Anyone one have any info/suggestion about using a junkyard 4L80 and transfer case (which one?) to replace a TH350 and NP203 in my '75 GMC?
Would it work and what are the most likely problems/challenges (in addition to the driveshaft replacements)?\\
Thanks.
If it's a 4WD, my NP208 required I rip out the 27 spline input to the T-Case, replace with a 32 spline.
It requires ripping the T-case down to the bottom and rebuilding the entire stack.
Took me three solid Tries, There's not much for manuals and quality pictures, so take a lot of pictures as you tear down.
Then you'll want a 3/8" spacer/clocking ring adapter.
There's a great seller from MT on eBay with the hardened studs/bolts and a solid ring that fits excellent between the T-Case and the rear of the 4WD version 4L80-E.
You'll also want to tear down the 4L80-E and add the rear reluctor sensor ring to the rear drum, or you won't have speedometer for the computer. If you see a sensor on the rear left sensor port, then you have a reluctor ring installed.
If not, then the computer could not see the speed sensor and may have damaged the transmission.
Newer NP246 T-cases have the reluctor inside the T-Case. Adapting them to older trucks will not work as they are driver's side drop for the modern GMT400 front axles.
Reverse servo piston's are known the crack from snow plowing and abusive drive to reverse without complete stopping of the entire 4L80-E center section of these transmissions. Hard brake, then shift, unless you add the reverse over-run system. (There's a couple of solutions for over-run to choose from. I used the Sonnax version for my build.)
The clocking plate/ring allows you to pair the cases without risking ruining the transmission output shaft by cutting off the 3/8" you MUST account for. You can cut it, however if you ruin the splines, connecting the Trans to the T-Case will be very difficult or impossible. The splines must be greased or bathed in fluid. Depending on which T-case you select, wt or dry output/input sections. I recommend plugging the rear 4L80-E case holes with tap and hex plugs.
There's 2 holes above and below the rear output shaft on the 4L80-E that require plugging.
The 6L80-E comes with these plugged, so this mod is known to be safe when converting to NP203 or NP208, forming a dry space between the cases over the tranmission mount/adaptor.
The hardened T-Case studs are a MUST, in my professional opinion. If you have to pull it three or four times to adjust and repair, you'll ruin the soft studs or strip them where the stress between cases is critical.
Watch out which threads you THINK fit these connections.
The SAE 3/8 are very close to the 15mm bolts. You will have both in the hardware bucket during the swap/mod. Mix up the hardware and you may strip and ruin a case or 2. Pay very close attention.
The 2 main shaft bushings, mentioned in this thread are specifically where the reverse issues beat the forward drum bushings and the entire case bronze bushings.
The drum requres machine work or a sleeved replacement in order to re-align the main shaft and keep the teflon seals from cutting the steel further. Any slop here is a recipe for down the line main shaft bushing failure. Buy the correct drum with the correct size hole.
Some rebuilder's ream and use oversized teflon seals and some ream and re-sleve and use standard teflon seals.
You MUST know the dimensions and use exactly what is specified here.
Knowing what has been done previously by measuring is critical.
You must not guess and slam in what came with the rebuild kit, if there is any chance the unit was previously remanufactured.
Then the real issue is verifying it was properly remanufactured with the correct parts for the year of the case.
Hack rebuilder's will mis-match internals and cause a lot of problems with year to year swapped internals.
You must verify every component and every measurement, if the transmission was replaced and is not 100% verified OEM specific for the year of the truck. Any variation between 86-97-99-2003 will cause a lot of incompatable internal parts. Every version has a different parts list with significant changes and spacing requirements.
Nobody can assist you unless you photograph, measure and document the tear down very carefully.
Mods to the Valve body plates must be measured using small numbered drill bits, if any of the OEM valve balls are removed. Plate plugs and modifications from different shift kits can cause compications, unless you stick to a specific recipe from one shift kit manufacturer. There's a lot of ways a rebuild can be ruined by a hack rebuilder.
Select you used transmission very carefully.
Low miles, OEM never swapped, never rebuilt... is best, if you can find one.
Just because somebody installed a shift kit does not mean they did it correctly.
You want to set the starting point as close to OEM as possible.
My OEM unit had incorrect machined drum bearings, according to every manual I own.
Even OEM has to be verified by careful measuring of every componant.
Last but not least is the angle of the unit if used for rock crawling or 4WD angled high country or mudding use.
4L80-E is known to run dry racing or on a steep grade, due to the angle towards the rear axle on 4wd and crawling rocks.
You should search for a rear intake plumbing solution from the rear of the pan for any HP, rock climber or steep mountain driving.
The intake/filter is positioned towards the front of the not-so-deep OEM pan.
Most aftermarket deep pan DO NOT place the filter lower. (This is why they don't show you the inside pictures for the expensive aluminum pans). They stand-off on thin aluminum sticks, to hold the stock filter above the pan bottom.
This adds liquid and does nothing to keep the filter down low OR TOWARDS THE REAR OF ANY DEEP PAN, to prevent sucking air into your pump.
You want extra fluid and the filter intake towards the very back of the deep 4" pan, (without the stepped rear portion), or you risk running the pump dry and burning up the pump, slamming first to 2nd shift, etc.
My 99 K2500 had 92K. I wanted to swap it as is.
When I tore it down the reverse servo was severely cracked from towing and plowing.
The front drum bushings were beat and Worn badly.
My front mainshaft bearings were worn and cut from the teflon seals.
If you use a junk yard donar, buy 2 and rebuild a spare ASAP.
Search my posts for more detailed info on my rebuild or ask Nick for his excellent advice.
His video's explain a lot.
Prepare to watch all of them several times and save them for the days you get stuck during the rebuild.
