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I'm setting up the synchro float in my NP420, and after reading the manual (TM 9-2520-232-35), I believe that there's an error, or at least a mis-leading instruction regarding the float measurement.

Par. 43b of the manual reads: "With two thickness gauges placed diagonally opposite each other, measure the space or clearance between the rear stop ring and the third speed gear. A clearance of 0.050- to 0.070-inch is necessary to give the synchronizer the proper amount of float."

Par. 43c then adds: "If the clearance between the rear stop ring and third speed gear is insufficient or excessive, remove the main drive gear and the main shaft ball bearing retainer and gaskets and drive the main shaft back far enough to remove the synchronizer parts. Replace the third speed gear shim with one of the required thicknesses. Shims are availble in thicknesses of 0.010- and 0.015-inch."





The problem is in measuring between the synchronizer rear stop ring and the third speed gear. Doing so ignores the thickness of the third speed gear shim, and changing the shim will have no effect on the measured clearance between the stop ring and third speed gear.

I'm assuming that the proper points of measurement for setting the synchronizer float are the back side of the rear synchronizer stop ring (as the manual states), and the front face of the third speed gear shim, not the third speed gear itself. Using the shim as the measuring point allows changing the shim to the other available thickness to actually change the synchro float.

And something else that the manual doesn't address: if the float measurement is excessive, and can't be corrected by changing to the thicker shim, then likely the aluminum stop rings in the synchronizer are worn out, and should be replaced.

I have no info about the measurements. I was wondering if gear cases are regularly painted? Yours looks nice, but does it serve a purpose? It seems like every square inch would be coated in gear oil, so rust should not be an issue.

Of the three NP420s I've disassembled, all have shown evidence of being painted on the inside, so I assume that New Process did it as part of the manufacturing.

The Glyptal paint on the inside is not to prevent rust, which as you note would be unlikely in any gearbox that is used semi regularly. And even for one that sits for long periods of time, rust is more of a concern on the machined surfaces of the gears and bearings than on the inside of the case. Rather, the paint is to prevent oil weeping through any porosities in the case casting. I hate leaks. Especially ones that are frustrating to diagnose.

When I set up the synchro float in my NP420 I followed the same directions. I took the tail off to add shims needed to match the measurement and when put back together the whole thing was way too tight.

I ended up re-doing the shims two more times until I got it where I thought it was tight enough and not binding. The manual to me was not very clear on this point. I knew binding was not a good thing though.

sturmtyger380 wrote:When I set up the synchro float in my NP420 I followed the same directions. I took the tail off to add shims needed to match the measurement and when put back together the whole thing was way too tight.

I ended up re-doing the shims two more times until I got it where I thought it was tight enough and not binding. The manual to me was not very clear on this point. I knew binding was not a good thing though.

Did you have the early 88950 transmission?

as stated the paint is to prevent casting leaks and it speeds up the flow of gear oil back to the bottom of the case and prevents metal particles from sticking to the sides as well you want the particles at the bottom asap.

Kaegi wrote:as stated the paint is to prevent casting leaks and it speeds up the flow of gear oil back to the bottom of the case and prevents metal particles from sticking to the sides as well you want the particles at the bottom asap.

Yep. Combined with a magnetic drain plug (http://www.mcmaster.com/#1072a14/=145zwhh) instead of the stock plug, it helps keep the ferrous debris from recirculating into the gears, bearings, and synchro rings.

Yes early transmission.

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