TH350 to TH400

[Locutus]

My truck has a fried TH350 in it. I was planning to have it rebuilt and beefed up a little, but a friend offered me a TH400 for free. I know the 400 is supposed to be tougher. What are the other differences. How much will it affect my rpms at 70-80(I am a volunteer firefighter so I occassionally do run 80 down the hiway). Right now around 70 I get about 3500 rpms. Will I use the same crossmember in the same location? I do know I will need a TH400 yoke. Is the length the same or will I have to get my driveshaft cut? I am trying to decide if the swap is worth the money and effort.

[CPNE]

Both are 1:1 in 3rd so rpms won't change when cruising. It's the same length so only slip yolk is needed, driveshaft won't need to be altered. You will need to hook up an electic kickdown, other than that it should bolt right in. Cross member is same and in same position. I've heard th400's rob more HP than a th350.

[Locutus]

I am not so concerned about hp. My engine is about 350hp @ flywheel. More than enough for a daily driver and to haul my race car and boat around.

[crazy longhorn]

there is a slight lenght difference between the short 350 t & the 400t. the 400 is 11/16" longer.......you may or may not need to cut the shaft. if your truck has a slip yolk in the center of the shaft, you may be able to "split the difference". Care must be taken to check that ther is enough clearance with the slipyolk in the trans that it does not bottom(or "slam" ) the tailshaft on the larger bumps/with load. minimum clearance ,with the weight on the truck(truck on the wheels/empty) should be right @ 3/4". slide the shaft in the trans(with new slipyolk), till it bottoms & mark it......when you pull it back to bolt up to the diff, & set the hanger bearing you should have 3/4-1" clearance. The crossmember is also the same, but not in the same position(the holes will probably be in the frame for the swap tho).Good luck,crazyL

[Randy70C-10]

I've heard that it takes about 30 extra hp to turn a th400, don't know if this is accurate or not though.

[jimfulco]

There are also different tailshaft lengths on T400's. Big GM cars in the early to mid '70's used one about 10 or 12" long.

[ira ashley]

You may have to get a cross bore starter. I converted from a 3 on the tree to a th400 to make the big trany work I had to get a cross bore starter. I believe it would be the same for the the th350 to the th400 (change starters).

[Locutus]

I thought the starter type was dependent upon how many teeth are on the flywheel/flexplate?

[COBALT]

That's true I thought. It depends on what motor the flexplate came off of or was attached to. Is it the 400 and/or the 454 that had different number of teeth??? Typically there's no issues with the 168 tooth for the small block.

A huge concern should be where the 400 came from. If it came from a 69-72 chevy truck you should be fine. If it came from any other chevy you might have an issue with the tailhousing. There were three different lengths. The chevy truck uses the shorter one for the most part. As long as the bolt pattern on the bell housing is the same you're off to a good start.

Sometimes the electric kickdown can be a pain. You'll either need to search for an original kickdown switch, or modify your gas petal like the '72, or go for an aftermarket unit. I had to do this and was lucky enough to find an original for a 2bbl, but when I installed my 1406 4bbl I went with a Lokar unit...

[ira ashley]

The th400 came from a 72 chevy truck; however I did hook it up to a 305 small block with 168 tooth flywheel.
I was told by a board member that all th400 must use a cross bore starter + you must use a 168 tooth flywheel.
He was right for mine.

[crazy longhorn]

Straight or staggered pattern on the starter, are related to flex plate size.......2 different sizes(153T takes a straight pattern starter, & the 168 t take a staggered pattern starter) . Generally the 400t has the larger converter & flexplate. the trucks will as a rule have the large flexplate....regardless of trans/converter. The 153 t flexplate was more something found on the cars. As Cobalt has stated, the 400sb, & the 454 will have the large flexplate(they are both externally balanced)......i beleive there are aftremarket small dia flexplate/flywheels available fo them tho. The rest of the sbs /BBs will take either size, & most aftermarket flexplates will have both converter patterns.....crazyL

[ira ashley]

I tried to use a small flexplate 153 tooth; the problem I had was the teeth on the flexplate was were you had to bolt it up to the torq converter (the small flexplate will not work) they may make a smaller torq converter for the th400 that will fit the small flexplate. Maybe someone on the board can tell us for sure.

[crazy longhorn]

Originally posted by ira ashley
I tried to use a small flexplate 153 tooth; the problem I had was the teeth on the flexplate was were you had to bolt it up to the torq converter (the small flexplate will not work) they may make a smaller torq converter for the th400 that will fit the small flexplate. Maybe someone on the board can tell us for sure.

I guess i wasnt quite clear on my last post The large dia (168t)flexplate will take either converter. Generally , the 400 t had the large converter & flex plate, but the smaller converter for the 350 t will fit the 400 t(will give a little more stall speed also). on the aftermarket 168 t flex plates, you will find both converter patterns. On the 153 t plate, you are right on Ira.....you can only fit the smaller converter. the larger converter has lugs welded on that are drilled/tapped for the bolts, wheras the smaller converter has 3 "wings welded on, & takes bolts & nuts to fasten the converter to the flexplate. im not positive on the sizes, but I beleive the stock converters were 12 & 13". crazyL