All M10's they had were 15mm heads. Looked nearly identical to what the OP showed with the exception of being full thread.

 

I found this from Trinity Logistics Trinity M10 - 1.5 X 25MM ANSI HEX HEAD CAP SCREW 10.9 ZNYEL Trinity Hardware Headquarters they don't sell to individuals, but there is a Fastenal store within 25 minutes. That cap screw, however has a 16mm head, so I'm not sure if there's enough room, might have to take some measurements.

Nice to have the GM part number. With as many of those bolts that GM seemingly used, you'd think they'd be easier to find. And as much of a stickler as I am for using exact parts, I really dislike the idea of paying that much for a screw, but at least I know that option is there as a last resort.

There is another fastener place in the same town, as well as a Kubota dealer. I tried Tractor Supply Co., but they didn't have anything close in 10.9 steel, only 8.8.

 

You could always tell folks you couldn't get the GM Parts, so you went to a Mercedes Benz dealer and got them there. I think the Genuine Mercedes Benz bolts are actually cheaper than the GM ones.

The socket cap ones you can get at Ace Hardware or Home Depot are identical to what MB used for the inner CV joints in the W126 (S class) and W124 (E class) cars from the 1980's and 1990's. Audi and VW use those same bolts as well (but with 12 point "triple square" sockets instead of 6 point sockets).

I've put over 200,000 miles on a set of Ace Hardware bolts in a hard driven Mercedes 560 S class coupe. My neighbor has over 300,000 miles on a set of Ace Hardware socket cap bolts in a 300E sedan that I reconditioned and traded to him.

 

Found these at Carolina CAT https://parts.cat.com/en/carolinacat/6V-4248

Got 10 of them (j.i.c.) for 60 cents a peice. Only spec difference from the OEM ones are that they are fully threaded and have 16mm heads. Shouldn't be big enough to cause a problem.

 

Well, the screws from CAT fit fine, with just enough room to get my socket on them. I can see why 17mm heads would not work. I used blue loc-tite and torqued them to 60 ft-lbs, going by what I found in my Haynes manual for "Driveaxle-to output shaft flange bolts".

I'd like to change the front different fluid, but Hayne's is not as helpful as I'd like. It says I need to use a suction pump to remove the fluid from the fill plug hole (which I'm not sure where that is) and refill with SAE 80W-90 GL-5 gear lubricant. Is that correct, or are there any differing opinions on brand or viscosity?

This is my first non-FWD vehicle, so anything else I should look out for? I have changed the transfer case fluid and fixed the 4WD (just in time for last winter's weather).

 

Two questions.

• Do you own a suction pump?
• Is there any leakage around the front diff cover?

If you don't have a suction pump, there is another way, by removing the cover..

If it's leaking around the diff cover, you might want to do it by removing the cover anyway. You'll have to do that to change the gasket (and maybe the cover as well).

Without a suction pump, you need lube, a new diff cover gasket (should be about $3), and you might want a new diff cover (should be $15 +/-).

Either method, you need to remove the fill plug first. It's on the top, back closer to where the drive shaft connects. It's a 3/8 inch square socket on the plug. Technically, you should use a 3/8 inch square plug key (like an allen wrench, but square instead of hexagonal), but any 3/8 inch extension for your ratchet will fit in the hole and get it out. I think I'm probably the only guy in the world who owns a full set of square plug keys.

If you're using a suction pump, suck the fluid out that hole and refill it. PUt a dab of silicone sealant on the threads of the plug and reinstall the plug.

Without a suction pump, read on...

Once you have the plug out, get a catch pan and put it under the diff. This gets a little messy. Then start loosening the bolts on the diff cover. Completely remove all the bolts across the bottom, and loosen the rest of the bolts. Gently pry the cover loose at the bottom. I use a plastic scraper usually, to avoid scratching/damaging the metal. The old lube will drip out the bottom. Wait for most of it to drip out, until it's not dripping much more. Then remove all the rest of the diff cover bolts and take the cover out. Use a plastic scraper to scrape off the old gasket and clean up the mating surfaces of the diff and the cover (don't bother with the cover if you're replacing it, obviously). Reinstall the cover with a new gasket, apply blue locktite on the bolts and install them finger tight first, then tighten them alternating back and forth across the diff. The exact pattern isn't critical, just be sure to tighten them all. The German torque spec, "gütentight" works for these. There probably is a torque spec, but I don't know it off the top of my head. Too tight can damage the gasket, though.

Use a funnel and hose or a turkey baster and hose to fill it with lube up to the bottom of the fill plug, and reinstall the fill plug. I generally use a bit of silicone sealer on the threads of the plug before installing it so it doesn't leak there.

For lube, I use conventional (not synthetic) 80W90. House brands (CarQuest, Napa, O'Reilly) work fine in these. 3 or 4 quarts will do both diffs, front and rear.

 

I don't have a suction pump, per-se, but I have a couple feet of clear fuel hose that I used to change the t-case fluid and a dispenser pump from a commercial size soap jug I got from Sam's Club.

I haven't seen anything dripping from it, but I had a high-pressure power-steering hose spring a leak several months ago, so a lot of things are still caked in dirt and oil.

 

Oh, and I may have a square 3/8 wrench. My grandfather taught automechanics and had a ton of different tools, I remember seeing some square allen-like keys out in the shop.

I don't think I have any silicone thread sealant, I do have some permatex aviation form-a-gasket (the black sticky kind) and permatex thread sealant with teflon. I thought I had some of the red permatex silicone gasket maker somewhere.

(Can you tell I have an affinity for Permatex?)

EDIT:

I don't know if this counts as a full set (for some reason there's no 1/2"), but I just found this drain plug wrench. It's seen better days; of course the size I need is the only one damaged, but it fits tight in a 3/8" socket so I think it work, provided there's enough clearance to turn it.

 

I use Permatex "Ultra black Maximum Oil Resistance RTV Silcone Gasket Maker" on diff plugs. Some folks use the teflon plumbers tape for this application, too. I find the more oil resistant sealants work better.
That's a cool antique tool you have in the picture. I'd keep that antique and not risk damaging it more or breaking it.

For this plug, any 3/8 extension fits the hole and works just fine. My set is a set of keys that fit on a 1/2 inch driver ratchet. They are the black oxide finish common on "impact sockets". I bought my set because I had to deal with some vehicles with 5/16 square socket plugs, and another application with some 1/2" and 5/8" square socket plugs. I got an 8 piece set that goes from 1/4" to 5/8".