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Originally Posted by Prowbar

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Been a month, not without its fair share of GMC action...

The bearing noise in the old SM420 started to get worse, figured it was time to do a transmission rebuild. Ordered a rebuild kit and new synchro's that arrived very quickly. Removed the trans and started the rebuild process. Used the solvent tank to clean all the parts, you'd be surprised how much old oil and sludge are present in every nook and cranny.

Disassembled everything, cleaned all parts, and assembled them with regular engine assembly lube, which easily dissolved in the gear oil.

Quite a fun transmission to rebuild. The rear ball bearing of the secondary shaft was pretty stubborn to get off, so I removed the outer ring and cage and welded up the inner race. Using a bearing puller it finally sprung loose.

All the gears looked excellent, but the bearings had a lot of play but did not look as bad as I thought they did. They were still the original New Departure bearings.

Ultimately there is no need to get all the fancy tools as described in the manual, if you use your imagination you can rebuild the trans with a shop press, some snapring pliers, and a set of bearing pullers.

The new transmission sounds and shifts great. Very satisfied with the results.

As for the HEI ignition, I had 2 modules fail in a short time. First the original developed a misfire the night before. The next day it would not start. Replaced with an OEM GM module, it lasted about 2 weeks, then quit on the same note. Now on the 3rd module. Installed it today and it runs well, hope it lasts a while this time. Checked all the wiring for shorts of problems, but couldn't find any. Getting a proper 12V signal to the BAT. Tach port is fine too. The coil and magnetic pickup all check out fine regarding resistance. The only thing that I can think of is the distributor ground, which, using the original hold down, could see why it would cause trouble. I modified the hold-down with some brass pins to improve the ground signal and hold-down ability with the HEI as the base is smaller in diameter than the points distributor.

Nothing else to report other than I am tweaking the jet sizes for gasoline. 73s were much too big, I dropped them to 66s and ran about 13.2 AFR in the main circuit at cruise. Still too rich for me. Probably change them to the original 61s to try and achieve get a 14.7-15 at cruise for fuel economy.

Also installed the new Walker 17613 turbo-style mufflers. These don't have the necked-down internal piping as the old mufflers did. They sound great but are a little loud, though. Might have to quiet them down some.

Seems the engine is fully broken in now, the plugs look great and the valve lash is stable.

Where did you get the rebuild kit and synchros from?