The Stovebolt Site Forums The Shop Area Driveline downshifting problems, SM420

My truck is a 53 half ton panel with the 4 speed, still have the torque tube. I swapped it out for another SM420 because of bearing issues. It works well, except it doesn't want to downshift from 4th to 3rd unless the truck is going less than 15 mph. Downshifting into second is not possible unless I'm at a dead stop. What should I be looking for?

Mine does that sometimes. It is worse when cold. I also have some noise at any speed above 40 mph. My thought? Worn synchros. And some ratty bearings. I drained the fluid and put in new, it helped so later I will drain and look for shiny stuff. I hope not....but I don't think my Karma is any better than any one else.

Second gear does not have synchronizers. Therefore you have to manually synch the engine speed to the driveline speed to down shift.

Fourth to third problems probably have to do with synchronizers.

My 1951 4 speed does have synchros for 2nd gear. I find that thicker oil helps the sychro operation. I use 85W140 for mine. You can also give it a little touch of gas as you are downshifting and that should help.

My other transmission always downshifted into third and second. I'm using the same oil, 80W90. Never messed with the clutch adjustments, just swapped transmissions.

Charlie, I think you are thinking of first gear. 2nd was synchronized in all the SM420s. First gear is so slow that you don't need synchros. As far as that goes, I don't know why one would want to downshift into second without being stopped anyway.

OK, I 'mis-remembered'.

Just a suggestion that I am going to pass along. The Ol' Muncie 3 speeds and 4 speed transmissions have CONE shaped synchonizers. I was told by a gentleman older than I am who worked for GM back in the late forties and fifties that these transmissions have to have 90 weight mineral based gear oil which has a higher ash content, or the synchonizers will not work properly. The new gear oil most parts houses carry in 80w 90 or 85w 140 is GL3 or GL5 rated. this gear oil is too slick which will make the synchornizers slip instead of engaging. I had the a similar problem with Muncie 3 speed in a 1958 Chevrolet Biscayne. He sent me to NAPA to get straight 90 weight mineral based gear lube. We changed the gear lube and WHAT A DIFFERENCE! You can purchase straight 90 weight mineral based gear oil at NAPA, or a farm supply store....like Tractor Supply. Old tractor that the farmers still use have to have this gear oil also.

Well that should start some kind of discussion...:)

You bet your 'ash' !

Just like Denny, I to have had Muncie 3 and 4 speed manual transmissions of various kinds throughout my 55 years, but always followed the manufacturers recommendations except after I rebuilt the Muncie 3 speed in my Chevrolet Biscayne. Speaking from previous experience with old Muncie transmissions. Wear out those synchos or keep crunching gears and a rebuild of that heavy chunk of metal behind the bell housing will ensue. Read that Bible we call the Chevrolet truck shop manual....transmission page 7-30 lubricant type: SAE 90. Transmission installation # 12. Fill the transmission with 6 pints of SAE 90 gear lubricant and tighten drain and filler plugs to 30-35 ft.lbs.

Synthetic gear lube just like synthetic motor oil has a lot of negative myths associated with it. DON"T BELIEVE IT!! Synthetic has nothing to do with syncho engagement. It all has to do with the GL rating and the ash content of the lubricant. Synthetic oil does not cause your crankcase to leak, It can be changed from conventional oil to synthetic and back with NO adverse effects and the list goes on and on. Watch the video on You Tube produced by Mobil 1. Myths about synthetic oil. Very informative and as I say....we ALL learn something everyday.

It's certainly easy to try a different lube. As long as we don't get that wet snow they're predicting, I'll get on that this weekend.

What....can't follow simple instructions? Denny maybe you need to take heed towards your last post. Good luck to you also. Derogatory comments towards new members does not promote a healthy environment in this forum.

nor does 'lecturing' old members

Bill

Bill,

I apologize if what i posted sounded like I was "lecturing." That was not my intention. Just passing on experience and knowledge. that Mobil 1 video on You Tube is quite interesting.

Nobody is saying you can or can't put whatever you want in your transmission. Personally I take advise that is given to me, especially when the person giving the advise is speaking from experience. I grew up bring around tradesmen. The first thing the apprentice learned was to watch, listen and follow the experience the journeymen and masters of the trade were showing them. I'm not saying that a new guy may not have a good idea or point to make, it just needs to be presented respectfully.
In my old transmissions I have the best luck using mineral based 85-90 gear lube. In my work trucks (Sterlings) I use synthetic gear lube which happens to be what the factory put in them when they were new.

I had no idea that SAE90 gear lube was so scarce. TSC has a whole wall display of lubricants, not one #90. NAPA not either. I'm sure I'll get it, just don't have the time today.

Yes, That 90 SAE 90 weight gear lube is not easy to find. NAPA and TSC usually carry it in the back in 1 gallon or 5 gallon drums. You will need to ask a clerk or store manager, or someone who has grown up on a farm.

From Novak Conversions: The Novak Guide to the GM Muncie SM420 Transmission:

"An 80W-90, API-GL5 or MT-1 rated fluid is very good. Some claim faster shifts from using a 50W engine oil in their transmission and we do not consider this to be contraindicated unless you operate your vehicle in a very warm environment."

Straight 50w at any Harley dealer. 60w at the same place. Been working in constant mesh transmissions for 100 years. But...the loads are much less in even a heavy motorcycle. How ever if Novak says you can use it then it must meet the minimum requirements...The things you can learn.

Mercury Quicksilver High Performance Gear Lube is SAE90 gear oil made for the lower unit of marine engines. It is available at any Walmart. You can find it with the marine supplies. It will not be in the automotive oil section. I use it in my SM420 tranny and my Eaton H)52 rear end.

Don't confuse this with its sister item--Mercury Quicksilver Premium Gear Lube which is SAE80W/90.

As stated in previous posts, SAE90 gear oil is definitley hard to find. It is extremely hard to find in quarts, easier if you want a five gallon pail or a 55 gallon drum. The Mercury Quicksilver SAE90 is sold in individual quarts at Walmart. Not cheap...about $12 per quart.

Maybellene

Maybellene....VERY WELL SAID!! SAE90 is still the way to go on these old Muncie transmissions.