I'm fixing to do a fluid change in my '49 GMC 4-speed tranny. The "Owners and Drivers Manual" had a caution NOT to use hypoid gear oil in the transmission. I've been all over town and cannot find a non-hypoid gear oil. This is leading me to think ya'll must be running hypoid gear oil... or have I missed something?
That's a new one on me ~ the venerable Muncie SM-420 tranny has been working hard for over 50 years with EP90 Hypoid gear oil so I'll keep on using it . I could reach under the counter anf pull out the factory shop manual for '49 Chevy (same tranny) and see what it says but it'd be a waste of time since _everyone_ uses Hypoid gear oil in there . Add a 1/2" NPT magnetic drain plug (FLAPS or NAPA) to keep it working well and do change it at least once a year . Add the new oil slowly as it needs to creep past the rear bearing and fill the u-joint ball . -Nate
Nate - Thanks again for the suggestions. My GMC Maint Manual came in the mail yesterday... didn't think to check it for lube specs. Have it in front of me now. It does NOT contain the caution about not using hypoid gear lube, but does have some conflicting info on type lube to be used in the 4-speed tranny. The Lube diagram/chart for the truck indicates lube symbol "MP" defined as SAE 80-90 multipurpose gear lub, BUT in the narrative page that specifically addresses the transmission it specifies lube symbol "ES" which is the previously discussed 50W engine oil that is specified in the lube section of my original owners manual for the truck. I'll go with your experience and fill the ole gal with SAE 90 hypoid gear oil. I've run the 50W engine oil in her for about a 100 miles so that should act as a good flush if nothing else. Will also pick up the magnetic plug when I pickup the gear oil.
Remember Mike , lubricants have come a long ways in the 56 years since our trucks were built . If it doesn't all leak out too fast , using synthetic gear oil is the way to go , I'm using it although I must needs top it up every three months . The rear end doesn't drip nearly that much I'm thankfull for that :7 -Nate
Nate - Thanks again for your thoughts and patience... I know there have been big improvements in the lubs & fluids over the years, but some of the improvements were made without consideration of the old iron they potentially coulv be used in. For example, GL-5 has additives that eat yellow metals (copper & brass) and would do damage if used in the transmission and final drive on the '41 John Deere. It's those kinds of problems I hope to avoid with the expertise folks like you. BTW, the Deere holds 4½ gallons of GL-1 in the tranny/final drive. Fortunately, no leaks at all in my transmission and rear end on the '49 GMC. It has an open drive line so not as many opportunities for leaks. Engine is a different story... I'm getting about ½ pint daily loss dripping off the bottom of the bell housing/clutch cover. Didn't leak a drop when I brought her home, then started leaking shortly after I did the first oil change... straight 30w. Hope to get the rear end drained and filled with 80-90w GL-5 tomorrow. What would you think about draining then filling with kerosene and driving a couple miles to get a good flush before going in with the new fluid?
RE: GL-5 Synthetic Oil You know Mike , I've never had a problem with that yet , I am told that they fiddled the chemical composition to aleviate the etching od the bronze parts , what can I tell you ? I'm still working my old '49 as a daily driver work truck . I don't cotton to flushing with Kerosene , certainly _NOT_ driving anywhere with it in the box , just change it in a couple months and then go to your annual / 12,00 miles whichever comes first changes and it should be good to go . 1/2" NPT magnetic drain plug in the tranny of course . Your rear crank seal is a rope typ and may have dried out else it's plain worn out . no fun to change in situ so think hard before deciding if it runs O.K. and just drips as is normal for a 1930's design or you want to go in and fiddle with it . -Nate
RE: GL-5 Synthetic Oil Nate - Got the rear end drained and refilled. Feel a bit uncomfortable about that in that the capacity of the rear end is 6½ pints and I doubt if I got 2 pints out of it in the draining process. Wiggled a screw driver around in the drain hole to make sure it wasn't stopped up. The refill took the advertized 6½ pints. Got the steering box and the knee action shock absorbers to go and will have replaced or topped-off all the fluids.
RE: Shock Oil The original Knee-Action oil was Glycerin but I use motocycle fork oil , it seems to work out well and keep the seals soft and relatively weep free . -Nate
RE: Shock Oil Nate - Glad to here there is an alternative to the Delco shock fluid. Only place I had found any was from Kanter and I was about to order at $10 for 11oz plus s&h. I know the bike fork oil has got to be less expensive than that. Do you recommend I add the fork oil to whatever I find in the shocks or empty the shocks and refil with fork oil?
