Thought I would increase our knowledge base by closing this out. I did purchase the 86774933 for my truck. Mounts to the transmission fine, but body of the unit has a slightly different angle where the lines mate with the unit, and it did not want to seal properly. My solution was to swap the new thermostat into the old housing.
Brad I think what happened in your case is you mistakenly bought the valve for the 6-speed when you have the 8-speed tranny. You need part #13511137, and that would explain why your lines didn't match up just right. It's weird though, there is no mention of the 8-speeds in the TSB. But if you look up the valves in the GM parts database, there's a clear distinction between the 6 and 8 speed valve.
Just my theory. I'm seeing numerous part numbers listed for the thermal bypass valve depending on application, but the 86774933 seems to be the right one for 6-speed equipped 1/2 ton trucks 2016-2018 (which all have steel cooler lines).
2019-2021 trucks have aluminum cooler lines. The correct part number for the updated valve is 85563685, again for 6-speed (MYC, MYD) equipped trucks only.
The correct "seal" or gasket thing is 23135703 across all applications.
The 2015 and older trucks use the quick release style lines and aren't even mentioned in the TSB. I'm not sure what you guys do.
I don't know. Seems the problem doesn't affect the 8-speeds, or at least GM doesn't think it does.
