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Hagerty Employee

Wrenchin’ Wednesday: Simple air-powered fluid pump for nasty diff oil

Everyone who’s ever serviced their differential knows how much of a joy it is to work with gear oil. It’s pleasant smelling, easy to clean and … who am I kidding? This stuff is among the worst mechanical substances ever. When cold, it’s like a gelatinous skunk spray. Even worse, differentials are filled from the side through tiny plugs and, since the whole apparatus is tucked up into the chassis, the opportunities for spillage are plentiful. Today, we’re going to break down the principal behind another cleaning tool and create our own air-powered fluid pump using the bottle in which the diff fluid originally came.


You could easily replicate this with any jar or container that has a sufficiently large lid, but stay away from anything made of glass. Metal and plastic containers are ideal. Even a thin-walled two-liter bottle can hold around 150 psi safely (they’re often used in hobby planes and rockets as flyweight air tanks), so the thick plastic sides of the diff fluid jug will be just fine.


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Pit Crew

I bought a 70qt version of this for doing maintenance on my bobcat and tractor, and man I gotta say, I will never crawl under a car again to do oil or fluid changes.  These things make it a breeze.  Stick the tub down the oil level stick hole, and suck all the old oil out. It makes it a breeze to do any thing now.


What a great idea! Thank you.

Intermediate Driver

Dang, I was just remembering the last time I filled the differential on the El Camino, lying on the ground (in the winter) squeezing 90w hyploid lube from the bottle into that small hole.  Why didn't I have this then!


Clever, but gear oil pumps that fit standard quart bottles are available at your local auto parts store, or on Amazon, for around $10.


The gallon size gear oil bottle I have came with a hand pump built into the cap.

New Driver

NICE!  Now how about a way to first suck all the old fluid out...

Intermediate Driver

When transferring automotive fluids such as siphoning fuel or, as in the case of this article gear oil, I prefer to use clear plastic tubing. 

For this application I would use a thick wall soft flexible tubing affixed to the cap by tightly installed zip ties. 

First placing the zip tie that will be in the lid, threading the tubing through a hole that is just barely smaller than the tubing, to act as a seal, then the second zip tie that goes on the outside of the lid. 

Clear tubing has the advantage of being able to see when the fluid is traveling through the tubing. 

(It also helps avoid that pleasant fuel taste when siphoning.)

Pit Crew

One could also control the air pressure with a cylinder leakdown tester.  I've used one before pressurizing cooling systems checking for coolant leaks, so it's easy to regulate the pressure.

Intermediate Driver

I did a similar thing. I used a 1 qt. metal can (paint thinner etc). I soldered a length of brake line to a hole in the top of the can. It reaches to the bottom of the can to collect the oil. I then soldered another small piece of brake line to the cover of the can for the compressed air input. A short length of flexible hose for the output completes the rig. I found that using an air pressure of 1-2 psi worked best since the can is completely sealed.

To suck out the old oil, I use my hand vacuum pump that is normally used to bleed brake lines.

Intermediate Driver

Safety Reminder:  Stay way from metal containers.  A retired engineer at work got tired of hand pumping up his old steel herbicide sprayer.  He installed a schrader valve, as in the story, pumped it up from his air compressor and blew his head half off.  A sad ending and although we're not dealing with high pressures in this story with an open tube, putting a control valve on the outlet and not monitoring pressure could lead to catastrophic results...    


Why not use a boat gearcase oil filler pump?