronze clutches.

Reduction

Any ratio greater than 1:1, where the output shaft turns slower than the input. Common 71C/72C reductions run 1.52, 1.88, 1.91, 2.10, 2.57 and 2.91:1. See identifying your transmission and ratio.

Regulator valve

The valve that sets system oil pressure and routes extra pump flow to the cooler. A jammed or sticking regulator shows up as low or no pressure in both forward and reverse. See low transmission pressure.

Sealing ring

The rotating seal design used from about September 1978 on, replacing the older bushings. A sealing-ring case and planetary hold a proper positive neutral. See Velvet Drive history.

Selector valve

The spool valve that routes pressurized oil to the forward or reverse clutch depending on where the shift lever sits. Grease packed into this area is a documented cause of pressure problems. See neutral creep and the selector valve.

Sump

The reservoir at the bottom of the case that holds the transmission's oil supply. It's where debris ends up: on one 71C with shattered reverse plates, Chris found a thick "strawberry milkshake" goo in the bottom of the sump, which makes the pump work harder and the transmission run hot. See Velvet Drive transmission fluid.

V-drive

A transmission that folds the drive back on itself through a gear or chain set, so the engine can be mounted backward in the stern with the prop shaft running under it. Velvet Drive's cast iron V-drives are the 1004 (71-series) and 1005 (72-series). See the 1004/1005 V-drive series page.

Related reading: Velvet Drive manuals, reading an ID tag, and Velvet Drive history.