The Stovebolt Site Forums The Shop Area Driveline Flushing/Filling SM420 manual transmission

Hello Everyone,

Just had a quick question to see what folks would recommend for flushing my SM420 transmission before I install it.

I drained it out and the fluid was pretty thick.

Also noticed there are two plugs, which I assume the one was the drain and the other a fill plug. They didn't seem to be that far apart and so I just want to make sure you only fill it with fluid until it pours out from the higher side plug or do you fill it some other way.

Lastly what type fluid and what viscosity should I use.

Thanks for the help.

Scott

It can be flushed using diesel or kerosine, no load, just run in place for a while.

Ed

What Ed is saying is, get both back tire off the ground and fill the tranny with diesel. The side plug is your fill level.
Run the truck in second gear for a minute or two. Drain and fill with 90 weight gear oil till it starts to come out the side plug.
The fill level does seem low, but it get the lower gears in the lube and splashes the rest of the gears and bearings for lubrication.

Don

Thanks guys

Right now it's not installed so I guess I can either wait to flush it out after I install it or maybe figure out how to turn the shaft while in gear.

Maybe what I can also do is turn it sideways where the side plug is face up and fill it with diesel and just let it soak for a few days and then pour it out.

What do you guys think.

Thanks
Scott

If you have a large drill chuck, you could possibly connect it to the input shaft and spin the transmission for the rinse cycle. But letting it soak in diesel would help, just not as much.

Gasoline is a better solvent if you can park it somewhere outside, in the sun if possible.

Ed

When I had my tranny out of the truck for a clutch job I pulled the side cover and top cover, splashed some gasoline in it a couple of times. Refilled with Valvoline MT manual trans fluid. Shifts like a dream.

When filling the gearbox with oil, I simply remove the shifter and pour into it's hole. Way much easier versus filling via the side plug. I still use the side plug to check oil level though.

Thanks everyone for all the good tips.

That should take care of this one.

Anyone know of a shop that rebuilds original starters.

I have an original 6 volt starter that I want to make 12volts and also change the flywheel engagement to work with a 1956 235 engine flywheel.

Thanks again for all the help.
Scott

No need to change the 6 volt starter, just don’t lay on it. That way you can continue to use your existing flywheel. 12 volt starters require a different flywheel.

Ed

6 volt starter will last forever on 12 volts if you don’t crank excessively. Plus, it will crank faster.

Hi Ed,

I have heard that you can use a 6volt starter in a 12volt setup.

The truck was a 51 but the engine was a 56 235, I think the starter is potentially from the original motor but not sure.

I read that if the tag is black (which mine is) that its 6 volts and I thought by 56, all the starters were 12 volt.

Just trying to figure out the correct combination of teeth are on the starter drive gear and how many teeth on the flywheel to know that I have the right configuration.

If not, can I just rebuild my starter and change that gear to match (starter being original it probably should be rebuilt, even though I tested it and it spins).

Seems easier than tracking down a new flywheel just to use with an old starter.

Thanks again
Scott

I agree with Ed. I've put 40,000 twelve volt miles on my GMC 228 with its stock 6v starter. But I never crank it for more than 5 seconds at a time. Spins real fast. Starts quick.