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Fluid leaks, one is puzzling
#1376905 Sun Sep 13 2020 03:33 PM
Joined: Feb 2008
Posts: 155
Ol' Red Offline OP
Shop Shark
Hi. Been a long time since my last post. I hope everyone is well.

I'm about to give my truck (1951 3100, all original old stuff, no modifications), the most rigorous workout she's had since I've had custody, and that is to haul a yard of topsoil form the dirt place to my house. In prep for that, I'm doing the oil-change procedure that Hotrod Lincoln suggested, which is to warm up the engine, add two quarts of diesel fuel to the crankcase and let it idle for 15 minutes, then drain. While under the truck I took stock of my fluid leaks, and I'd like opinions on these, please. Moving from front to back:

1) Crankcase oil drain plug - Surprisingly, no leaking.
2) Flywheel cover drain hole - Surprisingly to me, leaking/dripping what I assume is motor oil.
3) Transmission gear-oil drain plug - A small leak. I'm going to just tighten the plug.
4) Rear of transmission where propeller shaft inserts into the tranny - A leak that's been going on so long that the drips from the support bracket have solidified into grease.

The only one that surprises me is #2, the flywheel cover dripping oil. Should I be concerned about the source of this? Any suggestions on the transmission leaks? I think the rear connection (propeller shaft into transmission) has a rope seal that maybe needs replacing??

Thanks for your thoughts. I have pictures, but you all have probably seen these leaks a thousand times, so pics may not be helpful.


Last edited by Ol' Red; Sun Sep 13 2020 03:51 PM. Reason: Added vehicle description

'51 Chevy 3104 1/2-ton 216
Re: Fluid leaks, one is puzzling
Ol' Red #1376906 Sun Sep 13 2020 04:04 PM
Joined: Aug 2018
Posts: 261
Shop Shark
Hello, DJ.

Your #2 leak sounds like the rear main seal on the crankshaft if that's engine oil. Not uncommon, but best to rule out simpler leaks at the rear of the engine first, before jumping to that conclusion. Replacement of the rear main seal can be done with the engine in the vehicle using a rope-style seal (only one that seems to be any good these days is the Best Gasket brand, graphite-rope seal, "GraphTite" PN 6100). There is a Tech Tip on rear main seal replacement in the Technical section - which involves pulling the oil pan, loosening the main bearing caps to allow the crank to drop just a bit, then replacing the seal. If the drip isn't too bad, you may want to watch and let it go while monitoring.

The torque tube U-joint ball at the rear of the transmission has a cork or felt seal at the large gland nut that deteriorates over time. Again, not uncommon. The usual suppliers sell the seal kits for the torque tube ball, including a replacement for the large seal, and shim-type gaskets for the ball to transmission seal (which also set the "tension" on the ball - if not familiar with setup procedure, see the paragraph that starts toward the end of this page in the shop manual: Universal Joint Ball Adjustment []). If it's a long term seepage, and the transmission fluid level doesn't drop very quickly, you may want watch the fluid level and monitor, rather than opening up the seals.

One other thing to check. If you find the transmission is periodically low on fluid, it is worth checking to see if the rear end is actually overfilled. This happens when the seal internal to the torque tube for the propeller shaft deteriorates - it was cork, I believe - and allows transmission fluid to drain back through the torque tube into the rear end. An overfill condition can cause leaks at the axle seals, of course, which can contaminate the rear brakes. If the internal leakage is minor, you can monitor the fill levels. If it's a large internal leak, then the typical fix is to pull the front torque tube bushing, and replace with an Okie (sometimes misspelled, Oakie) bushing assembly (available from the typical suppliers) that includes a new-style seal. There are a number of threads in the forum covering that.

Hope this helps!


Last edited by drdoug; Sun Sep 13 2020 04:17 PM.
Re: Fluid leaks, one is puzzling
Ol' Red #1376907 Sun Sep 13 2020 04:47 PM
Joined: Jan 2013
Posts: 1,662
Shop Shark
I would expect even the best rope or lip rear main seal to leak with 2 quarts of diesel in the crankcase. Drain it and refill the crankcase with oil and climb back under there.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: Fluid leaks, one is puzzling
Ol' Red #1376911 Sun Sep 13 2020 05:19 PM
Joined: Feb 2004
Posts: 21,018
First of all, my suggestion is for cleaning a sludged-up engine, not for routine oil changes, and it's done at a fast idle speed of 1500 RPM or so, not curb idle. The crankshaft and connecting rods whipping down into the over-filled crankcase act like the agitator in a washing machine and splash the solvent-thinned oil around vigorously. If anything, this process might CREATE leaks, not solve them, especially with a rope seal that has shrunken over the years. The whole idea of the flush is to do a better job of dislodging sludge than can be done by taking the oil pan off and scrubbing stuff with a brush and solvent. Once the crud is gone, then fix the leaks the sludge was plugging up!

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The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Fluid leaks, one is puzzling
Ol' Red #1376914 Sun Sep 13 2020 06:04 PM
Joined: Feb 2008
Posts: 155
Ol' Red Offline OP
Shop Shark
Jerry, I didn't mean to misrepresent your suggestion; I was just telling the reader what I had done in case it was relevant. But I do appreciate your clarifications.

Thank you to Doug and SSstock for your suggestions.


'51 Chevy 3104 1/2-ton 216

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