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Transmission "upgrade" (downgrade)

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PatJ

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I bought a membership for this one question (and the inevitable follow-ups.) So, please be nice to me :)

I have a 1987 K5 that I use for some mild towing. I am paranoid that the 700R4 will someday leave me by the side of the road. Rational or not, I want to upgrade (downgrade?) this transmission to a TH400 or TH350 for my own piece of mind.

First I want to confirm what I think I have (because I am not as familiar with this “later” Chevy stuff.)


- 1-piece rear main seal (and its associated flywheel bolt pattern)


- 208 Transfer case



IMG_6169.jpg




Does this sound correct?

I would like to find a TH350 or Th400 that will fit in to my existing engine/xfer case combo (if such a tranny exists.) If it doesn’t exist I would like to find a transmission/part time xfer case combination that would require minimal modifications to engine, crossmembers, and driveshafts.

Where should I look for such a transmission?

I am familiar with the “earlier” 73-87 drive trains and I am comfortable with rebuilding a TH350 or TH400.

Thanks so much in advance for help with this project.
 
i can't see the pictures since i'm in school. but yeah most likely a 700r4 to a 208

you can find a 400 (the best idea) and then put a 205 behind it while you are it since it is stronger and will be easier to swap since either way you have to have your driveshafts extended/shortened.
 
Why don't you build a 700-r4 to handle towing and replace it when yours [heavan forbid] goes south? They have been known to be strong enough for towing in the later years. Plus, that sounds like the least amount of work too.
 
hell i got a 700r4 with a shift kit, people hate 700s but either way i'd upgrade once it dies, not before. get an old th400 and hod onto it for now.
 
Thanks very much for the replies.

If that 700 fails it will leave me by the side of the road. I would probably be towing my camper to a campground for a vacation. I would have my family loaded up and I would have gone through great pains to get the time off, get the camp spot, make all the preperations, load up, etc. I just wouldn't want a transmission that I knew wasn't really up to the task to potentially ruin a week-long trip like that. Like I said I might be irrational but it is more of a "piece of mind" thing for me.

I assume I can bolt on a TH400 from a 2-piece rear seal engine. Would I re-use the 700R4 flexplate with the 1-piece seal bolt pattern? Would I try and find a later 1-piece seal flexplate out of something that had a TH400? Will a TH400 TC bolt to the 700R4 flexplate?

Thanks again for the help.

Pat
 
Some of the flexplates have dual sets of holes for the different converters, If yours is OE, it probably doesn't, but a A.M. replacement flexplate should come with both sets of holes.
 
i would just geta new fly wheel because your doing eveyrthing anyways, then have a th400 built with a strong worked shift kit and everything... then you can bolt up an np205 and do everything first, stronger then the 208 which would be worse for towing.
 
Keep in mind that ANY tranny could potentially leave you stranded. If it were me i would have a 700R4 built for your intended use. If you swap over to a TH400 then you'll need a different input in your t-case or another t-case. If you swap to a TH350 then you'll need one with the correct length output shaft and appropiate adapter.
 
Thanks very much again for the replies.

Would I have to fabricate a crossmember with a 400/205 or is there something I can take one out of (like if I find a pickup with a 400/205 will the crossmember work in the K5?) How about a front driveline out of a 400/205 pickup? (I assume mine wouldn't work.)

Thanks again.

Pat
 
You would need to fab up or modify a crossmember if you go 400/205. You'll also need the front shaft from a 400/205 vehicle and also need to find a new rear shaft as well.
 
or you can have yours cut and modified, (and balanced while your at it) at any driveline shop... the 700r4 has overdrive which helps when towing and everything along with the lower first gear (same with the 208) which gives better towing response and everything. the 400 would be slow off the line and would have higher rpms at highway speeds.
 
habitatxskate said:
or you can have yours cut and modified, (and balanced while your at it) at any driveline shop... the 700r4 has overdrive which helps when towing and everything along with the lower first gear (same with the 208) which gives better towing response and everything. the 400 would be slow off the line and would have higher rpms at highway speeds.
Click to expand...

A TH400/205 will use a much longer driveshaft so he won't be able to use his existing one other than for yoke parts.
 
i vote sm465, they last forever and are a dime a dozen, pretty cheap as well.
 
Well i'm in no way a 700r-4 fan but for what you're going to put into converting over to 400 turbo you're better off just gettin a 700 built for what you need. To be honest it truly is your best bet IMO. The 465 you have a lot of work entailed...trust me. I just did it.

So my advice-go 700 built and don't look back but you sound set on the 400
 
Go th350. Teckpak makes an adapter for you. Is all you need is a 2wd short shaft th350 and the adapter. You can reuse the stock mounting x-member holes, driveshafts, and everything. It truely is that simple. Just one cut with the sawzall on the tailshaft of the th350.

If you build a th350 correctly it'll last just as long as anything else you put in there.


One of the guys, Destinbeachman has a 500hp 427 with a th350 and is happy as can be with it.
 
habitatxskate said:
or you can have yours cut and modified, (and balanced while your at it) at any driveline shop... the 700r4 has overdrive which helps when towing and everything along with the lower first gear (same with the 208) which gives better towing response and everything. the 400 would be slow off the line and would have higher rpms at highway speeds.
Click to expand...

This is what kills overdrive trannies. Why would you use overdrive for towing unless you're going downhill? Overdrive has the smallest clutch pack in the trans and is not meant for towing.:doah:
 
Thank you all very much for the replies.

Gaffed, that teckpak adapter you mentioned is very intriguing to me. Is it part number 7q-35a that you are talking about? Before this post I have never heard of teckpak. Do you have any experience with this adapter? Does it appear to be a quality piece?

It is very interesting and I am going to do some more reading in that direction. Thanks for the information.

Thanks again to everyone for thier help.

Pat
 
That is the set-up that I am running. Mine being an 89' was originally a 700r4 truck. After it puked the second time I swapped to a th350. Of course I only have 2.73 gears so it runs great down the highway.

Another part of my set-up is that I run a manual valve bodied tranny. I didn't have to splice the vacuum line or anything but I know that stuff isn't hard. I also don't have a kickdown cable. But if you read on the teckpak site you'll find the directions for all that stuff. Its a very easy swap.

Is all you are doing is adding a spacer so its very simple and using the bolts that come with the adapter. Then your transfer case goes on just like it was going into your 700r4. Just make sure the th350 you get is the same spline count as your 700r4. It would be rare that they would be different but you don't want to find out the hard way.

The most problem that I had with the swap was splining up the transfer case to transmission. Since you've cut the trans output shaft its now flush and no rounded edge to help you align it up to the transfer case. It took us about 4-5 tries to get it to align right.

Oh and yes that's the part #. Just remember your not going to have overdrive if you swap in a th350. Try driving around in 3rd gear for a little while b/c thats what its going to be like. I would at least try and find a th350-c which retains a lock-up converter for the highway. I figure I dropped 1-2mpg when I did my swap. I drive the living hell out of mine. It goes on a 540ish mile round trip at least once a month and then when I go off-roading it rarely comes out of 1st gear(2.73's). I also spent $118 and got a B&M torqmaster stall converter(up to 2,200rpms) and then installed a huge tranny cooler.

I love my set-up but I do want to go either built 700r4 or 5-speed swap if I ever get the money.
 
Thanks very much for the information.

That kit looks like the direction I will go. We do not use overdrive now while towing and we are happy with the highway performance. When towing, we usually won't exceed 55-60 mph anyway. We are willing to accept a small decrease in un-loaded gas mileage in exchange for the strength increase. This Blazer has 3.54 (I think, maybe 3.45?) I might even already have a TH350 laying around for this!

We have already added a large B&M stacked plate cooler and Auto Meter transmission temp gauge, which we will keep for the TH350.

Once last question. I noticed there are 2 "kits" for this, the "quick kit" and the "full kit." I understand the difference is the full kit includes the shaft (so you don't have to cut your own) and also a new dipstick tube and dipstick. Were you able to re-use the TH350's dipstick/tube or did you adapt another one?

Thanks again for the information.

Pat
 
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