Helping a friend out together a dependable 1975 blazer he can daily drive. He has a fresh rebuilt 400 small block chevy and I don't know enough about the transmissions to guide him well.
I assume he has a th350 and my dad ran one for decades with no issues behind the same motor. Is the 400 a bolt-in upgrade? Other recommendations with minimal effort and long-term reliability?
IIRC, the 400 may have a different flexplate for balance, but everything else should bolt right up.
Edit - that's for the engine 350 to 400 swap, not the transmission. The TH400 is a different length and will likely need the driveshaft length changed.
I vote sm465.
400 has a different mounting location, too, but it wouldn’t surprise me if there are already holes in the frame to move the crossmember.
In reply to jfryjfry (Forum Supporter) :
700r4 gets him overdrive and will bolt up. Just make sure the TV cable is set up properly. Bowtie overdrives is a good source of info and parts...
http://www.bowtieoverdrives.com/
Yeah, the 700R4 should fit, even with the stock crossmember, maybe a bit of opening up the mounting holes. I assume they know they’ll need new driveshafts if they go to a different transmission?
What about a locking torque convertor for the 700r4 and controlling it?
How much better is the th400 or 700r4 vs the th350?
since I'll be putting it in and likely diagnosing future issues I'd like to make it as easy for me while giving him a reliable unit.
also, who to get one from? Tci?
In reply to jfryjfry (Forum Supporter) :
I've always run my 700r4s bone stock. Never really needed lockup... could you? Yeah. I just never saw the need.
BowTie overdrives is well known in the squarebody world, and that's all they mess with. Haven't heard of any bad experiences with them. I believe you can order different levels of transmission build from them.
Since K5, assume 4WD. Don't forget to account for transfer case and they usually bolt straight up to the trans, tail shaft not on the 4WD trans. Most that vintage had the TH350. My 81 K5 did. Later K5's did have the 700R4. Different lengths although will bolt straight up to the engine. Different drive shaft lengths to account for trans length difference. Agree what was said about lockup. I had to have my TH350 rebuilt and had a TH400 torque converter and shift improver kit installed. Torque converter was the reason for the rebuild after several years of Alaska year around driving and off-roading.
Any idea of tire size and axle gearing?
The TH350 is the easy button, go for the 700R4 if you hit the freeway regularly. Bowtie over drives will show you how to wire the O/D, it's very simple, no computer needed.
iansane (Forum Supporter) said:I vote sm465.
Saw topic thought this. Still considering converting my K30 from a TH400 to a SM465.
Ooowwww. 1975.
I have nothing to add
The last year the K5 had the full convertibile top.
In reply to Gearheadotaku (Forum Supporter) :
The 700r4 is fully hydraulic. No wiring for the OD. Maybe some for neutral lockout/reverse lights, but that's it.
Gearheadotaku (Forum Supporter) said:
The TH350 is the easy button, go for the 700R4 if you hit the freeway regularly.
Agreed!
If it's got a TH350 in it now that needs to be rebuilt, there should be no shortage of people willing and able to build it about any way you want it and I don't think there is a less expensive option out there to have built (and there is no easier option since it's already in there...I expect)
If this is an occasional pleasure driver, I wouldn't bother swapping it.
Right now, I wish my old suburban didn't have a 700r4 in it.
Recon1342 said:In reply to Gearheadotaku (Forum Supporter) :
The 700r4 is fully hydraulic. No wiring for the OD. Maybe some for neutral lockout/reverse lights, but that's it.
The 700r4 (and the last of the TH350 transmissions, I believe they were designated TH350C) have an electrically-actuated lock up torque converter. I'm sure that's what Gearheadotaku intended.
The guy who built the 700r4 in one of my projects said it wasn't necessary to utilize this function. But it lowers the cruise RPM a bit when locked up which is obviously beneficial.
NV4500.
What kind of axle gears are in this thing? If it's got 3.08 gears and tall tires, an OD transmission will be a waste.
ShawnG said:NV4500.
What kind of axle gears are in this thing? If it's got 3.08 gears and tall tires, an OD transmission will be a waste.
Exactly. 700r4 would be horrible with the wrong gears, and swapping a 4wd to 3.73 would be an expensive proposition. Use the th350.
Really appreciate the input. Sounds like th350 or 700r4 are the most popular.
I don't know final hearing or tire size yet.
but I'll talk to him about what he would want and post it.
frankly I'm strongly leaning to a solid th350 just to keep it simple and stock. But I'll talk to him about it!
In reply to jfryjfry (Forum Supporter) :
I would also go th350/400 for an additional reason- you say "solid daily driver", but if that ever becomes "rock crawler" or "prerunner" a fancy built replacement transmission is just a matter of money and swapping it out.
If it has a th350, keep th350.
A little late to the party, but I'll chime in.
First, gotta know tires and gears. GM automatics are pretty much interchangeable. Just make sure they come from a V8 or a 90 degree V6 (like the 4.3L). All Chevy bolt patterns will be the same up until the LS vehicles. Even those would bolt up, you'd just have an extra unused hole in the bellhousing.
Many GM vehicles will also be ready to accept any transmission that was offered from the factory. This gets hairy with trucks because there were frame differences between half ton and HD trucks, but it's pretty easy to check.
His truck probably came with a TH350. The easy button is... you guessed it, TH350. Starting somewhere around the late 70s or early 80s, GM did a TH350 with a lockup converter, sometimes called a TH350C. It is possible that frame combo also was available with a TH400, but they were never offered with a lockup converter. Both of those (with the exception of a kickdown and reverse light in some) are completely hydraulic and don't need wiring.
Quick rundown:
Metric trans family:
- THM200. Light duty 3-speed. Junk. Mostly used in intermediate cars like G-bodies when the most HP you could have was 140, and even then they didn't last long
- THM200-4r. Light duty 4-speed that is suprisingly not junk. Surviving big torque and abuse takes some careful parts choices. Narrow ratios and steep OD make it not a wise choice for a truck.
Non electric trans:
TH350. Standard fare 3-speed, narrow ratio, with or without lockup
TH400. HD 3-speed, narrow ratio, no lockup
700r4. basically a TH350 with OD and lockup. Wide ratio.
4L60. exactly the same as a 700r4, GM just changed how they name transmissions
Electric trans:
4L60E. As you would assume, a 700r4/4L60 that now needs a computer to operate
4L80E. This is more or less a TH400 with overdrive, but they never made a non-electric version. When they added the OD, they added a computer.
If it came with a TH350, the only direct bolt-ins would be a TH350. In a 2wd, the THM200-4r and THM200 would be direct bolt-ins, but as far as I know the metric trans were rarely offered with 4wd. TH400 will be a bolt-in if you have additional holes drilled for the crossmember and just get driveshafts from a TH400-equipped blazer.
You can probably also source a 700r4 and driveshafts from a later blazer (think like late 80s) if you go OD. Be aware that the 700r4 (and subsequent 4L60 and E) are a very wide ratio. This can be helpful for a heavy truck, but the 1-2 shift is the widest ratio drop of any automatic trans out there. Terrible performance choice, but often does well in trucks. OD is pretty tall at .70
With all of the above non-electric transmissions, you'll find that they employ any number of three connections for control:
-TV cable. Not a coax that delivers Nickelodeon signals, it is a physical cable from the throttle to a valve in the transmission
- Kickdown switch. This is a rotary switch and a 16 ga wire. The switch is usually under the dash on the accelerator lever, but sometimes on the carb.
- vacuum line to a modulator.
Any of those combinations can be easily added if you switch between families of transmission.
Also, looking at EastsideTim's picture...
Make sure you're getting a trans from the correct vehicle. Notice the optional tailshaft differences. Chevy usually used the shortest tailshafts. It's rare, but if you just get (for instance) a 4x4 TH400, it is possible that it has the 9" tail instead of the 4". It's not the end of the world as everything will still bolt up, it just means there are no bolt-in driveshafts available.
Interestingly, the 13" tail TH400 is usually called the TH375. More common in BOPC cars. There is also a TH475 which is an extreme duty TH400 used in severe applications. It basically took a TH400 and added straight-cut gears, more clutches, and a bigger converter. Medium duty trucks, ambulances, tow trucks, etc.
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