Replacement 4th-5th gear synchronizer assembly, part 312766X, for Spicer TTC CM60-series manual truck transmissions without overdrive.
A medium-duty truck with a Spicer CM60-series five-speed that grinds going into 4th or 5th, fights the lever on the top two gears, or kicks out of 5th on a pull is usually wearing out the 4-5 synchronizer. This is the complete 4th-5th synchronizer assembly, part number 312766X, for Spicer (now Tremec TTC) CM60-series transmissions in the direct-drive version, without overdrive. It is the full assembly with the center hub and blocker ring, not a loose ring, so you are not rebuilding a worn hub around new brass.
The CM60 family are medium-duty constant-mesh manuals built for vocational, utility and municipal trucks. The 4-5 synchronizer sits on the mainshaft between 4th and 5th gear. When the driver moves the lever, the shift fork pushes the sliding sleeve, the keys push the blocker ring onto the gear cone, and friction pulls the gear to mainshaft speed. Once speeds match, the ring lets the sleeve pass and lock the gear. In a direct-drive box the top gear locks the input straight through to the output, so the 4-5 synchronizer carries the shifts the truck makes most often on the road. Big speed differences on upshifts under load, a clutch that is out of adjustment, or the wrong oil wear the cones and round off the blocker teeth.
Check the clutch first. A clutch that is worn or out of adjustment drags and makes shifts notchy, and running it that way long enough is exactly what wears synchronizers. On Spicer self-adjusting and manual-adjust clutches, the OTC 7028 Spicer clutch adjusting wrench makes the adjustment check simple. If every gear grinds, it is the clutch. If only 4th and 5th grind or jump out, it is the 4-5 synchronizer or the mating gear teeth. Our synchro wear guide and popping out of gear guide cover how to tell rings, teeth and forks apart.
This assembly is listed for Spicer/Tremec TTC CM60-series transmissions without overdrive, 4th-5th position, replacing 312766X. Overdrive CM60 versions use a different synchronizer, and Spicer used several CM-series models and suffixes across Ford, GM and other medium-duty chassis over the years, so the model letter on the tag matters. Before ordering, read the model number off the transmission tag, confirm it is a direct-drive unit, and match 312766X to the part number on your old synchronizer or in the parts book for your serial number. If the tag is gone, send us the truck year, VIN and any numbers on the case and we will check it. Other heavy-duty internals are in hard parts.
For the tools a heavy truck transmission job needs, see the heavy-duty transmission tools guide, and for budgeting the repair, our manual vs automatic repair cost breakdown.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
No. This 312766X assembly is listed for direct-drive CM60-series transmissions without overdrive. Overdrive versions use a different synchronizer. Check the model on the transmission tag before ordering.
No. It is the complete 4th-5th synchronizer assembly with the center hub and brass blocker ring, which is usually faster and more reliable than rebuilding a worn hub with loose parts.
Probably not. Grinding in every gear points at a clutch that is worn or out of adjustment and not releasing. Adjust or repair the clutch first. This synchronizer fixes grinding or jump-out that is limited to 4th and 5th.
Only if they are worn. Inspect the clutch teeth and cones on both gears. If the teeth are chipped or rounded or the cone is scored, replace the gear, or the new synchronizer will wear out early and the truck may keep jumping out of gear.
Use the lubricant specified on your transmission tag or in the service manual for your model and serial number. Spicer medium-duty units were specified differently by era, so do not guess from another truck.
