Flushing NP435 manual transmission?

[ryanschillinger]

In my 67 F250, the NP435 manual transmission had some really, really nasty gear oil come out when I changed it a while ago. I have no idea when it was ever changed in the past, if ever. The truck pretty low miles, but I don't know much of the maintenance history. I did some searching about flushing manual trans, but didn't find much.
Is there some chemical I can put in the trans to help drain all the old crud out without harming the transmission? I was thinking maybe put some seafoam or something in there, jack up the rear end, start it in 4th gear and just let it idle a little bit to spin the trans without any real load on anything. I've heard of people using diesel oil or kerosene, but never talked to anyone who has done it firsthand. Don't want to ruin my transmission, but the very noticeable shifting improvement after the drain and fill has me thinking it would be good to get all the junk out.

Thanks!

 

[GREBOW]

I have used a quart of MMO and filled rest of way up with whatever oil your transmission calls for. Then go put maybe 50 miles on it where you can shift through the gears a bunch, then flush again and refill with correct oil. I am not saying this is the best way to do it,just what I did and it worked good for me also this is for older trucks like yours,probably wouldn't recommend for a newer model manual transmission.

 

[SwampSurvivor]

Those are pretty much indestructible so I don't think you're going to harm it. But, with that said, I'm not sure it would need a "flush". I'd just fill it with the correct oil, run it a bit, then drain/fill again.

 

[SHOZ]

On my Genesis Coupe I used some Redline lightweight shockproof oil once. It's a aqua green color. Changed it out for Redline MTL and in 2 weeks drained it to refill again. The MTL came out as green as the shockproof. I filled the gear box with some mineral spirits and then ran it through all the gears at idle speed for a few minutes. Then drained that and let it sit open for a day. Refilled with the rest of the MTL.

 

[RoadRaceJosh]

If you put gear oil in the NP435 that's your flush. The box will shift better with SAE50 motor oil and that's what Ford and Dodge both specify.

 

[sw99]

Yea just some fresh gear oil and then do another a year or so down the road.

 

[Alex_V]

That transmission is nearly indestructible. If I felt the need to flush it I'd probably use kerosene and just pour it through with as much volume as possible (more sloshing to wash the gunk out) but a fill of good GL4 gear lube on one short change should remove anything that'll do it any harm. The one in my '67 Suburban had over 250,000 mi. put on it and I doubt it had the fluid flushed at less than 100,000 mi. intervals, and I'm sure it was a GL5 if it ever was changed - dad wasn't really meticulous about stuff like trans./diff/ service or GL4 vs. GL5.

 

[ryanschillinger]

I did a lot of research before using 80w90 and it shifts a lot better than before. I just might do another drain and fill after a freeway drive so it gets warm...