The th350 tranny in my 78 k5 blazer wont stop leaking atf!!  I leaks out from between the transmission and the transfer case adapter where the crossmember bolts to.  last week i removed to HEAVY np203 to look at the tranny and all there was, was the rear output shaft and hollow casing, i didn't see no seal or stuff like that so i just put silicone on the mounting surfaces of the tranny and t case adapter thinking this was all it needed, but after running it, it slowly leaks though not as much as it did before i serviced it.  I went to napa and asked if there was supposed to be a gasket there but they said no, so why is my tranny leaking and how do i fix it, i really dont want to have to pull the HEAVY t case out more than i have to!!!Please HElp btw the way the truck has 400 ci v8  

there's no gasket......there is a o-ring seal that belongs between the case and adaptor. I generally use silicon also along with the old seal if I don't have another one to replace it with. the t-case side (203) has that big ol' paper gasket that you have to spend a little time scraping off if it's been there a while.  

i saw the o ring ur takling about and its in there along with silicon, so my tranny is leaking because the silicone didn't seal all around or what?  I cant believe i have to pull the HEAVY t case out again! ugh!  BTW do you know what i would have to do to install 700r4 instead of 350, like will the 700 bolt up to the 203 or will it require new shafts and what about the adaptor can i use my existing one?  What else would  i need besides driveshaft, will i still use my stock crossmember.  Thanks for ur help  

I was at one time going to swap a 700 into my 75' blazer, still hasn't gotten done, I'm pretty sure there is a kit for that, I made a spacer block and cut down an output shaft, just never finished putting it together, the 350 is still plugging along. the easyiest would be to use a 208 or 241 t-case, and a stock adaptor on the 700, now i'm thinking about it again...I'll see if I can find that kit.

 

using a 208 would be alot lighter and would get rid of full time four wheel drive but is it as strong as 203?, i really dont want to lose 203 becuase im trying to use least amount of money and buying another t case dosn't sound cheap, but please keep me informed of that kit your talking about  

Have you considered that there may be a crack in the adapter that is flexing under torque and causing leakage????  

I did consider that maybe i had a crack but i looked and found nothing, Anyways anyone here done a 700r4 swap before?  I feel like using this leaking tranny as an excuse to get the 700r4.  Isn't the 700r4 electronic, so do i need a stand alone computor for it?  Where would be a good place to find a 700r4, Should i find one in the junkyards and have it rebuilt or should i just buy a rebuilt one, If i have it rebuilt what should is ask for to make sure it is able to withstand 4wd abuse of 35 inc tires.  I read there are certain upgrade from later models that can be install to older model to make them stronger.  Also what is the difference between a 2wd  and 4wd 700, i have access to a 700 2wd for cheap what would it take to make it into 4wd.  PLEse give me info i am serious about this swap.  SO PLEASE REPLY WITH ANY INFO. Thanks            

no electronics on the 700, the 4x4 case is stronger in key areas.  

So i should forget about the 2wd 700 and find a 4x4700 and rebuild it?  I heard someting about a tv cable, what is it?  specifically what do i need to do to mate 700r4 to np203, including adaptors, crossmebers, shafts and splines...  WIth the 700r4 od and 35 inc tires i think and 4:56 ratio in the axles shold be about right?..now have 4:10 and 35s with th350.  INFO IS GREATLEY APPREICIATED!!!     THANKS  

http://www.transmissionpartsusa.com/350_4X4_To_TH700_R4_4x4_Switch_Kit_p/609-00000370a.htm


about the only thing that stinks here is a 208 or 241 can be found cheaper than the switch kit. A good skid plate makes the 241 nearly as durable as a 203.  

Geo450racer:
So i should forget about the 2wd 700 and find a 4x4700 and rebuild it?  I heard someting about a tv cable, what is it?  specifically what do i need to do to mate 700r4 to np203, including adaptors, crossmebers, shafts and splines...  WIth the 700r4 od and 35 inc tires i think and 4:56 ratio in the axles shold be about right?..now have 4:10 and 35s with th350.  INFO IS GREATLEY APPREICIATED!!!     THANKS

I would say a 4.56 with a THM350 is about "right" and a 4.88 to 5.13 with a 700R4 with 35's. Remember that truck came with about 29's and a 4.10 with 35 is like a 3.42 with 29's, not a good power combo. You now it is more than just equally gear ratio here and bigger tires and lifts take more power to roll so you want to be deeper in a effective ratio than before and if you think you might save a little gas with taller gears and 35's, guess again. If you are worried about MPG you will not run 35's and a lift. Also a 5.13 with a 700R4 will cruise at about 2250 RPM in OD or about 300 RPM LESS than a THM350 and a 4.10 and have awesome power in lower gears too and about 35% more power in reverse too (deeper reverse and deeper axle gears) Your first gear starting ratio before converter stall with 4.10 and a THM 350 is 10.32 to and figure on 2x from converter so about a 20X torque boost in first gear at stall. With a 700 R4 and a 5.13 you have a 15.7 to one plus a 2x converter boost for 31X at stall (50% more power!!) In second gear it is 6.2 over all for THM 350 and 4.10's and 8.2 for 700R4 and 5.13's. Are you getting the big picture here yet? You can have your cake and eat it too!  

I know that if i was really worried about my mpg, i wouldn't have a lifted truck with 35s, but im just trying to get the most mpg i can out of this setup, ur suggestion of 5:13s sounds really deep to me for only 35s, i want to have some high speed ability in the truck and with 5:13s i don't think it will go over 90mph, i know a lifted truck isn't an ideal high speed rig, but i only have a 4 inc lift and i don't want to have a truck that tops out at90mph, im thinking 4:56 or 4:88 at most, any ways i looked at that site mud posted and i only know what 3 or 4 of the things listed are, can someone help?  Is this all that is need to get 700r4 to hook to np203 in place of th350?  THe kit dosen't say anything about a tv cable and i still don't know what it is???  

LOL.....actually at 3000 rpm you' be doing 87mph (on paper) and will do 145mph at 5,000rpm if your motor has the balls.

I run 4:88's with my 34's, they aren't "deep" enough, I thought they were to low as I was trying to make up my mind on the correct ratio, after driving it the "shoulda coulda, woulda" was rolling off my lips, It'll be getting 5:38's in a couple of months now after running 4:88's for 3 years. power to weight ratio,(uless your motor is over 350hp) the jeep has more power than the blazer.

with 5:13's @65 you'll be at 2250, the 4:56's will still be riding the stall and the 4:88's marginal, and will be constantly shifting between 3rd and 4th at the slightest incline or throttle presure,... If you drive over 70mph for extended periods of time, then the 4:88's may be best for you. I'm sure the 5:13's will score the best mileage also for normal driving.

back to the kit,.. it has everything except the tv (the detent may be refering to he tv though) cable as it appears, and has the output shaft, adaptor, just there was no photo in the listing. I mentioned before the cost is enough that I'd consider using a 241.  

Well put Mud! Some are scared of by numbers but then forget about OD and how tires size play a roll too. (35's with 4.88 will turn about same RPM as 29's with 4.10's) A simpler way to do this, if you have a 700R4 which has a pretty tall OD, you want to take tire size and divide it by axle ratio and if you have a answer of 7 or less you will be in good shape. (35 / 5.13 = 6.8) Also as stated above you will be turning about 2250 at 65 in OD and you do not really want to be turning any slower if you expect OD to really be fully usable with lift and tires. (I do not think 4.88 would do any better MPG wise at higher speeds because there is a LOT of drag to overcome and it takes power to do that and 5.13's would be best match for this. BTW, you could cruise 3200 RPM all day with a SB and not hurt a thing but for some reason many seem to think that it will blow up or something.  

hmmm..... u guys have got my attention about the gearing, your saying the 5:13s, are a perfect macth for my truck with only 4 inc lift and 35s, show me how u are commiing up with these speeds and rpms please , u might convince me to use 4:88s or 5:13s  im not afraid of my motor blowing up at 3000 rpms, i just afrain that crusing at that rpms will yeild bad gas mileage, im going to college soon and will put alot of freeway miles on the truck during the week so im looking for a good crusing gear cuz my offroading is high speed stuff not slow crawling.  Also what do you guy think is a cheaper and easier swap from the th350 the 700r4 or an NV4500 stick shift??