This is for the PTU. 1 inch in front the seam where the case COVER meets the HOUSING. I've drilled 3 of these in this location.
The RDU can be drilled right on that case rib.at the bottom rear.
I wouldn't worry about aluminum shavings too much. The hardened hypoid gears will emulsify the very soft aluminum to the point where its more of an antiwear additive like zinc. The gears wont even know its there if there's just a small shaving floating around.
Plus I always run some brakekleen or atf thru the sump to drain out any sediment or shavings.
I use the same drain plug that our transmissions use. It's a 1/8 27 NPT.
Tools needed
- Drill
- Assorted drill bits from about 1/8 to 21/64. Make sure they're sharp. This is important.
- 1/8-27 NPT Tap (requires 21/64 drill bit)
- drain pan
- sockets, torch, penetrating oil, etc to remove front downpipe
I usually drill it with the gear oil still in the sump, with the bit poked through a plastic bag to shield the drill. But you can extract most of it before drilling if you wish.
Start with the 1/8 bit, drilling slow speed with steady pressure on the drill. Most of your shavings should look like a steady helix or heli(?) vs many little aluminum flakes. Make sure you're drilling straight. If its almost straight, you can usually correct it as you step up the drill bit size.
I would have at least 4 gradual bit steps from 1/8 to 1/4 to 19/64 to your final 21/64 size for your tap.
The only bit size that's definite is the final 21/64. The others are round about incremental sizes.
The gear oil will keep the bits lubed. After your first bit, let the gear oil drain out, make sure your fill cap is removed.
Remember, sharp bits, slow speed, steady straight pressure on the drill.
Next, the Tap.
So you've got a nice straight 21/64 hole. Get your tap and t bar style tap handle (do not use a tap handle that uses the ratchet, use the T)
Press the tap firmly and straight as you are turning. Keep pressure and Do not rely on the tap to pull itself until you are sure you have about 4 good full turns/threads tapped.
After you get your 4 first turns, My tap method is a 1/4 turn forward (cutting) and 1/2 turn back to clear the tap. After about 6-7 full turns, you can back the tap out, clean it off, and then complete the threads. After this you should be test fitting your drain plug every time you tap 2-3 additional full turns. You don't want to tap too far and bottom out your drain plug. You probably want about 4 threads of the plug exposed AFTER it's tightened up. That will give you a good safe sealing margin for when you future fluid service.
Flush some brakekleen through, then some fresh ATF or gear oil. Put your drain plug in, use a little sealer if you like, snug it up, fill with about 16-18oz of 75w140 gear oil, make sure your magnet is clean. Remove/clean your vent with some brakekleen, and reinstall your downpipe.
If your fluid is really nasty, I run 2oz of ATF with 14oz of 75w140synthetic gear oil. That mix is safe to run for 5000-7500 miles or 1 engine oil change. Ive run this mix, and so is my buddy whose Ptu had 4oz of fluid in it (the rest was black metalic paste). I have him running this mix permanently to dislodge the paste, changed every 15k. He likes to idle his car all the time, roasting the ptu from the cats.
I ran 4oz Amsoil ATF with 2oz of Amsoil engine flush with 10oz of Amsoil 75w140 in his car as a Hail Mary to see if we could clean save his Ptu. (I don't recommend this mix long term.) He drove that for a week (~300miles). Came back, I popped out the drain plug and all the gunk sludge that was stuck the top of the case had been broken down and flowed right out. I also like to drain it warm. I flushed some atf through to clear the sump, and put 2oz ATF in with his 75w140. I change this ever 2 oil changes for him. The fluid is looking pretty decent. Very little metal on the magnet, fluid looked dark brown, similar to the pictures of mine posted here. I anticipate his next change will be even cleaner, since im sure we're still cleaning up the gunk.
The pics here are from mine in 2017 when I drilled the plug. I extracted 10oz of fluid and filled it about 7500miles before installing the drain plug and taking these pictures. I flushed the sump. Reflilled. With Amsoil 75w110. This was 2017. 20k miles later oil still looked clear as the day I put it in.
The RDU was done in 2016, fluid had 100k on it in and look brand new. But that cover was a PITA, lowering the exhaust, cleaning the surfaces of the diff with little clearance between the subframe. Arrrrggggh! So I drilled for a drain plug to ensure I would never have to do this again.
-Fill the PTU with 75w140syn
(Im catless, using 75w110syn as an experiment)
-Fill the RDU with 75w90, 75w85 or 80w90. (Synthetic)
Interval: every 2-4 oil changes or 15-40k miles. Depending on climate, abuse level and IDLE time. Idle time being most important, abuse second. If you're catless, you can sleep better with the least maintenance. If you drive aggressively, idle alot in a hot climates you should be changing every 2 oil changes or 15-25k miles.
Anyway, I hope this helps anyone that wants to be able to service, maintain or possibly save their PTU or RDU.
I'm going to make a fluid maintenance guide at some point.
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