TH400 in a 1975 C20?

[Theoriginalsilverado]

Hey guys. I had assumed that my transmission was a TH400 3 speed automatic in my 1975 C20 Camper special. But I've been getting some conflicting information. I need to buy some new transmission cooler lines for it, and when I looked up the option as listed on the sticker (M40) it says TH400. But summit racing says that my truck didn't come with a TH400, it came with a TH350?

Can you guys help me out?
I attached a picture of the options where it lists the transmission

[Theoriginalsilverado]

I've also been thinking about getting a lokar automatic transmission shifter. I really want one and I have a few questions about those.

[gmachinz]

GM never put a TH350 behind any factory big block. M40 is the code for a TH400 transmission. Summit monkey just doesn't know what he is talking about. But, because of how old your truck is, identifying it is easy-just look for either a square pan w/a notch @ one corner-that is a TH350 trans. If yours is fairly oblong then you have a TH400. Also, you should have a kickdown switch mounted in your accelerator pedal bracket and there is an orange wire that sneaks out of the firewall through a 3/8" grommet near the distributor and it snakes along the engine side of the trans tunnel to your kickdown switch mounted on the drivers side of the transmission-that is your kickdown trigger wire.

[MikeB]

TH400 pan:


TH350 pan:

[Theoriginalsilverado]

Originally Posted by gmachinz

GM never put a TH350 behind any factory big block. M40 is the code for a TH400 transmission. Summit monkey just doesn't know what he is talking about. But, because of how old your truck is, identifying it is easy-just look for either a square pan w/a notch @ one corner-that is a TH350 trans. If yours is fairly oblong then you have a TH400. Also, you should have a kickdown switch mounted in your accelerator pedal bracket and there is an orange wire that sneaks out of the firewall through a 3/8" grommet near the distributor and it snakes along the engine side of the trans tunnel to your kickdown switch mounted on the drivers side of the transmission-that is your kickdown trigger wire.

Yes but unfortunately I only have the small block 350. Although I'll check to make sure it's a TH400. As for the kick down switch- what is it's purpose, how does it work, and is it important? I did a complete tear down on my truck and I can't recall seeing that particular wire.
Thanks for your help

[gmachinz]

If your truck came equipped with a trailering/towing package, it very well could have a TH400 with a 350 engine-its just that GM didn't build them the other way around. The kickdown switch activated off your accelerator pedal serves to shift the transmission down to when you're at crushing speed to provide increased speed for passing. Its basically an electrical shift solenoid inside the valve body.

It could be that a PO has changed the running gear so you no longer have the original setup anymore-pretty common with these old trucks.

[slotard]

While a 400 is possible, my truck is a C20 Camper Special (not sure how different that is from trailer/tow package) and I believe it had a TH350 factory (and certainly does now). It's best to just look at the pan, who knows what's happened in the past 40+ years.

[Theoriginalsilverado]

Hey guys. I understand what you're saying about anything could have happened in the last 40 years, but considering my grandfather bought it new, and I'm the only one who's had it other than him, and it's only got 30k miles on it, I'm pretty sure everything is original. I'm gonna have to go ahead and take a look at my pan to see for sure. I'll let you know!
Thanks guys

[tlawre87]

My 74 C20 is a 350 with a TH350. It has RPO code M40. My uncle bought it new in 74 and its still all original.

Quite awhile back i came across information that if you ordered a 350 on a 73-75, you got a TH350. Even if it was a camper special. Starting in 76 the RPO code M40 was truly a TH400. For the life of me I can't find that link though.

Attached is my SPID sheet.

[Theoriginalsilverado]

Originally Posted by tlawre87

My 74 C20 is a 350 with a TH350. It has RPO code M40. My uncle bought it new in 74 and its still all original.

Quite awhile back i came across information that if you ordered a 350 on a 73-75, you got a TH350. Even if it was a camper special. Starting in 76 the RPO code M40 was truly a TH400. For the life of me I can't find that link though.

Attached is my SPID sheet.

Wow. Very informative. Thanks for the information. I'm glad I didn't go ahead and get the TH400 shifter. I'll definitely have to check on what it actually is now. Thanks a lot I appreciate the help.

[gmachinz]

The info below was gleaned from a Corvette website where they discuss a similar issue. I just copied/pasted what was relavant to this discussion.




I agree with 7T...it would not surprise me to find an L48 married to an M40 but the trend began in 76 at the factory installing the M38 with the L48. M40s were reserved for the L82.

In doing research on build sheets, you can see where the dealer on behalf of the owner, ordered an M40 (its on the window sticker) but the build sheet shows the order was filled with an M38.

The difference is obvious from underneath. The tunnel was designed (in 68) for the M40. Its the largest tranny that could fit in there and an M40 will fill the tunnel. M38 is smaller.

but the distinguishing characteristic is the oil pan. The M38 will have a cut off corner while the M40 has four corners. I used to have a schematic somewhere that showed the difference but I'll have to look for it.

[Theoriginalsilverado]

Originally Posted by gmachinz

The info below was gleaned from a Corvette website where they discuss a similar issue. I just copied/pasted what was relavant to this discussion.




I agree with 7T...it would not surprise me to find an L48 married to an M40 but the trend began in 76 at the factory installing the M38 with the L48. M40s were reserved for the L82.

In doing research on build sheets, you can see where the dealer on behalf of the owner, ordered an M40 (its on the window sticker) but the build sheet shows the order was filled with an M38.

The difference is obvious from underneath. The tunnel was designed (in 68) for the M40. Its the largest tranny that could fit in there and an M40 will fill the tunnel. M38 is smaller.

but the distinguishing characteristic is the oil pan. The M38 will have a cut off corner while the M40 has four corners. I used to have a schematic somewhere that showed the difference but I'll have to look for it.

Wow.. ok so this post has been both surprising and very informative. I wish my truck was in my own garage I would have checked by now. But unfortunately it's in my family's shop and I'm really busy with exams and studying. Hopefully after my last exam I can go take a look to solve my mystery.

Thanks again guys

[geezer#99]

You don't need to look at the pan to see what it is!
Look for the vacuum modulator. If it's on the passenger side pointing towards the frame rail it's a T400. Easy to see down beside the motor/tranny from above.
If it's on the back pointing towards the rearend it's a T350. You gotta dirty your knee to see it though.

[tlawre87]

Originally Posted by geezer#99

You don't need to look at the pan to see what it is!
Look for the vacuum modulator. If it's on the passenger side pointing towards the frame rail it's a T400. Easy to see down beside the motor/tranny from above.
If it's on the back pointing towards the rearend it's a T350. You gotta dirty your knee to see it though.

Correct me if Im wrong, but even cleaner way to look at the pedal for the electric kickdown?

Well, I guess this way still causes a dirty knee too.

[hatzie]

Originally Posted by gmachinz

The info below was gleaned from a Corvette website where they discuss a similar issue. I just copied/pasted what was relavant to this discussion.




I agree with 7T...it would not surprise me to find an L48 married to an M40 but the trend began in 76 at the factory installing the M38 with the L48. M40s were reserved for the L82.

In doing research on build sheets, you can see where the dealer on behalf of the owner, ordered an M40 (its on the window sticker) but the build sheet shows the order was filled with an M38.

The difference is obvious from underneath. The tunnel was designed (in 68) for the M40. Its the largest tranny that could fit in there and an M40 will fill the tunnel. M38 is smaller.

but the distinguishing characteristic is the oil pan. The M38 will have a cut off corner while the M40 has four corners. I used to have a schematic somewhere that showed the difference but I'll have to look for it.

If it's on the window sticker the original purchaser paid for the optional M40 transmission... and GM installed the base M38 instead. Nice.

[Theoriginalsilverado]

Ok guys... looks like I've got the TH350! Very surprised at this after assuming it was the 400 all this time. I attached a picture of my pan just to make sure you guys agree with me.

[GRADYS Performance]

That is a TH350 pan.

[lkfldredneck]

I was going to buy a truck off of my buddy that he got off of an old guy that bought the truck new in 76 to haul stuff to the dump and go camping every once and a while back when he was 30. The truck was a 77, 350 with a Farmer 4 speed. the truck only had 80,000 kms on it, but the RPO code for the transmission on the original SPI card was M40 Turbo Hydromatic Tran. Vin matching and everything. I doubt the guy would have swapped a transmission, clutch linkage and everything in a truck that would have had a near mint transmission. Not to mention, wasnt a car guy or truck guy that would want to wrench on it much.

So, would that be a factory error or some weird event where someone wanted the dealer to swap it over before purchase? It got me thinkin a bit

[68c10owner]

FWIW my old 1975 GMC SWB was originally a 454/400 truck. I got it with a I6/400. I swapped in a 350 v8 and then a TH350 after the th400 went out. Cooler lines worked just fine on both transmissions. Slight tweaking but no kinks or anything like that.

[Theoriginalsilverado]

Originally Posted by GRADYS Performance

That is a TH350 pan.

Thanks for your input

[Theoriginalsilverado]

Originally Posted by lkfldredneck

I was going to buy a truck off of my buddy that he got off of an old guy that bought the truck new in 76 to haul stuff to the dump and go camping every once and a while back when he was 30. The truck was a 77, 350 with a Farmer 4 speed. the truck only had 80,000 kms on it, but the RPO code for the transmission on the original SPI card was M40 Turbo Hydromatic Tran. Vin matching and everything. I doubt the guy would have swapped a transmission, clutch linkage and everything in a truck that would have had a near mint transmission. Not to mention, wasnt a car guy or truck guy that would want to wrench on it much.

So, would that be a factory error or some weird event where someone wanted the dealer to swap it over before purchase? It got me thinkin a bit

It also has me thinking. Everything on my truck in terms of the vin and options list says it's a TH400 but it turns out it's a 350? I'm wondering why I haven't heard about this before. Is it a common or isolated problem?

[Theoriginalsilverado]

Originally Posted by 68c10owner

FWIW my old 1975 GMC SWB was originally a 454/400 truck. I got it with a I6/400. I swapped in a 350 v8 and then a TH350 after the th400 went out. Cooler lines worked just fine on both transmissions. Slight tweaking but no kinks or anything like that.

Alright thanks I'm glad that option is available. I live in Canada. Any idea of the cheapest place to get those lines? I need both