L31/6L80e

[618 Syndicate]

There is no difference between a TH400 and a 3L80. GM simply renamed them to bring them in line with then current nomenclature.
While I don't dispute smaller output shafts existed in some of these transmissions, that's not the difference between the two.

 

[97Centurion]

Hey all. Sorry to bring this thread back. Erik the Awful did mention that I needed to log in and give an update and so I've read all the posts since I last posted. The 6L80 is still running strong! I haven't had any problems with it at all. I did pull a stock 2013 Corvette A6 tune from the HP Tuners repository and copied those tables over a while back and the transmission seems to love this tune much better than previous tunes. 99% of my driving has been pretty mild so I haven't really romped on the throttle a whole lot. I am going to pull the trans at some point to rebuild it with some higher quality parts to get ready for my lift/wheels/tires. This will come after I build my 2014 Silverado's new motor. Here's a recent video I took back in April of light-ish throttle.

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[Schurkey]

both G20s have 87-90 TH400s that GM named 3L80 that used TH350 output shafts.

Decades ago, a 400-style transmission with a 350-style output shaft spline count was considered a "375". Even had "375" cast into the tailshaft housing.

They were a light-duty version of the 400-family transmission. Fewer clutch discs in the drums, and the smaller tailshaft/tailshaft housing.

If I ever have to rebuild them it is likely they will get TH400 output shafts and output housings, but they are both holding up well.

Get the drums that hold more clutch discs, too. IF (big IF) you grab a 6-clutch Direct drum from a Toronado or Eldorado FWD TH425, be sure to flip the sprag upside down, so it freewheels in the correct direction for the RWD trans. A 425 Direct drum with a roller-clutch won't work--freewheels the wrong direction.

Roscoe originally had a 454, which means that it also had a TH400. When I pulled the driveshaft, it had the smaller TH350 front yoke and I was wondering if it was actually a 350 truck. Now I know, it didn't have a "TH400", it had a "3L80".

As said--I don't know what GM may or may not have changed in the "3L80" years and applications, but decades ago--1970s--when the 400 trans got a 350-style output shaft, it was considered a "375".

For the record, the 375B was an extra-heavy-duty 350 with more clutches in the drums, and a 250 was a 350-family transmission redesigned to be cheap and low-torque. Seems to me they deleted a clutch pack and used the band instead. I've never actually seen one.

Of course, the TH200 was a Metric piece of crap, failed "on schedule" at around 50K miles. I'm told they're getting good use out of them on the slower-class cars on the dirt circles; they're the "new Powerglide" with favorable ratios and not much parasitic drag.

The TH180 was also called the "Tri-Matic", used in Chevettes and--maybe--Vegas. Built in France, I think.

There is no difference between a TH400 and a 3L80. GM simply renamed them to bring them in line with then current nomenclature.
While I don't dispute smaller output shafts existed in some of these transmissions, that's not the difference between the two.

I'm just plain surprised to discover that GM is still cramming the smaller output shafts into the 400-family transmissions. I thought that was "all done" a long, long time ago.

 

[350ss]

thanks for writing this up. i'm considering this for my tahoe and can't seem to figure out how things worked out with the bellhousing/flywheel/converter combination.

i'm still trying to figure out what works with what so bear with me.

so the short of it as i understand (maybe i'm wrong) is the ls style transmissions including the 4l60e (and 6l80e i think) have a longer pilot on the torque converter (compared to the l31 transmissions) where it centers into the back of the crankshaft. this works on the ls motors because the rear crank flange is shorter. (in both cases the flywheel ends up in the same place relative to the bellhousing flange by using a flat flexplate for the l31 and dished flexplate for the ls) there's a good write up on this here

All about Flexplates, Converters, Transmissions and Spacers - LS1TECH - Camaro and Firebird Forum Discussion

Automatic Transmission - All about Flexplates, Converters, Transmissions and Spacers - When installing a different (non-stock) transmission there is often confusion over what flexplate to use and whether a spacer is needed. When swapping an LS engine into a custom Hotrod, there is even more...

ls1tech.com

on gen1 converters, the pilot extends about 0.25" past the flexplate bolt flanges
on gen3/4 converters, the pilot extends about 0.60" past the bolt flanges

so when using an ls 4l60e or 6l80e behind a gen1 motor l31 i would think the converter pilot would bottom out on the crank flange before the bellhousing touches the back of the block. there are kits for mounting an ls trans behind a gen1 block and they have 0.400" spacers for the bellhousing flange and flexplate to converter flanges.

so why did this not happen in your case? i see there was a lot of clearance between the flexplate/converter bolt flanges, but was there any play where you could move the converter back and forth? i would be worried it is pushing against the back of the crank and would wipe out the thrust bearing or the trans pump...

what am i missing?

attached pics of a 4l80e gen1 converter vs 6l80e converter to show the difference in pilot lengths

 

[L31MaxExpress]

thanks for writing this up. i'm considering this for my tahoe and can't seem to figure out how things worked out with the bellhousing/flywheel/converter combination.

i'm still trying to figure out what works with what so bear with me.

so the short of it as i understand (maybe i'm wrong) is the ls style transmissions including the 4l60e (and 6l80e i think) have a longer pilot on the torque converter (compared to the l31 transmissions) where it centers into the back of the crankshaft. this works on the ls motors because the rear crank flange is shorter. (in both cases the flywheel ends up in the same place relative to the bellhousing flange by using a flat flexplate for the l31 and dished flexplate for the ls) there's a good write up on this here

All about Flexplates, Converters, Transmissions and Spacers - LS1TECH - Camaro and Firebird Forum Discussion

Automatic Transmission - All about Flexplates, Converters, Transmissions and Spacers - When installing a different (non-stock) transmission there is often confusion over what flexplate to use and whether a spacer is needed. When swapping an LS engine into a custom Hotrod, there is even more...

ls1tech.com

on gen1 converters, the pilot extends about 0.25" past the flexplate bolt flanges
on gen3/4 converters, the pilot extends about 0.60" past the bolt flanges

so when using an ls 4l60e or 6l80e behind a gen1 motor l31 i would think the converter pilot would bottom out on the crank flange before the bellhousing touches the back of the block. there are kits for mounting an ls trans behind a gen1 block and they have 0.400" spacers for the bellhousing flange and flexplate to converter flanges.

so why did this not happen in your case? i see there was a lot of clearance between the flexplate/converter bolt flanges, but was there any play where you could move the converter back and forth? i would be worried it is pushing against the back of the crank and would wipe out the thrust bearing or the trans pump...

what am i missing?

attached pics of a 4l80e gen1 converter vs 6l80e converter to show the difference in pilot lengths

What you are missing is the pilot hole is way deeper than the depth the converter pilot inserts into it. I did a lot of part number chasing and found out the 4L85E and 6L90E Duramax applications use all the same parts to physically bolt them to the Duramax van. 6L90E from a Duramax van should physically bolt right to a Small block or Big Block setup for a 4L80E. The TCM will need a Gas vehicle flash though. What you are also missing is the fact the spacing changed once again on the GenIV engines with a 6L80E atleast from what I have seen.

 

[350ss]

1. when you say the pilot hole is deeper than the depth of the converter, i'm missing why then you can't install an ls based 4l60e on an l31 engine without a bell housing spacer? or maybe you can, i've never tried but i also haven't seen anyone else do it.

2. what year vans are we talking about? i believe the pilot hole is deeper on the back of the crank on the newer 4.3l motors in vans than the l31...which is why they can accept the longer pilot ls transmission... ill attach a couple pics

3. i don't know enough about the duramax crank/flexplate/converter situation to give any useful thoughts/questions. i do know though and you're probably aware that the 4l80e behind a genIII or IV motor requires a crank spacer because it has the short converter pilot. the same trans behind the l31 doesn't need a spacer...same flat flexplate.

4. regarding spacing...the plane that the flexplate teeth are in are all the same from what i know... i'm assuming this is the case due to the fact that the same dished flexplate is used (pn 12654640) for silverado 1500 with genIII or IV motors from 1999-2013 for both 4l60e and 6l80e transmissions. same for the v6 motors though they use a flat flexplate (pn 12555640)

i did just see that the pilot on a 6l80e torque converter is shorter than the ls 4l60e version (1.000in vs 1.100in). maybe that's the difference but i wouldn't think that would be enough but maybe it is. this still doesn't make sens to me though as the original poster of this threat states that there's a big gap (1" though it looks more like 0.5" in the pics) between his flexplate and torque converter mount pads.

from the link i posted previously
"The pilot for all non-LS converters extends only .250" past the mounting pads (versus about .600 for LS-converters)."

so if the 6l80e pilot is 0.100in shorter maybe that is all that is needed.

something still doesn't add up for me...maybe i'm just dense lol

pics attached are l31 5.7l and lu3 4.3l...not the best pics but i think you can see the different pilot depths...the rear of the crank flange should be in the same plane/position relative to the bellhousing flange (i think)

 

[Erik the Awful]

The tl;dr I got from @97Centurion was that there's a lot of misinformation about fitting a 6L80 to a small block. It was definitely a f* around and find out situation for him, and he was pleasantly surprised that it didn't need billet spacers or any other stupid stuff. It pretty much bolted together and worked. YMMV.

 

[350ss]

i agree it has worked...just trying to understand how it did without wiping out his engine thrust bearing or trans pump.

 

[Supercharged111]

i agree it has worked...just trying to understand how it did without wiping out his engine thrust bearing or trans pump.

It should be obvious when you pull the converter against the flex plate. If it just slides right in then you know that the crank is drilled deeper than the pilot. In fact you could measure that before you even try to bolt it up.

 

[350ss]

i was hoping to nail this down before i bought a 6l80e. maybe i can find one pulled at a yard and take some measurements.