I just picked up a '71 F100 Custom and there is what looks like some kind of oil on the underside of the bed, just above the rear end. So, I have a couple of questions:

1. How can I confirm where it is coming from and stop it?

2. Is there a seal between the two pieces of the housing and if so, what does it take to replace it?

3. What is the procedure to add fluid?

4. If I just add fluid and then drive it for a while, what am I risking?

 

Here's an answer to your list:

1. How can I confirm where it is coming from and stop it?

The easiest way would be to be able to clean off all the oil mess with a power washer. You can also take some wiping rags and solvent and clean it up around the housing and rear universal joint and make sure its dry. Then drive it and check it every couple of blocks and see where it is getting wet with oil again and the location of the leak.

2. Is there a seal between the two pieces of the housing and if so, what does it take to replace it?

There is a yoke on the rear part of the u-joint attached to the rear differential. This yoke attaches to the pinion shaft and gear. There is a pinion seal pressed into the diff housing that seals against the pinion shaft. There is also a nut on the end of this pinion shaft that you can see inside of the u-joint which must be tightened to a torque specification to make sure it stays tight. If this nut loosens, it can cause the pinion seal to leak. This is the only place the diff can leak in the center. Remove the driveline by disassembling the back u-joint, remove the nut on the end of the pinion shaft and slide the yoke behind the nut forward and off the pinion shaft. Use a small bar to pry out the old seal. Clean the groove where the seal is housed to remove any old gasket cement, then apply a light coat of Permatex around the outside of the new seal and carefully tap it into the seal groove with a small ball pean hammer making sure to not get it in crooked. Light small tapping from side to side will generally seat the new seal straight. Watch and tap where it is high and keep it straight until it completely seats. Reinstall the yoke and torque the nut, install the driveline and u-joint. Check the fluid level and fill if necessary. You're done!

3. What is the procedure to add fluid?

On the back side of the differential, (when crawling in from the rear), you will see a large square plug. Take a large crescent wrench and remove this plug. Unscrew it slowly. When it is almost all the way unscrewed, if gear oil starts to leak out, screw the plug back in and tighten it. It is full if it will run out of the plug hole. If it doesn't run out, then use a squeeze bottle of gear oil to squirt gear oil into the fill hole. Allow the oil to sort of "settle" as it is thick and the entire axel will have oil in it making it take a few seconds to level out. When it is full enough to begin leaking out the fill hole, install the plug and tighten it as it is full.

4. If I just add fluid and then drive it for a while, what am I risking?

Depending on how fast the oil is leaking out of the leak, you would be risking running the differential out of oil completely. That would cause dangerous metal to metal contact, causing extreme heat and rapid wear to the bearings and gears and other internal parts. Complete failure of the differential assembly would probably occur. However, it is highly unlikely this would happen in.....say a day or so. There is close to a gallon of gear oil in the rear differential and it would take an extremely bad leak to run that much oil out in a single day. But, any leak is not desireable. Recommendations would be to fix it. It is not a costly operation and is relatively simple to accomplish. Good Luck!! ...............

 

Before you assume its a leak, look carefully at the u-joint, what you see splatter on the bottem of the bed may simply be grease from greaseing the u-joint.. first pull the fill plug and check the level. if its just about to leak from the plug hole, it most likely dont have a leak

 

Could be someone pumped grease into a zerk and it spun it out.

Look at that.



John

 

Thanks for all your input. You guys are a great help to a newbe like me.