RocketRonnie

Hello. I'm in the market for a bop th400. What's the difference between a 68 and post to mid 70s th400. I've heard that the valve body in the 70's are more responsive. But interchangabe so that's not an issue. But is there anything else I should know about let's say a 68 th400 (from a firebird).
Are they weaker?

matt69olds

Sometime in 1969, Hydromatic made a design change to the center support by machining 0.040” off the locating lugs. To make up the difference a 0.040 thick snap ring was fitted in the case, the center support would sit on the snap ring. The change was made to prevent wear in the case from the center support smacking the case when the intermediate clutch applied. About the same time GM decided to remove the lube restriction to the intermediate shaft. It’s a small freeze plug with a 0.060 hole drilled in the center. It’s easliy removed by knocking it out with 1/4 inch all thread. The bearings were also upgraded with more rollers, if you order a bearing kit it will include the later better design bearings. Around 71 hydromatic changed the design of the direct drum from a smooth roller clutch that took a 16 element sprag (easily upgraded to a 34 element with no changes) the later drum use a roller clutch which is much weaker and can’t be upgraded. In the mid to late 70s the pistons were changed from cast aluminum to stamped steel, they are pretty flimsy and aren’t the preferred choice for performance application. The accumulation piston in the valve body was change from aluminum to plastic, Don’t put the plastic piston back in! Replace it with a earlier aluminum piston. If it isn’t broke on tear down it will almost certainly break when it’s put back in service. There were also lots of valve body calibration changes. This covers most of the big design changes.

matt69olds

Also forgot to mention the switch pitch 2 speed torque converter used from 64-67. The converters are hard to come by, you can use a standard converter if you swap in a standard forward clutch and into it shaft assembly, and a standard pump.

midnightleadfoot

did olds use that or was it a buick only thing?

RocketRonnie

So the 68 and 69 are more prone to cracking bellhousing from what you said.

When you said "Around 71 hydromatic changed the design of the direct drum from a smooth roller clutch that took a 16 element sprag (easily upgraded to a 34 element with no changes) the later drum use a roller clutch which is much weaker and can’t be upgraded"

What years have the weaker un-upgradable clutch or drum?
Thanks for all the good info.

RocketRonnie

i think olds did it in thier big cars. I know cadillac did it in fleetwoods

matt69olds

Buick and Olds used the switch pictch, I don’t know if Cadillac or Pontiac did. None of the cases are more likely to crack than others, some of the big car cases have some extra webbing around the upper 2 bolts. I guess the absolute best core to use would be a 69 (good center support) thru 71 (earlier had the good drum). The 4l80e direct drum built from 89 (I think) to 98 used a direct drum that came factory with a 34 element sprag.

RocketRonnie

thanks for your time Matt. Sorry to bug again but the 69-71 had the good drum?

RocketRonnie

There is a 69 in my area too, look at that.

don71

I didn't read where anyone brought up the length. This alone might me important to one's choice of Th400, for their given application.

Valve body calibrations already considered, The 400 was Gm's heavy duty automatic of the day.

If the fluid runs clean and not smoked or burnt, put a new front/rear seal and maybe a pan gasket,and run it. You'll know what you have pretty quick.

RocketRonnie

Thanks you Don.

george landis

This is to Matt69. Sent this to Joe sometime back:


Re: switch pitch torque convertors


Quote:
Originally Posted by george landis
Joe, Dacco Detroit carries variable vane TC's. They are in their catalog on page 149 for the B-2 13" for the T400, and on page 160 for the B-1 12" for the ST300 "Jetaway. They were $100 each delivered to my local transmission shop. I have the B-1(12") behind my 400E/Th400 and it's high stall is 2800 rpm. I thought the price was great so I bought both the B-1 and the B-2.

That's great info! Thanks for the heads-up. Have a great Thanksgiving.


__________________
Joe Padavano

RocketRonnie

Ok so if you end up with late 70's th400 what is one to Do? Upgrade to stronger pistons and ditch the plastic parts?

matt69olds

the best option would be use early aluminum pistons. They are getting kinda hard to find, my parts supplier sells them almost as fast as the get them, and most of the aftermarket performance trans builders don’t sell them or make their own. If it’s a stock rebuild or street performance a better option to the stamped steel would be 4L80E bonded pistons. They are much thicker, with the seals bonded to the piston. Make sure you get the return spring retainer, the stock 400 stuff won’t fit. Use bonded pistons from the 4l80E direct drum in forward and direct clutch of the 400. You will need to eliminate the seal in the forward clutch housing, it’s no big deal other than a slightly firmer engagement into drive. If your direct clutch doesn’t have a check ball in the drum, you will need to drill a vent hole. With the drum on the bench sprag side up measure from the outer diameter .410 inches and drill a 1/16 hole, you either need very steady hands or a drill press. If you measure accurately the drilled hole will be just inside the machines sealing area of the piston. The very small leak is no big deal, the 3-2 downshifts are much “cleaner” and the small spray of oil helps keep the sprag and 2nd clutches cool. Hope that helps.

Vistabrat72

I don't think Pontiac ever did use the switch-pitch in their 2 or 3 speeds of the day. As stated, yes, Cadillac used the switch pitch, 1967 being the last year, in at least the Fleetwoods. I do know somebody who had a Coupe De Ville, 1967 with this feature as well. It was a clean, unmolested car so I am going to lean to the fact other Cadillacs would have the Switch-Pitch too. The THM-400 is pretty much the cat's meow for strength and reputation of the 1960's three-speed units. Switch pitch sweetens the deal, love it on our '66 Ninety-Eight.

RocketRonnie

I am still stuck on the whole drum thing. What's the importance of the drum. Why is one superior over the other? Why can't they be swapped or upgraded( or can they).

Update.. I passed on the 68 and the 69 and am now looking at a 73 from a 455 pontiac. So the drum comes into consideration when thinking of purchasing it.
Thanks for everyone's time.

matt69olds

The 71 and up direct drum uses a roller clutch with a race that has steps for the 8 rollers to ride on, the 70 and earlier use a drum with a smooth race for the sprag to ride on. The 71 and later drum can’t be upgraded. They are reliable to around 400-450 hp. The sprag style drums have 16 sprag element, the aftermarket offers sprags with 34 elements that will easily handle 1000hp. When the 4l80 trans came out GM went back to the smooth race drum, the later 4l80 drums also have 34 element sprags factory. Your 73 core will be a excellent core to use once you add either a early 400 drum and 34 element sprag, or a later model 4L80 direct drum. If I knew how to post a picture I would show you the difference in designs, google 400 direct drum interchange and you will find plenty of info.

Are you going to rebuild this transmission yourself? If so, there are a ton of cheap durability upgrades you can do easily. If your doing it yourself there is no need to buy a shift kit, it’s very easy to get a firm shift with nothing more than a couple cheap setscrews found at any decent hardware store and a freeze plug from NAPA.

RocketRonnie

Matt thanks for sticking with me till I got it. I am going to be rebuilding it myself. I have purchased a HD th400 off a RV just for the gears and have that as an option to beef up my future bop th400..
So the drum,,,,
Correct me if I'm wrong, the direct drum can not be upgraded but can be tossed for the smooth style drum.
I'll be reading up on "direct drum interchange" thanks. Last night I was going to buy the 73 but just had to leave untill I knew more.

Thanks again

matt69olds

Make sure the RV has a conventional extension housing and output shaft. Some have the parking brake mounted on the extension housing, it looks like a big brake drum.

Some RV use straight cut gears. The straight cut gears reduce the force the helical gears exert on the case and bushings. The straight cut gears are a little noisy compared to helical gears, but they will work just fine.

RocketRonnie

Thanks for all your help. I ended up purchasing the 73. The RV transmission I have does have a break drum on it. I am interested in saving money on the shift kit and much more. I'll be purchasing a book soon to help with the rebuild. Thanks man