As usual, I don't have the 99 Tahoe here in front of me at the moment which I'll be working on, so I have no way of looking at this myself.

The front differential drain and fill plugs... if memory serves, one is larger than the other. I checked with the dealer and they say the fill plug is M20x1.5 and the drain plug M12x1.5 yet they list the same nylon washer for both of them. This can't be right (same washer for both), so I've been trying to source some nylon washers for the upcoming diff service since those washers have been re-used several times already and are seeping, most likely having been over-torqued at some point.

This is such a small detail it's probably a long shot, but if anyone has a spare diff they can look at or has recently serviced one and happens to remember the respective sizes, could you verify this info?

I replaced mine but don't remember if they were the same. Its not time for me to service diffs. I did my rear because I had a leaking seal.

I bought a k2500 front diff but it's the end of tax season and I won't have time to check it for an other 2 weeks

JR

Had some time so I looked through a bunch of drawings for on on this. It's the weirdest thing, the diagrams all show 2 plugs with different numbers and they are obviously different sizes. However, on the parts listing underneath it only show one of the numbers from the diagram.

Some diagrams will list the top plug and some the bottom plug so maybe you could get both part numbers by looking at different diagrams. I am assuming the washers probably come with the plugs.

http://parts.nalleygmc.com/productSearch.aspx?&ukey_make=1024&modelYear=1999&ukey_model=14544&ukey_trimLevel=15077&ukey_driveline=6246&ukey_Category=0&numResults=50&sortOrder=Relevance&isOnSale=0&isAccessory=0&isPerformance=0&searchTerm=front%20differential

Thanks for the input guys.

z71gmc06, you and I think alike! That link is exactly where I was looking before deciding to go to the delaership (I hate that we lost Comp Nine). I did notice the different diagrams which have the numbers for the plugs, but yeah, same as you, I only found a part number there for one of the washers. I don't know if the plugs come with the washers, but that is a definite possibility. Didn't think about that.

Since I can't seem to figure this out circuitously, I'm just going to measure them when I have them out and try to source somethig locally, if possible. I know the ones which are on there are original, so the dimensions will be correct. As such, I will definitely post up the dimensions and a source for the seals once I get the info, as I'm sure I'm not the only one here who would probably like to have the ability to change them out with new ones every now and then.

You could probably use copper washers too. My son's BMW has a copper washer on the oil drain plug and every time we get a new filter it comes with a new washer.

Was actually thinking that. Also, my old Toyota beater truck used soft aluminum rings instead of the nylon washers. Another option.

Should be able to just take them to your local parts house they should have a variety of them you can look through in any material you prefer

While you are at it, look into replacing the breather hose vent for an updated unit as well.
I don't have more info handy on it now as this link:
http://www.dieselplace.com/forum/showthread.php?t=129812

1999 Tahoe 4x4 wrote:

While you are at it, look into replacing the breather hose vent for an updated unit as well.
I don't have more info handy on it now as this link:
http://www.dieselplace.com/forum/showthread.php?t=129812

Awesome. Thanks a million, I had NOT heard about that. Glad to know this beforehand. I'll definitely be picking one of these up.

Yes, they are different sizes. I was able to find the correctly sized washers right with replacement oil pan drain plugs at my local AutoZone after the dealer tried three times to get me the correct ones! GMSPO obviously has the part numbers wrong.

Just put one of those updated vents in last week on an 03 they thought the had an axle seal leaking that I had replaced last year turned out the the o ring was junk on it, weather its from synthetic lube or not I dont know but it looked like something had chewed it all up

Now that this has been brought to my attention, I'm wondering if perhaps that's not where some/most of the oil film all over the housing (in the pic above) came from. I had just assumed it was the seals on the side and had planned to re-seal the front diff housing when I have the front end apart to do ball joints. Will be interesting to see how much seepage there is after I put in the new vent at this next service.

BTW, this truck has had Mobil 1 75w-90 in the front and rear diffs since new, so this would make sense if in fact these seals are sensitive to synthetic lube.

An update to this post:

While servicing the front differential on the 99 Tahoe last summer (2012) and also replacing the vent plug with the updated version as suggested here, I measured the factory fill and drain plug gaskets. The dimensions are below. As stated previously, they are two different sizes between the two plugs, although the dealereship (erroneously) lists both plugs as being the same size. Yes, the fill plug (top) is significantly larger than the drain plug (bottom). Since these are apparently Nylon plastic and therefore compressible, note these thicknesses are for old gaskets, so they are compressed and possibly flattened out (larger diameter) from their new thicknesses/diameters (they appear to be the same thickness for both plugs though despite the obviosuly differing OD & ID diameters). You will therefore have to keep in mind when sourcing new gaskets that they will be of a fresh, uncompressed state, so the dimensions will be slightly skewed to reflect that.

Fill Plug (M20x1.5 - 15mm Head):

Gasket:
OD = 1.274" (32.36mm)
ID = 0.792" (20.12mm)
Thickness = 0.084" (2.15mm)

Drain Plug (M12x1.5 - 13mm Head):

Gasket:
OD = 0.9675" (24.57mm)
ID = 0.510" (12.95mm)
Thickness = 0.083" (2.11mm)

It should be noted that I am positive these are the factory drain plug washers, as they have never been replaced. Also, it turns out the seepage around the case has returned a year after replacing the vent plug with the updated part number. Although this new plug is not leaking, I can clearly see that the differential case is leaking at the output shaft seal on the driver's side and also at the pinion seal at the back. I plan to take this apart this summer and re-seal. Since I now have the dimensions of the drain plug gaskets, I will also be replacing those as well.