The Syncro drive shaft transfers power rear to front with a one piece unit with two U joints, a rubber coupler and a sliding shaft and bushing set. The drive shaft rarely goes bad, but when it does, one hears a rumbling noise and may feel the vibration. Know that it is normal to hear a rhythmic clanking noise from the Syncro drive shaft if you try to lug the engine down to low rpms in the lower gears. The drive shaft is easily removed, although getting to the nuts may prove dicey. We use a special wrench made by Snapon that is a cross between an open end and a box end. It grabs one corner, preventing the nut from stripping, as the required torque is high. Notice that the stock bolts are hardened, and have a 8 x 1 thread pitch, all other 8 mm bolts on the car are 8 x 1.25. When removing and reinstalling a drive shaft, either the front differential or the engine/transmission assembly needs to be loosened to allow the drive shaft to slide out freely. Prying on the drive shaft to get it out will likely ruin it. See your Bentley manual. Then read it.
Front Axles: A Syncro Thing
It is worth a paragraph to cover the Syncro front axles, just to mention a few differences from the rear axles that they share with the 2WD vans. Unknown to most, the front inner CV (constant velocity) joints have a spacer between the joints and the drive flanges. Can’t be seen, so easy to miss. Shops lose these spacers, not customers. Since the longer bolts required do not come in the modern 12 point triple square variety, first we check to see if shorter bolts have been installed, as they may all break at once if the coupler engages suddenly. If the bolts look wrong, we measure the distance from the inner boot to the flange, as the difference between right and wrong is just 5mm. We also double check if it looks like someone has removed the axles previously, and most have been removed before, and about half of those are missing one or both shims. And that side axle may have both CV joints worn out because of that. The inner CV joints are the same as 2WD CV’s, the outer front CV joints for Syncros cost over 3 times as much.
Quick Clue: All CV bolts must be torqued to their exact specification, especially on a Syncro drive shaft and axles. We see several sets of CV bolts a year coming loose, or with broken bolts. Many of those vehicles get towed in. Every Technician at Karmakanix has several torque wrenches. All our Technicians know when to use them.