TH350 K-Case vs HD Case

[dyeager535]

I suspect we need to keep the TH400's out of this.

TH400 was a beefier transmission by design, so it may not be accurate to compare.

You could be right that later on all TH350 cases were built identically,
when it saves money, GM does it. But without owning the truck since new, I would hesitate to assume that whats in them now is original. No reason you couldn't swap guts, and in fact is done quite commonly at transmission shops.

Those that have parts manuals can probably look the TH350 cases up and see why the part numbers are different, and if they ceased to be different at a certain time.

 

[merc359]

So then...

That info and photos is all awesome. Thanks guys.

So then... anyone know where I can buy an HD case (preferrably with the support rods)? I just need the case. It would have to be shipped to Washington state too... ugh...

Rowan

 

[thatK30guy]

That info and photos is all awesome. Thanks guys.

So then... anyone know where I can buy an HD case (preferrably with the support rods)? I just need the case. It would have to be shipped to Washington state too... ugh...

Rowan

Good luck, I'm trying to find a 350 K/HD case, too.

 

[BGKYK5]

I had thought that the K/HD cases were phased in for a certain year too and that "K" refered to the letter cast into the belhousing not whether it was 2 or 4WD.

I can't help with sourcing cases, but I think I have an extra set of support rods if anybody is interested.

Does a specific motor mount plate need to be used with the support rods? That is where they attach.

 

[thatK30guy]

Yep, there are triangular shaped braces that sandwich between the clamshell steel motor mount and the block bosses. Both are different in size and shape. I have several of these braces and can take pics of them but will see if someone else already has some.

 

[kennyw]

I would also be very careful to find the problem that caused the first case to crack. Bolting it to the motor should not cause that unless you didn't have it flush to the block in the first place.

 

[merc359]

In this case, I bought the transmission and t-case separately. Never went together. It cracked when it was sitting on the floor getting bolted together and the bolt was only 1/3 of the way in. The metal cracked in 4 spots, and I think it was just an inherent metallurigical flaw.

 

[Wasatch K5]

merc,

I'm curious. When our Blazers were new, gasoline was less than $1.00 a gallon. Now it's about almost four times that. If your rig is a daily driver, why not replace that turbo 350 with a 700R4 and get a 30% overdrive fourth gear and improved fuel economy?

 

[merc359]

It's not a daily driver. It is a full rebuild from the ground up. The transmission was built to handle the 400 hp that the engine is pushing while revving to damn high RPM. I COULD swap over to a 700R4, but with 40" tires, the weight of all the 1 ton axles, 205, frame supports, roll cage, and other crap I've put into this truck, I am a little gun shy on the 700R4 as I've destroyed 2 of them in one of my other muscle cars with a near identical motor but far less weight. I'd really rather not spend the huge dollars required to build up the 700R4 when I've already sunk the money in the TH350 and this thing will see 5000km per year MAX... just a toy for going camping and making sure I get home again.... otherwise, I'd LOVE the fuel savings. Somewhere along the line, this truck went from a solidly-planned rig to a little bigger and heavier than I thought.... surprise surprise....

 

[wasted wages]

why not replace that turbo 350 with a 700R4

because 700R4's suck the big one....

I'd put in a T-400 and be done with it...

The 4 bolts on the back of the T-350 are notorious for cracking the case/stripping out. If you insist on sticking with the 350, I would stud the case w/grade 8 hardware and red locktite.

Then bolt up the adaptor /transfer case....

 

[merc359]

Stud it... hmm... I like the sounds of that. Got me lots of grade 8 hardware, and th TH400 is definitely the next thing going in, but for now it'll do with the TH350. When I destroy it, and it may happen soon, I'll be ready to throw more good money after bad! lol

 

[thatK30guy]

Don't forget Stephen Watson ran the 700R4 in Wally during the TTC back in....uh....can't remember the year.

 

[wasted wages]

Stud it... hmm... I like the sounds of that. Got me lots of grade 8 hardware,

IIRC,,you can get nuts on the backside of the top two studs threaded thru the case,,,you just have to use a longer stud.

take a 3/8 tap and run it all the way thru the top two holes of the case,to clean up the threads. wash the holes out with brake cleaner. let dry.

take two studs long enough to thread thru the case,, douse the studs with red locktite,,and thread them through the case.

slide on a lock washer and nut on the backside of the case flange and tighten it down. let it sit overnight,,then bolt on the adaptor plate..

I'm not sure if you can use studs on the bottom two holes,,but the same applies...
grade 8 hardware,bolts,washers lock washers,,,,wash the holes with B/C and apply the red locktite.

 

[merc359]

Found one...

Ok so I found a core locally and have managed to swap out all internals over to the new case. Still need to track down a bolt-on dust cover and the rods. What is a fair price for those items do you think?

What a hassle! I called like 8 core suppliers all over the city and only 1 place had 1 of them. I hate living in Canada and building these trucks!

 

[Wasatch K5]

wasted wages,

I agree that the early 700R4's were not so great, but to say that they all are poor is, well, ignorant.

GM, to their credit, recognized the deficiencies in the early 700R4s and incorporated more robustly designed parts as the years went on. The later ones are substantially stronger and better than the early ones. Any "quality" rebuild will take advantage of the beefier re-designed parts and will result in a better transmission.

In the initial post merc 359 did not mention what he had for an engine, gearing, tires, etc, nor did he say how he intended to use the vehicle.

If you read my post again you'll see I asked "If your rig is a daily driver..."?

Why do you need a turbo 400 just to drive a relatively stock vehicle on the street?

 

[troutbum_mt]

<snipped a lot of Wise Wasatch's post for space>

If you read my post again you'll see I asked "If your rig is a daily driver..."?

Why do you need a turbo 400 just to drive a relatively stock vehicle on the street?

For towing? I am making my blazer into a dd and to use during the summer as my guiding vehicle. I will be towing a driftboat for 4+ months a year and then there is towing snowmobile trailers in the winter, etc for fun. Not trying to argue, but people always have their reasons....

This thread has been really interesting. I can't wait to take some pics tomorrow of the two trannies I am dealing with. Both are TH350 and both are from 4wd models. One is from 75 and doesn't have any of the bracing and has an thin metal inspection cover. The 78 has a brace that runs from the bellhousing to the transfer case and has an aluminum inspection cover or whatever you want to call it. The bolts for that cover are different too I think. I may be missing some of the bracing because the guy that rebuilt the tranny from the 78 was a complete retard and had stuff all messed up (which is why it failed and why I have to swap it out). The TV cable wasn't even hooked up and the inspection cover wasn't on just to name a few things.....

Not knowing sh*t, I am curious as to which case is which or if it is just a model year difference. I'll snap some pics tomorrow. Hopefully they will help the discussion in how to identify what. Lord knows I would love to learn myself.

 

[wasted wages]

Why do you need a turbo 400 just to drive a relatively stock vehicle on the street?

In his own words,,,,,

but with 40" tires, the weight of all the 1 ton axles, 205, frame supports, roll cage, and other crap I've put into this truck, I am a little gun shy on the 700R4 as I've destroyed 2 of them in one of my other muscle cars with a near identical motor but far less weight.

I "get " the O/D thing with the 700R4. believe me,,,BUT, from my past experience with slushboxes,,( and I've rebuilt all 3,, 350,,400,,700r4 )
The T-400 is the brute of them all. I run a fairly stock T-400 behind my blown big block with no problems. I wouldn't even think of putting a 700r4
in the same application.

Yeah, I know we all don't drive around with blowers sticking through the hood, but still my point is,, the 700r4 will never take the place of a T-400
for the same bucks. Everyone I know that has been into a 700r4,,it has cost them 1700 bucks or more,,and they still come away with issues.

now take about half of that,,,buy a stock T-400 throw some seals and a shift kit in it..and your good to go.

For a low cost heavy duty trans it's hard to beat a T-400.

Unless of course you can find a T-475 w/straight cut gears...but that's another story...

 

[wasted wages]

BTW,, most case failures on our K5's can be traced back to several different things....

#1 not enough slip in the front driveshaft... a hard stuff on the front end while wheeling causes the front driveshaft to "stack" solid and forces the transfer case to pivot at the adaptor plate.
Result...broken case at the back of the transmission or pulled threads in the case where the adaptor bolts on or a broken adaptor plate. Real common on the 350/205 combo.

#2 solid motor mounts w/ stock trans mount...or vice versa...sure fire way to crack a bellhousing.

#3 missing braces..engine to trans..and some models also have a transfer case to frame brace. missing braces,,,another way to bust the bellhousing.

 

[troutbum_mt]

BTW,, most case failures on our K5's can be traced back to several different things....

<snipped for space>

#3 missing braces..engine to trans..and some models also have a transfer case to frame brace. missing braces,,,another way to bust the bellhousing.

I bought my first Blazer (75) and it has no braces. I bought a 78 and it had one brace that runs from the transfer case to the bellhousing. The 75 had two crossmembers, the 78 had one. So what braces are SUPPOSED to be there? Lord knows I don't want to deal with another tranny failure.

 

[Hogback Fabrication]

Don't forget Stephen Watson ran the 700R4 in Wally during the TTC back in....uh....can't remember the year.

Stephen also broke a 700R4 output shaft in Moab last year

Of course, he's not particularly easy on his rig either