The Stovebolt Site Forums The Shop Area Driveline Rebuilt sm420 3rd gear synchros

Had the transmission rebuilt by a pro a couple years ago, replaced everything easily sourced, polished gears etc.
Have filled it with Dino 90W gear oil. Have issues if rpm's aren't exactly matched shifting into 3rd. Upshifting or downshifting.
Was reading a post about "bench testing sm420" and Jerry mentioned 90W Ford tractor oil. Anything I should look for, and would my dino oil be okay for it? Son's a heavy equipment operator and gets it free from work as it's used in his Cats.

Get rid of the 90W "modern" gear oil and go get the " 90 WT mineral gear oil" as orginally recommended in the manual . Tractor supply has it as Traveller Brand Ford Tractor Transmission Fluid Mineral Gear Oil. The word is that the modern oils are too slippery and do not allow the right amount of friction for the synchro's to work correctly. Thats the brand I run in my SM420 in my 1954 3600.

As well as being too slippery, anything higher than GL4 gear oil has components in it that will attack the brass synchronizers and other components in older transmissions.

One of the problems with the SM420 and similar transmissions is the sheer mass of some of the gears. The synchronizer has a difficult time getting enough of a grip on the gears to match the speed of the two parts that are trying to mesh due to the inertia of the spinning parts. Super slippery lubricant defeats the purpose of the brass friction rings in the synchronizers.
Jerry

I had used hypoid gear oil in mine for years before I realized it was wrong. It never ground into 3rd but It took too long for the syncro to work and was slow shifting. Being an old 2-stick trucker I would take it out of gear without the clutch, wait for the RPMs to fall then drop it into gear with a dab at the clutch, just like I did on the non-synchro big trucks. This went on for so long that I forgot about it completely until it was time to change the gear oil. Then I read about the evils of modern gear oils and mended my ways. It now has SAE90 mineral oil in it and it shifts much better but I still use the old technique as it saves the synchros and one of the brass ones for the older SM420 is no longer available. I forget which one. GM changed the design in the mid fifties and the parts aren't interchangeable IIRC,

So, since there's no Tractor Supply north of the border I'll have to source some proper oil then. Suggestions for alternatives appreciated.
Would this oil be better for the vacuum 2speed rear too?

Shell Spirax S4 TXM Tractor Transmission & Hydraulic Oil, available at Canadian Tire says it's GL-4 rated, which is OK for your transmission.
For your rear axle, you can use any Hypoid gear oil, even GL-5 rated, although the same stuff I found above would be OK.

NAPA and O’Reilly’s carry GL-1 rated straight mineral oil which is what you want to use. I think (but may be wrong) any GL1 rated oil in 90 weight will work.

Looks like O’Reilly’s bought out Vast-Auto recently so you may have one around you.

Anything GL-1 thru GL-4 is OK for synchronizers. I haven't been able to find GL-1 in stock at either local NAPA or O'reilly's, but they can probably order it.

As long as it’s not synthetic because it’s too slick for the synchro’s to operate correctly.

I doubt that you'll find synthetic in anything but GL-5. There's just not enough demand for the lower grades of gear oil that the manufacturers just make GL-5.

I found this Motul product works OK in my 1967 SM420. Bought it at the local auto parts store.
Gear oil

I''d be a bit leery of that stuff Gord. It says it contains Molybdenum bisulphide, a sulfur compound, and is rated API GL-5. It says mineral oil, but additives to get it to GL-5 rating may not be good. They do claim it's safe for yellow metals, but it may take a long time to do damage to brass synchronizers.

I turned 70 this year, so I think it should outlast me!

Gord, I hear you about things outlasting you (and me).