Installing a transmission dipstick for the 6L90 (L8T gas trucks) using factory fit GM parts.

Epsilon Plus · October 28, 2022

A direct fit transmission dipstick is available for 2020-2023 HDs using part numbers from a 2019 model. You will need the following:

Dipstick tube: 24265546

Dipstick (indicator): 24271875

Seal/grommet for dipstick tube: 15796802

(1) Flange Nut or lock/flat washer & nut: M8-1.25

Start by crawling under the truck and removing the heat shield between the pass. side catalytic converter and the transmission. Use a 10mm socket (if you can find one ) for the front bolt and a 13mm deep socket for the rear flange nut. A 1/4 inch drive ratchet works well in these tight spaces, but a box end wrench will do as well.

Next, dampen a paper or shop towel and reach up and clean around the area of the dipstick tube boss to limit dirt and grime from entering the transmission during tube install. Now, raise the center pin in the factory installed plug to prepare for removal. I was able to use my fingers but a small flathead screwdriver or trim removal tool would work if yours is stuck.

Next, we are going to pry up the remainder of the factory installed plug. I used a trim removal tool but a flathead screwdriver would work as well, just be sure not to gouge or damage the boss area. The factory plug has multiple ridges on it to keep it seated so it might take some working. After a good pry with my trim tool the entire plug shot into orbit in one motion.

Now the fun part. Getting the tube to drop down just right can be tricky, especially as there are a few wires to dodge on the way down. I found that angling the fill tube towards the driver side and twisting as it went down from the top was the best way. Avoid jamming the tube with force that could bend it, tear the heat blanket on the firewall, or sever an existing wire. Use patience (and a step stool) and you'll get it.

Once you've got the tube down, grab your seal/grommet and give it a light coat of Dexron VI ATF to ease installation.

Now take the seal/grommet and slide it on to the dipstick tube.

Move the tube over to the boss and slide it in, making sure that you have achieved a full contact/seal with the transmission. A cellphone or mirror can help here.

Align the tube bracket slot with the extended stud from the transmission bellhousing. Using one hand to give some downward pressure on the tube, work a M8-1.25 flange nut onto the stud. My local parts house was out of flange nuts so I used a lock washer, flat washer, and nut combo. Keep downward pressure on the tube while you tighten with the 13mm deep socket.

Reinstall the heat shield that was removed previously. Almost done! Grab your dipstick and give it a light lube around the seal area with Dexron VI...

...and send it home!

Now as far as a proper hot check, I've always used a minimum transmission temperature of 145'F. The GM check procedure for these trucks says 86'F-122'F, but that's using the inane check plug thingy on the back of the transmission pan. Previous to that design, the hot check temp for Dexron VI was always 145'F min to 190'F max (ATF expands with heat). Perhaps there is someone out there with a service manual for a 2019 and can verify, but there you go folks. This truck should have had one of these sticks from the factory, and we HD users can debate with the non-believers (they do exist) why below or in different threads, but luckily we have a factory fit solution using original GM parts with about ~$60 bucks and some elbow grease!

Special thanks to @64BAwagon for doing the leg work on the part numbers and the initial install. I found his thread (on a diesel site) while pondering if this was possible and his previous work made it a breeze.

Get to dippin!

Edited October 28, 2022 by Epsilon Plus

Pryme · October 28, 2022

This is fantastic and looks extremely easy to do. It won’t void any factory warranty will it?

without this the procedure seems nearly impossible without a lift unless someone wants to put their 7-8k pound truck up on four jackstands making it level and run the engine while being under the truck to check the fluid.

Epsilon Plus · October 28, 2022

1 hour ago, Pryme said:

This is fantastic and looks extremely easy to do. It won’t void any factory warranty will it?

without this the procedure seems nearly impossible without a lift unless someone wants to put their 7-8k pound truck up on four jackstands making it level and run the engine while being under the truck to check the fluid.

Never say never. If you have a stingy A-Hole dealer I suppose they could argue tampering. Thing is, this can be reversed without a trace. Keep the factory plug, 10mm & 13mm wrench in the truck and you could put it back to stock on the side of the road if need be (once the exhaust cools down).

64BAwagon · October 28, 2022

I dont see how any dealer could take exception to this upgrade. It really doesnt modify anything other than the fill plug which is a PITA in the first place.

Its not like we will ever know the REAL reason these were deleted but my guess is like everything else good that is gone it all comes down to saving a few $$$$$ Just like the rear diffs that used to have the 1/2" pipe plug drain in the bottom of them making a gear oil change a 10 minute job instead of over an hour.

Core-Lokt · October 28, 2022

Thanks for this excellent write-up and to the member contributing the PN research. I had no idea there was no ATF stick (truck is somewhere in the process). I just found the tube/stick/seal for a total of ~$45.00 shipped on ebay and they're ordered.

64BAwagon · October 30, 2022

Next up is finding a 3 1/2" inlet and outlet muffler to replace the giant suitcase under my truck. I have to believe some manufacturer recognizes the potential market for a muffler upgrade on these trucks. Please, someone come take my money !

Pryme · October 30, 2022

16 minutes ago, 64BAwagon said:

Next up is finding a 3 1/2" inlet and outlet muffler to replace the giant suitcase under my truck. I have to believe some manufacturer recognizes the potential market for a muffler upgrade on these trucks. Please, someone come take my money !

I’ve heard trucks that simply remove it and run a straight pipe in its place. Sounds really good.

64BAwagon · October 31, 2022

The ones I have seen done have not been a true 3 1/2" they were necked down to a 3" pipe apparently because the typical bending machine doesnt go higher than 3". This defeats the purpose of flowing more. The delete also has a bit of a drone and my truck is used primarily for towing our 5th wheel on the expressway which would be where the drone is worst.

Still waiting.

Pryme · October 31, 2022

1 hour ago, 64BAwagon said:

The ones I have seen done have not been a true 3 1/2" they were necked down to a 3" pipe apparently because the typical bending machine doesnt go higher than 3". This defeats the purpose of flowing more. The delete also has a bit of a drone and my truck is used primarily for towing our 5th wheel on the expressway which would be where the drone is worst.

Still waiting.

I would think a performance type shop could bend 3.5” but calling around would let you know. But yeah a drone could get tiresome towing in a long trip.

Edited October 31, 2022 by Pryme

Epsilon Plus · October 31, 2022

My truck @ 7,200mi was about 1/2 quart low BTW. Not surprised after reading about these trucks shipped dry on front end grease and low in the diffs.

RedLT · October 31, 2022

On 10/28/2022 at 7:19 AM, Epsilon Plus said:

A direct fit transmission dipstick is available for 2020-2023 HDs using part numbers from a 2019 model. You will need the following:

Dipstick tube: 24265546

Dipstick (indicator): 24271875

Seal/grommet for dipstick tube: 15796802

(1) Flange Nut or lock/flat washer & nut: M8-1.25

Start by crawling under the truck and removing the heat shield between the pass. side catalytic converter and the transmission. Use a 10mm socket (if you can find one ) for the front bolt and a 13mm deep socket for the rear flange nut. A 1/4 inch drive ratchet works well in these tight spaces, but a box end wrench will do as well.

Next, dampen a paper or shop towel and reach up and clean around the area of the dipstick tube boss to limit dirt and grime from entering the transmission during tube install. Now, raise the center pin in the factory installed plug to prepare for removal. I was able to use my fingers but a small flathead screwdriver or trim removal tool would work if yours is stuck.

Next, we are going to pry up the remainder of the factory installed plug. I used a trim removal tool but a flathead screwdriver would work as well, just be sure not to gouge or damage the boss area. The factory plug has multiple ridges on it to keep it seated so it might take some working. After a good pry with my trim tool the entire plug shot into orbit in one motion.

Now the fun part. Getting the tube to drop down just right can be tricky, especially as there are a few wires to dodge on the way down. I found that angling the fill tube towards the driver side and twisting as it went down from the top was the best way. Avoid jamming the tube with force that could bend it, tear the heat blanket on the firewall, or sever an existing wire. Use patience (and a step stool) and you'll get it.

Once you've got the tube down, grab your seal/grommet and give it a light coat of Dexron VI ATF to ease installation.

Now take the seal/grommet and slide it on to the dipstick tube.

Move the tube over to the boss and slide it in, making sure that you have achieved a full contact/seal with the transmission. A cellphone or mirror can help here.

Align the tube bracket slot with the extended stud from the transmission bellhousing. Using one hand to give some downward pressure on the tube, work a M8-1.25 flange nut onto the stud. My local parts house was out of flange nuts so I used a lock washer, flat washer, and nut combo. Keep downward pressure on the tube while you tighten with the 13mm deep socket.

Reinstall the heat shield that was removed previously. Almost done! Grab your dipstick and give it a light lube around the seal area with Dexron VI...

...and send it home!

Now as far as a proper hot check, I've always used a minimum transmission temperature of 145'F. The GM check procedure for these trucks says 86'F-122'F, but that's using the inane check plug thingy on the back of the transmission pan. Previous to that design, the hot check temp for Dexron VI was always 145'F min to 190'F max (ATF expands with heat). Perhaps there is someone out there with a service manual for a 2019 and can verify, but there you go folks. This truck should have had one of these sticks from the factory, and we HD users can debate with the non-believers (they do exist) why below or in different threads, but luckily we have a factory fit solution using original GM parts with about ~$60 bucks and some elbow grease!

Special thanks to @64BAwagon for doing the leg work on the part numbers and the initial install. I found his thread (on a diesel site) while pondering if this was possible and his previous work made it a breeze.

Get to dippin!

WOW! Nice write up.

Pryme · November 1, 2022

9 hours ago, Epsilon Plus said:

My truck @ 7,200mi was about 1/2 quart low BTW. Not surprised after reading about these trucks shipped dry on front end grease and low in the diffs.

I have a hard time believing your trans was low. You sure this dipstick mod is accurately measuring the level?

i haven’t seen anything about diffs being low, I’ll have to look into that.

I’ve heard of the zerks and I did top all those off.

Edited November 1, 2022 by Pryme

64BAwagon · November 1, 2022

I did not check the trans level using the prior method of warming it and pulling the plug on the pan. I didnt want to burn my arms on the cat since I dont have a lift and the cats are screaming hot when the trans is warm enough to check. My dipstick is very close but IIRC not exactly all the way to the top of the full mark.

As for the diffs, I have seen this discussed for a very long time. The old tried and true method of making sure they were full is to fill them until the fluid runs out of the fill plug. If you dig up the American axle service manuals however they usually say that full is about 1/2"-3/4" below the fill plug. As is the case with other fluids, too much isnt always better.

The greaseable fittings arent full because the parts were covered in lube when they were assembled and that is sufficient until the first service.

Edited November 1, 2022 by 64BAwagon

mtbadbob · December 13, 2022

On 10/31/2022 at 7:37 AM, 64BAwagon said:

The ones I have seen done have not been a true 3 1/2" they were necked down to a 3" pipe apparently because the typical bending machine doesnt go higher than 3". This defeats the purpose of flowing more. The delete also has a bit of a drone and my truck is used primarily for towing our 5th wheel on the expressway which would be where the drone is worst.

Still waiting.

Get the MBRP cat-back system for under $500. $ well-spent & lose about 30 lbs!

mtbadbob · December 13, 2022

On 11/1/2022 at 4:53 AM, 64BAwagon said:

I did not check the trans level using the prior method of warming it and pulling the plug on the pan. I didnt want to burn my arms on the cat since I dont have a lift and the cats are screaming hot when the trans is warm enough to check. My dipstick is very close but IIRC not exactly all the way to the top of the full mark.

As for the diffs, I have seen this discussed for a very long time. The old tried and true method of making sure they were full is to fill them until the fluid runs out of the fill plug. If you dig up the American axle service manuals however they usually say that full is about 1/2"-3/4" below the fill plug. As is the case with other fluids, too much isnt always better.

The greaseable fittings arent full because the parts were covered in lube when they were assembled and that is sufficient until the first service.

I always try to get my fluid levels to not run out the fill holes, but where I can stick my pinky in and pull it out with fluid on the end. Figured that was plenty full.