[Prowbar]

Installed the "new" SM420 today. Before installing I cleaned her up and painted her black. Also replaced the rear (yoke) oil seal, as the spring wore through the rubber.

When I pulled the shifter from the old tranny, it turned out that it had the later SM465 shifter! I know these interchange but the 465 is a 2 piece shift fork design to keep the vibrations to a minimum. Turns out the roll pin that secures the fork to the shifter was so loose it created the sloppy shifting.

Anyway, when removing the old tranny the bellhousing and yoke bolts were snug but not tight...

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When turning the transmission by hand you can hear the noisy ball bearings.

Anyway, out with the old and in with the new. I used the yoke and speedometer connection from the old tranny as the one on the 'new' tranny were for 4.56 or even lower. Also reused the old 'donut'. See photos. Cleaned the throwout bearing and greased her up, when through the whole clutch mechanism to properly clean and grease the moving points.

The new tranny shifts a whole lot better! The 2nd gear synchro works like it should and the tranny is tight.

However, when shifting from 3rd to 4th and the RPMs drop to 800 or so and you accelerate there is a strange noise... Above a certain rpm (about 1400 I guess) it is gone, no matter the load applied to the transmission.. the old tranny had the same thing, so it probably is not the tranny but I'm not sure what it is. It sounds like it comes from the tranny, not the rear axle or something like that. I'll try to record a video capturing the noise.

Have been cruising with the GMC some lately. According to my calculations, I'm getting 13.4 MPG. Was expecting worse so not bad. Been gathering parts for the propane conversion as that will be next, together with the IPE bedwood.

Would like to upgrade the exhaust though, at least switch the muffler. Right now it has the factory muffler which is quiet, a little too quiet. Hoping to find a muffler that is a little louder but lets you hear the V6 rumbling. Right now I can barely hear the engine in the cab. I still have the factory single exhaust and muffler. Any suggestions please let me know

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