Jake the SSnake

iTrader: (11)

I've got a friend who has been working at a local tranny shop for about 12 years he knows his **** when it comes to th350s' and 400s'. So I am having him build me a 400. He says he can put straight cut gears in it apparently they came in hd 4x4s and dumptrucks.He says they are the same dimensions as the regular gears and weight just a different cut he's never used them in a build but wants to try them.They whine a little bit but that doesn't bother me my main concern is are they worth it? Are they stronger?And do they eat up more hp from frictional losses?.....Should I try it and see what happens or can anyone talk me out of it?Let me know what you think.

James B.

iTrader: (1)

The straight-cut gears are stronger. GM still uses them in the HD version of the 4L80-E found in medium-duty truck chassis. Those little Isuzu Diesels you see all over the place with small boxes or stake beds have them.
Straight-cut gears handle greater loads because contact is made evenly across the entire width of the gear as it meshes with the other one. With helical gears forces move back and forth across the width of the gears and they rotate. Also, less thrust load is placed on the thrust washers throughout the geartrain, especially with the planet gears.
You will find the straight-cut gearsets to be louder and have a little more play in them, but they are stronger and place less strain on surrounding parts.

I don't know the efficiency numbers. I would have to guess they are MORE efficient since thrust loading is significantly reduced.

Jake the SSnake

iTrader: (11)

Cool thanks for the info I think I will try it since it wont cost anymore.

Automatics

Straight cut gears are not necessarily stronger, as I have not seen either set shear themslves, but they just take the load off of the thrust washers. They do run slightly hotter, and eat a little more horsepower.

BOWTIE

I supposedly have straight cut gears in my TH400, but they are also supposed to be a lower gear ratio (2.73 or something like that compared to 2.20 or so for the stock ratio). They supposedly came out of like a Frito Lay/beer truck type truck (10 wheel type tractor trailer set up with an auto). This tranny has never been run yet, but we will see how it works eventually. All I know is, if everything is true, second gear should come pretty quick (TH400 with a low 1st gear set, trans brake, 5500 stall, and 4.56 gears).

john kilgore

gm made only one ratio, spur or helical, 2.48-1.48. l made many ratios for the general and hoped they would go into production, but thet never did. spur gears are very noisy good luck

BOWTIE

I am no tranny expert, but I believe you are incorrect. It is my understanding that the TH350 had the 2.48 (or close to that ratio) but that the TH400 was less. This was one of the reasons a lot of drag racers preferred the TH350 (in addition to the decrease in parasatic losses), in cases where they could survive. As far as who made this gear set, I have no idea, I always assumed they were factory GM gears unless they used aftermarket gear sets and installed them in new (well new back in the 80's) heavy duty trucks. The guy I got the tranny from owned his own tranny shop and originally built it for his 69, but then he wanted an old truck I had and we traded. When I decided to go with a transbrake and manual valve body, I took it to another shop and had them freshen it up and make the changes. That builder also commented about how he would like to have that ger set.

Badzracing

iTrader: (12)

The "t-400" with straight cut gears was called the turbo-475.

BOWTIE

So was it still the exact sane tranny except for the gear set? Or did it use a different case and stuff as well? Do you know what the gear ratio of that low gear was? Perhaps that is where my gear set came from??? Honestly, this is a new one for me, I had never heard of a TH475. I used to have a TH375, which from my understanding was basically a TH350 with a lock up T/C.

john kilgore

the 375 was a light duty 400 with only 4 clutches in the forward and direct. the 475 had 5 clutches in the forward and 6 in the direct. it also had a sprag in second. gm no longer makes spur gears they went to longer pinions and 5 of them. by longer pinions l mean from .745 long to .875 and these are helical. l hope that helps. by the way the 475 has the best valve bodies

S_J_H

Are you the same John Kilgore who makes the super lite th400?

john kilgore

yes, and man do you have a hot rod

S_J_H

Thanks, it's just a Hobby for me.
I recently built my own th400 and when I was researching various th400 modifications I came across your site. You have developed quite the specialized th400 transmission!! I am sure we can all learn a lot from you.

Steve

fastta

Straight cut gears, specifically in the planeteries, are in fact stronger and they are more efficient in the transfer of power. There are less frictional losses associated with a straight cut gear as oppposed to a helically cut gear.

john kilgore

you are almost right. you have to take into consideration the ratio of the gear set. power-glides would NEVER work with a 2.50 first gear, but with a 1.76 the thrusting is so low it is fine. plus you must take into consideration cost of manufacturing. cutting a straight gear is cheaper by leagues then helicals. my internals are cut in detroit, no one in so cal has the tooling to cut internal helicals.

ZCode400

John Kilgore,

I am aware that you can use the TH475 planets in a TH400. My friend Tim Beebe turned me onto that years ago. However, he also used a sprag, some roller thrusts and clutches from a Allison (AT540???) if I recall and that was his recipe for a stout TH400.

I always use a 4-Speed, but have been wanting to gather parts & info along these lines for a TH400 I am building for customer in the near future.

Could you set me up with a list of the parts that will interchange???

Thanks for your time...Robert 10851man@gmail.com