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can I flush my transmission?
#1376922 Sun Sep 13 2020 07:29 PM
Joined: Feb 2008
Posts: 155
Ol' Red Offline OP
Shop Shark
Hi. 1951 3100 216, no modifications. I checked the level of gear oil in my transmission, and it was very low (I drained it and it only had about 11 oz of the 24 oz capacity (per the shop manual)). Since the shop manual says if you add any gear oil, be sure to use the same kind, and since i had no idea what was last put in there and I know it was over 20 yrs ago, I decided to drain the gear oil and use new.

However, while trying to make sure I had room to tip the bottle (like a dry run), the tiny little plastic cap off the gear-oil bottle dropped into the fill hole.

So my question is, can I use something like diesel fuel to flush the transmission gear oil reservoir, to try to flush out the plastic cap? I noticed some very fine metal shavings in the drained oil, so I'm thinking a flush would be good anyway.

Your thoughts, please?

Edit1: [I just had an idea to blow compressed air into the fill hole to try to blow the plastic cap out of the drain hole. Standby.]
Edit2: [That didn't work.]
Edit3: [I found the cap lying on the driveway. So either the compressed air did work, or the cap simply fell of the bottle and never really fell into the oil reservoir. I AM still curious about the idea of flushing the transmission oil reservoir, though.]


Last edited by Ol' Red; Sun Sep 13 2020 07:59 PM. Reason: Resolution

'51 Chevy 3104 1/2-ton 216
Re: can I flush my transmission?
Ol' Red #1376934 Sun Sep 13 2020 09:55 PM
Joined: Sep 2011
Posts: 747
Shop Shark
Drive it enough to get it up to temp, drain & r refill again.

1960 Chevy C10 driver 235 T5 3.73 dana 44
1949 GMC 250 project in waiting
1960 C60 pasture art
G.M dealer tech since 1980 & counting
Re: can I flush my transmission?
Ol' Red #1376996 Mon Sep 14 2020 11:51 AM
Joined: Jun 2011
Posts: 3,630
Shop Shark
Or fill it with diesel, run it in neutral 'til warm, drain and refill, while you are at it, epoxy small magnet to drain plug.


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
Re: can I flush my transmission?
Ol' Red #1377002 Mon Sep 14 2020 01:46 PM
Joined: Feb 2000
Posts: 4,177
Shop Shark
Flushing it out won't hurt a thing, fill it till it runs out the fill hole and with the rear tires off the ground, slowly turn them to rotate the gear in the transmission. A helper on the clutch pedal will be needed. Drain and do again till you feel it's clean enough. Use diesel fuel, mineral spirits, lacquer thinner, or even gasoline, just some form of solvent that will dry on its own over time. I would stay away from Purple Power or the other water based cleaners due to the acids they contain. You don't want to hurt the bearings.

Re: can I flush my transmission?
Ol' Red #1377032 Mon Sep 14 2020 07:34 PM
Joined: Feb 2004
Posts: 21,033
That plastic cap wouldn't hurt a thing even if it stayed in there- - - -the gears would have chewed it up if it happened to get picked up with the lube, and they won't even burp. I'd add a couple of quarts of Diesel fuel and run the transmission in neutral for a few minutes, then drain and refill with 90 weight mineral oil like the Ford tractor transmission lube sold by Tractor Supply Co.

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