i have a 1990 GMC reg Cab Stepside i built the engine in it shes got about 400 hp but the Transmission hasnt been holding up since the first day i drove it i really didnt expect it too i wanna take her out when i come home on R and R everytime i hit the throttle hard she snaps into neautral anybody got any ideas of what i can do to just make sure she lasts i was thinking just find a junkyard tranny to beat on for the 2 weeks im home till i have the money to get a new TH 350 that will fit in place of a 700R4 which is what i have now  

Why in the world would you go to a 3 speed tranny? (many make this foolish mistake and replace a tranny with a 4.3 to 1 ratio range with one with a 2.52 to 1 range) There is nothing wrong with a 700R4 as long as you gear it right for tire size. Let me guess, it is lifted with big tires and stock gears and tranny is cooking. Also you will loose performance with a 350 because if has a 2.52 first gear vs 3.06 in 700R4 and a 2.08 vs 2.29 rev in 700R4. Also BTW, unless you are running about 10 to 1 CR with a cam that power peaks past 6000 RPM and 89 or better octane fuel, you do not have no 400 HP motor either. The real cure here is get another 700R4 and regear axles and it will run like a bandit and last too. Many never bother to figure out why 700R4's fail and never consider the big tires, stock gears and OD. Furthermore if you ran 700R4 in "3" rather than OD you would basically have a THM350 with a deeper first, second and reverse gear.  

like snoman said even with 400+hp that 700R4 should be fine. you got somethin else goin on there. too much tire and not enough gear maybe? that will eat up the best tranny. don't give up on a 700r4 you don't have too much motor for it.  

Im running stock suspension and 32 in tires but i didnt think about re gearing the axles thats somthing ill look into doing shes got the normal 3 73. see i dont know too much about the 700R4 what gear ratio would you think i need to go to to be able to help my problem cuz when i come home i think imma re gear the axles and buy a new 700R4 for her cuz you made a good point about the TH350 which i only wanted to go to cuz thats the Tranny i know about and trust  

Many do not realize that a 700R4 has a 30% OD which is pretty tall for a OD in a automatic and it can cause tranny to ride stall on lot in OD under load with bigger tires and stock gears. To some a 3.73 might sound okay with 32's but with a 700R4 it is a bad combo because the final drive ratio "looks" like about 2.56 to 1 in OD and places a lot of strain on tranny at times. A 4.56 would be a nice combo with a 700R4 and 32's or 33's  

Thanx for all the help man, see im a engine and suspension guy i really dont know anything about transmissions and axles, i was wondering to change my ratio to 4.56 i would have to buy all new gears right and if so do you know a good place where i can order them from the internet so there ready for me when i get home im in Iraq right now and i plan to be fixxing up the *** out of my truck when i get home im pretty sure my 700R4 should prolly still be good but i just need her to stay in gear when i mat the power down on that thing cuz in past id be going through a hole and i would mat the gas down and instead of just winding it would just go to neutral and the engine would rev up and now that i think of it it does make sense to change the rear end but i appreciate all your help bro  

Lately I have been getting gears on Ebay from a couple of vendors selling there. If I might make a suggestion, look for a 9.5 semifloater 14 bolt rear axle (not a 10.5 FF) to swap in to trucks rear as it can be had with 6 lugs and is about the equal of a D60 and a lot stronger than a stock 10 bolt and buy gears for it. Your 90 can have one of 2 versions of 10 bolt, the early one with 1.19 dia axle or later one with 1.29 axles because GM changed over between 89 and 91 and was not fully into newer one until 92. gears are the same in both though but spiders have different splines of the different axles. A 9.5 is pretty bullet proof up to about 38's so 33's or so would not be a issue with it while 33's and more is pushing it with a 10 bolt if you play hard. A 9.5 also has a 3 ton capacity rated housing vs 2 ton for 10 bolt.  

So a 9.5 Semifloater rear axle then buy the gears to put inside the new rear and matching gears to put inside the stock front so i shouldent have to change the front punkin as long as i have matching gears in both right. i really appreciate this imma look on ebay and see what i can come up with worst come to worst ill just use the stock 10 bolt i believe i got the 1.19 i totaled this truck 5 years ago and the guy before me replaced half the truck i think i remmember him telling me it has a 89 rear axle on it pretty much my truck has parts anywhere from 89 to 93 shes got 240000 miles but a new crate GM 350 with my rebuild on it with about 70,000 on the engine i thin kthe tranny has about 100,000 on her my truck has seen some *** lol but i love her shes my baby so imma look into finding the price for one of those axles thank you for all the help bro

oh one more question do you know what the front one was in 1990 its cool if you dont ill try to get one of my buddys to take a look at my truck to find out but if you happen too it would be awsome like i said i dont know to much about axles or transmissions,  

You have a 8.25 IFS drive axle in front. No other axle will bolt in and GM has used it on 1/2 ton trucks from 88 to present. There is not problems regearing it and there is no carrier break requiring different carriers or gear thickness for different ratios. Same with 9.5. BTW, GM has used the 9.5 from mid 70's to present day in various models and it was used in 6 and 8 lug versions (5 lug too in for old 454 1/2 ton 2wd suburbans and P/U's) If you find a 5 or 8 lug one, you only need to change axle shafts to convert it to 6 lug.  

Yeah i love my IFS cept the prices of lift kits lol im prolly for now just gonna go with getting the 4.56 gears but when i get out of Iraq and the army imma look into finding one of those axles when i lift and put bigger tires on the truck i couldent find anything on e-bay im prolly just going to look for  chevy 4.56 differentials at some junkyard when i get home theres millions of junkyards in my home town cuz i dont think i wanna pay the price of brand spanking new diff gears but if i can find some decent prices ill go for it im looking in 4x4 mags for companies that might sell something thanks for all you help man  

I would suggest if you plan to lift it more latter that you go with at least 4.88's instead. Even those gears are not too deep with 32's and OD tranny. BTW< you will have a hard time finding 4.56 geared axle in boneyard because it was not a factory option in 10 bot or 8.25 IFS.  

well the main plan for the truck of course if i ever get the money was a 6 to 8 in lift with the 33 TSLs then later on i was planing on converting it to a straight axle and trying to get a insane amount of lift on it  but thats years and years down the road but i like to think ahead and maybe your right then 4.88's if  there not 2 low for the 32s or the 33s im putting on her i might as well go with those then im pretty sure gear prices are gear prices anyways i just need to find a company or something that would sell me the gears i need so i can get them when i get home i have all the tools i need a hoist and everything in my grandpas garage i have everything there my great grandfather built the garage back in like 1920s. is you possibly have any sugestions on where i could find those gears i couldent find them on e-bay thanx bro  

4.88's with 33's will yield just a little over 2200 RPM in OD at 65 MPH which is really about right for good OD performance in  a lifted truck. You get much lower (RPM wise) than that and OD performance will suffer a good bit. Also it will make even 35's more tolerable as well (moreso than 4.56's)I would suggest you stay with 33's until you upgrade front and rear axle if you want it to hold up fairly well playing hard.  

hell yeah im looking into this Drivetrain warehouse to see there gear prices besides the gears what else do you thin kwould be good to change while i have the diff torn apart  

In the front, with a 8.25 there is nothing more to really do.