I am going to change my diff fluid to synthetic, and I went to buy a gasket and they offered 2 of them. Something about a pinion gear..7 or 8?? How do I know what I have? Or can I use gasket maker or rtv sealant???

 

the diff. with the 7" ring gear has a more square shaped gasket, make sure not to overtighten the cover bolts or youll rip the gasket.
part # at pepboys is RDS55072

 

You can use RTV but you'll have to wait until it cures to refill the rear diff. Besides a '98, what do you have?

If you have a ZR2 you definitely have the 8.5" rear end.

If you have a non-ZR2, it is highly likely that you have the 7.625 or 7.5" rear end (the same for this purpose). Just to make sure, have him pull the gasket out of the box and compare it to your rear end. If it is more oval shaped with a flat top and bottom it is the 7.625/7.5" rear. If it is more round shaped it is the 8.5" rear.

 

Before you even get started, Make sure you can get teh Fill Plug off and make sure you figured out a way to get fluid in that plug.

This may help

https://blazerforum.com/forum/steering-suspension-drivetrain-36/rear-end-gear-size-7-5-8-8-1-2-a-28619/


Perfect for refilling. Any Auto store has these. http://www.jcwhitney.com/OIL-SUCTION...9064_10109.jcw

 

So you cannot access the fill without one of those pumps??

TripleBlack, I have a 98 LS 4 door with 4wd. I have 212,000 on it, and want to keep it running strong, so i am slowly doing a tune up on it.

 

Trying to remember my daughters '99. I think the fill is on the Driveshaft side of the Diff. and I think you Might be able to fill it with a quart bottle and screwed on long tube ( http://www.hopkinsmfg.com/10106.html )
With the pump, you don't have to try to get the bottle high enough to let gravity feed the oil in. With the pump, you suck fluid out of the bottle , put the tube in the Fill and push the handle to force fluid up and in.

See what you need to do before dumping the old fluid.