ZF S6-650 Input Shaft 23-Tooth 10-3/8 in Ford 6-Speed

Owners of 1999-2003 Ford F-250, F-350 and F-450 Super Duty trucks with the 7.3L Power Stroke or 6.8L V10 and the ZF S6-650 six-speed usually find out about the input shaft when the clutch is out: stepped clutch splines, a grooved seal surface, or a blued synchro cone. This USA Standard Gear shaft is a new replacement with a 23-tooth drive gear and a 10-3/8 inch overall length, the ZF6 shaft used behind those engines.

What the input shaft does in a ZF S6-650

The input shaft is the only path from the clutch into the gearbox. The clutch disc slides on its front splines, its nose rides in the pilot bearing in the crank, and the front bearing and retainer support it in the case. At the back, its 23-tooth drive gear meshes with the countershaft to feed every gear except direct drive. In the 1.00:1 direct gear, the synchro locks the input shaft straight to the mainshaft through the cone on the back of this shaft, with the 0.72:1 overdrive above it. That makes the shaft a wear item at both ends: clutch damage up front, bearing, gear and cone wear at the back.

Symptoms of a worn input shaft

• Clutch chatter or a disc that hangs up even after a new clutch, caused by a stepped spline
• Gear whine in the indirect gears that quiets in direct drive
• Grinding or pop-out going into the direct-drive gear from a damaged cone
• Transmission fluid in the bellhousing from a front seal riding on a grooved journal
• Noise in neutral with the clutch out that stops when the pedal goes down, often the input bearing
• Galled pilot nose or pocket bearing bore after a dry pilot bearing

How to confirm before buying

Rule out the outside causes first. A worn flywheel, a clutch that does not fully release, or weak hydraulics cause most ZF6 grinding complaints, so check pedal travel and the master and slave before blaming the shaft. With the transmission out, slide a new disc onto the splines and feel for rock, run a fingernail across the seal surface and the spline faces, and inspect the pilot nose, gear teeth and cone. Our ZF S6-650 input shaft replacement guide walks through the bench inspection, and the ZF S6-650 problems and cost guide covers the other failures.

Fitment

This shaft is 23 tooth and 10-3/8 inches long. The same spec is listed as interchanging with Ford F81Z-7017DA and ZF 1319 202 026 for ZF6 units whose tag begins 1319 050 and ends 004, 005, 006, 022, 023, 024, 025, 036, 037, 045, 046 or 049, behind the 1999-2003 7.3L diesel and V10 in F-250 through F-550 trucks. The 2003-2007 6.0L trucks use the S6-750 with a different shaft, and the GM version of the ZF6 uses its own shaft, so neither is covered here. Read the ZF tag on the case, count the teeth and measure your old shaft before ordering, or send us the year and VIN. Parts for the rest of the job are in transmission hard parts.

Install notes

• Press the front bearing off the old shaft and install a new bearing; do not reuse the old one on a new shaft.
• Replace the pocket bearing in the shaft, the direct-drive synchro ring that runs on the new cone, and the front retainer seal.
• Check input shaft end play with the shims at reassembly before the case is closed.
• Install a new clutch disc and pilot bearing; a disc that ran on a worn spline has a worn hub. An OTC 1172 pilot bearing puller gets the old one out of the crank.
• While the transmission is out, refresh the clutch hydraulics with the 7.3L Super Duty clutch master and slave cylinder and tighten up the shifter with the ZF6 short throw shifter kit.
• Ford specified Motorcraft MERCON V ATF in the ZF6 for these years, not gear oil. Fill to the bottom of the fill hole with the truck level and check your owner guide for the exact spec.

Is 23 the spline count?

No. The 23 is the tooth count of the drive gear on the back of the shaft. Count the gear teeth on your old shaft and measure its length to confirm the match.

Does it fit a 6.0L Power Stroke?

No. The 2003-2007 6.0L trucks use the ZF S6-750, which has a different input shaft. This one is for the S6-650 behind the 7.3L diesel and V10.

Will it fit a Chevy or GMC with the ZF six-speed?

No. GM trucks used a different S6-650 input shaft. This shaft is the Ford version.

What should I replace with the input shaft?

The input bearing, pocket bearing, front seal, direct-drive synchro ring, clutch disc and pilot bearing. They are inexpensive next to the labor of pulling the transmission again.

What fluid does the ZF6 take?

Ford specified Motorcraft MERCON V automatic transmission fluid for 1999-2007 ZF6 trucks. Gear oil makes cold shifts stiff and slows the synchros.

Can I replace the shaft without splitting the case?

No. The transmission has to come out and be disassembled to remove the input shaft and set end play.

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