High Pinion or Low Pinion?

[mrw]

Thinking of upgrading my 2000 TJ Sport diff covers. My Jeep has the factory Dana 30 and 35. The Spicer/Dana covers I want to use have two fill plugs. Upper plug for high pinion and lower for low pinion. My Jeep has a 2 inch lift if that matters. I believe I have low pinions as the drive drafts enter at the lower portion of the diffs. Is my thinking correct on this? And.....would I hurt anything if I filled to the top fill plugs which is meant for high pinion. I'm not going to do that, I'm just trying to learn. Would the extra gear oil hurt anything on the low pinion? Thanks

 

[Strizzo]

all TJs got low pinion 30s from the factory, so unless it was swapped for an XJ front axle yours should be low pinion.

If you overfill, when the diff warms up and the fluid expands, it will push gear oil out of the vent tube.

 

[machoheadgames]

It wouldn't hurt anything but it might make a mess. I would say fill it to the lower plug.

 

[Woodrow]

As above, but one thing to note regarding a lift: usually a lift will rotate the axle and make both the fill plug lower and pinion bearings higher. This shouldn't be a big deal with a 2" lift but you may want your diff fluid a little higher than the stock fill plug to compensate.

 

[LONGJP2]

The Spicer/Dana covers I want to use have two fill plugs. Upper plug for high pinion and lower for low pinion.

The higher hole is usually to accommodate a tilted pinion.
No guarantee either hole will be the correct amount.
Ideally, the fluid level should be close to stock, which is up to the bottom of the axle shafts.

 

[jjvw]

Given a choice, I would prefer the higher fill hole no matter what. Then I fill to whatever volume the Factory Service Manual says and let any excess spit out the breather tube over time.