Im doing the rear diff. fluid on my 02 2dr 2wd blazer and when i went to go buy agasket it came back with 2 sizes. 7.5 and 8.5. does anyone kno which one i need? They only had the bigger one in stock so i just bought that one, and then i bought RTV silicone gasket maker incase it doesnt fit. if it doesnt fit is it ok to use that gasket maker? Or should I wait and buy the right gasket?

 

You don't need a gasket at all, RTV will be just fine.
Use the BLACK RTV.
Btw, I believe the 8.5 is wrong size unless you have a ZR2.

 

As mentioned, you can use black RTV in a pinch however, there is an improved design gasket that allows better fluid flow to the wheel bearings and no RTV sealer is recommended or required. There is a white sticker about 2.5" x 3.5" on your vehicle. It may be in the glove box, tailgate or in a door jamb. On the sticker are several 3 character RPO, (Regular Production Option) codes. Post all codes that start with "G" and we can tell you what ring gear and axle ratio you have. There is a difference in fluid also, depending on which axle you have.

 

the rtv i have is ultra black. is that perfectly fine forsure? or should I get the correct gasket? I got valvoline 80w-90 high performance gear oil

 

Permatex has a new RTV made specifically to perform better for gear boxes, Permatex Gear Oil RTV Sealant (#81182). I haven't found it in local stores yet, but I haven't been around to them all. I have used Permatex Ultra Black RTV prior to hearing about their new stuff and haven't ever had a problem. But if a gasket is available and on the store shelf, I would rather install a gasket.

As far as gear oil goes, any GL5 80w90 (or 75w90 if synthetic) will work in your rear diff. No additives required even if you have the G80 RPO code which is the factory automatic locking rear differential.

 

the stuff i have is permatex and it says ultra black oem Hi-temp rtv silicone Gasket maker. Im just debating if i should wait and get a gasket, or am i making to big of a deal out of this? the valvoline gear oil says API services GL5, MT1. so this is forsure fine to use?

 

The gasket vs RTV is completely up to you. With the RTV, you really should let it sit for 2-4 hours before putting the gear oil in and after that, at least 12 before putting it into service. With a gasket, you can dump in gear lube and drive the truck just as soon as you've tightened the last bolt.

That gear oil will work fine.

The link below should be helpful:
Rear Axle Service

 

Well I plan on doing this is my autoshop class cause im in Highschool, and I only have a 2hour block of it. So im deffinantly gonna do the gasket then. Ill have autozone order one for me. Thanks for the link!

 

Some will say you can tighten the bolts, pour in the fluid, and drive away using RTV. Whenever possible, I try to adhere to the manufacturers recommendations for setting time & complete cure time to some degree.

If you only have a few hours total and have the time to wait for a gasket, I would go that route.

 

Yea, I can run back up to autozone and have them order it so ill have it by tomorrow night, then I can do the fluid change on friday. Its not something that needs to be done urgently, My truck only has 50k miles. But I figure with its age, it forsure cant hurt anything. Thanks for all the advice

Just got back from autozone and ordered the correct gasket. Itll be in by tomorrow and i can do the fluid change friday in autoshop. thanks again for all the help.