Just how much of a moron am I?

[Ovals_and_Bowties]

Well I finally decided after having my square for over a year. Whilst knowing there was a rebuilder's plate on the block to, ya know, actually check the damn numbers on the block... And, uh, well, it explained a few things. Namely why my gas mileage wasn't what I thought it should be and why the engine wanted the carb jetted so rich. My 305 was, in fact, a 350 in disguise the whole time. Specifically out of an 83 C10, which also explains why my 85 has a TH350C in it. Someone gutted an 83 to keep the 85 alive sometime back in the 90s.

 

[jcperformance]

Haha, classic
Basically an upgrade you didn’t even know you had — definitely explains the fuel mileage and tuning.

 

[TotalyHucked]

Was probably just the engine that got changed. My '85 had a T350C in it from the factory

 

[Blue Ox]

Well, going from a 305 to a 350 is certainly better than the other way round.

 

[DoubleDingo]

Nice unknown upgrade

 

[Rusty Nail]

too bad you dont got 305 heads on that 350...getcha some #416s and 91 yeeaaa boiiii
then youd really have somethin.
instead you've got an 83 350 with a #2 pencil for a camshaft and prolly #462s on it.

i want to offical vote for #416 heads and 91 octane - it'll get better gas mileage...

 

[Ovals_and_Bowties]

Was probably just the engine that got changed. My '85 had a T350C in it from the factory

You know any of the parts catalogs I've looked in don't list the TH350C as an option for 85. The RPO code for the trans doesn't come up with anything other than some dribble about marketing.

Maybe it was just a really uncommon option?

 

[Ovals_and_Bowties]

too bad you dont got 305 heads on that 350...getcha some #416s and 91 yeeaaa boiiii
then youd really have somethin.
instead you've got an 83 350 with a #2 pencil for a camshaft and prolly #462s on it.

i want to offical vote for #416 heads and 91 octane - it'll get better gas mileage...

Hmm, better mileage but more expensive gas, that sounds like a wash to me.

Honestly she picks up and goes pretty good, enough to startle the wife anyway. It's mainly a work horse so if anything I'm more interested in mods that bump up the low and mid range torque. Engine will probably rarely, if ever, see above 4K RPM.

 

[Jrgunn5150]

I probably would have never checked at all lol.

I'm more surprised when I find something original than changed at this point.

 

[legopnuematic]

I had an 85 C10 I parted out, it too was a 305/350C combo with 2.73 rear gears.

1985 truck brochure:

You must be registered for see images attach

MX0 and MX1 would be the two codes used for automatic transmissions, the same code(s) were used for multiple transmissions, for example a th350 and 400 would be the same rpo code, as both are 3 speed autos.

Keith Seymore has gone into detail before on how all that works.

 

[TotalyHucked]

You know any of the parts catalogs I've looked in don't list the TH350C as an option for 85. The RPO code for the trans doesn't come up with anything other than some dribble about marketing.

Maybe it was just a really uncommon option?

Don't quote me on this but seems like I read somewhere that all '81-up 2wd C10s that got a small block/TH350 combo got the TH350C? Or at least the ones with ESC did. Maybe? I know mine had it, we got it working again and you could feel it but all it really did was make the truck shudder, so we unhooked it again.

@Keith Seymore could probably shed some light on it.

 

[Bextreme04]

You know any of the parts catalogs I've looked in don't list the TH350C as an option for 85. The RPO code for the trans doesn't come up with anything other than some dribble about marketing.

Maybe it was just a really uncommon option?

They don't call it a 350C, but it does list the 3sp automatic option for both the 305 and heavy duty trucks. If you have MX1 on your RPO sheet and it was originally a 305, you would have gotten the TH350C. MX0 would mean it originally had the 700R4.

You must be registered for see images attach

You must be registered for see images attach

I think 85 was the last year for that. After 85, it was either TH400, 700R4, or manual only.

 

[Ovals_and_Bowties]

They don't call it a 350C, but it does list the 3sp automatic option for both the 305 and heavy duty trucks. If you have MX1 on your RPO sheet and it was originally a 305, you would have gotten the TH350C. MX0 would mean it originally had the 700R4.

You must be registered for see images attach

You must be registered for see images attach

I think 85 was the last year for that. After 85, it was either TH400, 700R4, or manual only.

Rockauto, Autozone, and O'Reillys all swear 1983 was the last year for the TH350 with only TH400 or 700R4 being listed for an 85. Seems like GM kept it around a bit longer and forgot to tell anyone.

 

[Ovals_and_Bowties]

Don't quote me on this but seems like I read somewhere that all '81-up 2wd C10s that got a small block/TH350 combo got the TH350C? Or at least the ones with ESC did. Maybe? I know mine had it, we got it working again and you could feel it but all it really did was make the truck shudder, so we unhooked it again.

@Keith Seymore could probably shed some light on it.

Someone at some point on mine actually ripped the wires out of the tranny for the lockup solenoid. Valve body also had aftermarket gaskets so at least the valve body has been off before. Had a crap ton of clutch material in the bottom of the pan I suspect used to be the converter clutch.

 

[Ovals_and_Bowties]

I had an 85 C10 I parted out, it too was a 305/350C combo with 2.73 rear gears.

1985 truck brochure:

You must be registered for see images attach

MX0 and MX1 would be the two codes used for automatic transmissions, the same code(s) were used for multiple transmissions, for example a th350 and 400 would be the same rpo code, as both are 3 speed autos.

Keith Seymore has gone into detail before on how all that works.

I believe the code is MX1 which given any parts places I looked only listing TH400 parts under the year 85 made me believe that it was supposed to have the TH400 in it. Somebody somewhere messed up either in the aftermarket cataloging or at GM.