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Since I'm assembling the main shaft on an NP420 that I'm rebuilding, I dug the "NOS" synchro assembly out of my spare parts stash, and went to install it. It fits on the synchronizer clutch gear just fine, but it's definitely different from the three synchronizer assemblies that I've removed from other NP420 transmissions, which are identical to what's in the TM9-2320-212-34P parts catalog.
Here's what the synchronizer assembly looks like from the parts catalog:
(2) are the steel synchronizer outer stop rings, which rub against the appropriate aluminum stop ring (3) when shifting into 3rd or direct gears in order to synchronize the speeds of the two gears before engagement;
(3) are the aluminum synchronizer stop rings, which rub against their matching steel outer stop ring (2), and have three permanent pins that engage through holes in the sliding clutch (4);
(4) is the synchronizer sliding clutch, which is engaged by the 3rd and direct drive (i.e. 4th gear) shifter fork (the one that is supposed to have the little nylon inserts so that the steel fork isn't banging against the sliding clutch) around the outer circumference. When the sliding clutch is pushed forward or rearward by the shifter fork, it presses the appropriate aluminum stop ring against its matching steel stop ring, bringing the two shaft speeds close enough to engage without gear clash, i.e. synchronizing;
(5) is the snap ring that holds (6) on to the mainshaft; and
(6) is the synchronizer clutch gear, which fits over the splined front end of the mainshaft. The gear teeth on the inside bore of the sliding gear (4) slide forward and backwards on the teeth of this clutch gear, allowing the sliding gear (4) to move along the axis of the mainshaft, while always turning at the same speed as the mainshaft.
As can be seen from the diagram, the synchronizer assembly is not held together in any way except when assembled onto the synchronizer clutch gear on the mainshaft. When removed, the individual parts of the synchronizer just fall apart, like this:
The new synchronizer assembly, however, is not constructed the same way. While the basic design is similar (a center sliding clutch that engages the shift fork, two aluminum stop rings, and two steel outer stop rings), the aluminum stop rings and the center sliding clutch are permanently assembled, and cannot be taken apart without destroying the parts. Instead of the aluminum stop rings each having three pins that engage through alternating holes in the sliding clutch, there are three pins that permanently attach the two aluminum stop rings to each other, allowing the sliding clutch to float between them. There are also three floating pins between the aluminum stop rings that engage through the other three holes in the sliding clutch, and are held onto the aluminum stop rings by a large wire circlip on the inside of each aluminum stop ring. Unfortunately, I couldn't get a photo of the floating pins or wire circlips without disassembling the synchro, which would have destroyed it.
Here's the farthest that the "NOS" synchro assembly can be disassembled:
And here's a comparison of the two synchro assemblies. The one on the left is the original style, with one of the outer steel stop rings and its matching aluminum stop ring removed, showing the sliding clutch. The one on the right is the "NOS" assembly, without the outer steel stop rings; the peened ends of the three pins that permanently hold the aluminum stop rings to each other can be seen.
Dimensionally, the two assemblies are almost identical. Functionally, however, they work a bit differently. In the original style, the sliding clutch floats on the pins between the two aluminum stop rings, and is held in the neutral position only by the shifter fork. In the "NOS" assembly, the three floating pins (held by the two wire circlips) have indentations in the middle of their lengths, which keep the sliding clutch centered between the two aluminum stop rings, almost as if there's a magnet. While almost no pressure is needed to move the sliding clutch off center on the original, a fair amount of force is required to push the sliding clutch off-center on the "NOS" design.
So here are the questions:
1. Is the "NOS" synchronizer clutch above really for an NP420? Or is it for something else, such as an NP435?
2. Since it appears to be dimensionally the same as the synchronizer assemblies in the NP420 transmissions that I have on hand, can it be used in an NP420, and if so, is it an improvement over the original design? Is that centering action on the "NOS" sliding clutch plate something that the engineers at New Process came up with to fix a problem with the free-floating sliding clutch in the original NP420? Or will it make it harder to shift into 3rd and direct drive, accelerating wear on the shift tower pins?
