[dennisthemenace]

I'm looking for recommendations on rebuild kits to freshen up and upgrade my '68 TH400 transmission behind a '71 Pontiac GTO 400 engine. The engine has 8.5:1 compression, so it's not that hot at the moment. Eventually the engine will be reworked, perhaps with a 455 Eagle crank, so I want the transmission built to suit some future HP improvements. It also needs to be comfortable to drive on the street. My wife will drive the car too, so the automatic stays.

There was a moderate vibration in the mid rpm range that went away when the torque converter was disconnected from the flexplate (3 bolts removed) so I'm guessing that the converter has a balance problem. That is to say, the engine revs smoothly through it's range with the converter disconnected from the flexplate. It had this vibration in park as the engine was reved and car not moving. It does this in drive and in motion too. A new harmonic balancer made no difference at all, which is why we tried disconnecting the converter.

If this vibration can be caused by something else, let me know. I don't know how long the car was driven with this problem. The problem may have been caused during an engine swap or minor accident. The rear main seal leaked badly too. Would a bad rear main bearing cause vibration problems?

I don't see any cracks on the flexplate. I've been told that the Pontiac 400 is an internally balanced engine. Stuff has been swapped around by previous owners so there might be wrong parts installed. I figured I would replace both the converter and the flexplate.

So, I need recommendations for a replacement converter and a performance TH400 rebuild kit. I've considered the kits at 400rapture.com among others. The technical video on the TH400 and Haynes transmission books show the steps and the rebuild looks practical.

Performance wise, I want it quick off the line and with nice firm shifts. I want it to downshift quickly when the pedal is mashed.

I'm also looking to buy a kickdown switch that mounts up to the quadrajet. The previous owner seems to have lost that part. Where can I buy this part?

Should the stall of the converter be specified stock or for a higher rating? Is there any point changing helical cut gears for straight cut?

TIA

[Everett#2390]

The rear main seal is the effect of an out-of-balance engine. Later, if driven hard and long enough, it can flatten out the rear main bearing.

Overhauling a THM400 is pretty straight forward, alot easier than a THM350 and a Powerglide.

Some torque convertors are cheap and this is what you get, cheap convertor, not balanced very well if any.

IMO, since the young lady will be driving it, I'd go for an 11 inch convertor, 2200-2600 rpm. I'd install a shift kit and a deep oil pan with drain plug.

I'd contact a fellow member, Oldani, and ask him for assistance and parts.

There is an electric switch and with bracket bolts next to the gas pedal and is used for down shifting a THM400.

[Oldani Motorsports]

"Overhauling a THM400 is pretty straight forward, alot easier than a THM350 and a Powerglide."

Lol...I can think of a few hundred who would beg to differ with this statement! IMO the Glide is one of the easiest trannies to go through. You have two separate endplay clearances to set with the TH-400 and TH-350, and many more parts to stuff in the case. As to a converter, stay conservative with stall speed as you have a torque monster there that will absolutely die with too much stall speed, trust me on this one....and, the OEM gearset, etc. is FINE, no need to waste money on an aftermarket. The only upgrades to beef up a TH-400 for even a pretty stout street or street/strip combo would be the early drum with 34-element sprag, and possibly if you really wanted to make it nice, a Torrington to set rear thrust vs the OEM washer and shim setup. For very strong stuff use a later 4L80E steel forward clutch hub, as I have seen them break in race applications using the OEM cast hub. Pay attention to the reaction carrier surface and rear band too, along with the int band if you have an OEM or manual/auto valve body setup. If you need more info on stuff, just shoot me an email and I would be happy to help you out...

[dennisthemenace]

Thanks for all the help. Steve, I appreciate your comment about building the TH400 vs TH350. I think I can do it with the video and book handy by, but not in my sleep and certainly not quickly.

Everett, it looks like the rear main seal leaked for quite a while because the inside of the bell housing was filthy with dust and oil. The underneath of the car is filthy too, but I guess I shouldn't complain because that means it's not rusted.

I was thinking that the previous owner made some sort of assembly mistake or parts substitution goof that caused the vibration problem when he swapped the engine out. He wasn't much on details.

I was wondering he might have torqued the converter unevenly or without it centered up on the hub. I guess I should have tried rotating it 120* and trying again but I didn't. Oh well. Everett, when you say the leaking rear seal is a symptom of an engine balance issue, do you mean specifically the engine or are you including the assembly of the flex plate and converter? I think the engine is internally balanced, but I don't know this as a fact. Anyway, I'm sure he continued to drive the car with the vibration problem which is worst right at crusing rpm. Is there likely to be other damage besides rear main bearing wear or perhaps the pump bushing? I'm hoping there is nothing cracked around the main.

The transmission worked and there was no burned odor with the fluid which looked pretty good. I can't say if it worked well because the car was not safe to drive. We drove it on and off the flatbed, but that was all. The pitman arm joint is literally falling apart and the A-arms are sitting on the steel cross shafts. The brakes don't hold fluid either. I didn't test drive because of these problems. It did pass state safety inspection four years ago, lol.

Steve, I'll give you a call concerning some parts and a converter. I'm not sure how to measure on a converter; this one seems to measure 13". The improvements you talk about are the sort of thing I had in mind.

[Everett#2390]

You said when the t/conv was unbolted, the imbalance went away. Your t/conv is out of balance, not the engine and the engine included the flex plaste. Remember?

The rear main bearing, if the imbalance continued and/or is tremendous, will eventually flatten the bearing. Will it damage the block? Probably not, at least until the imbalance wore away the bearing enough to pound on the bearing saddle. But, I'd think the noise would be so loud, no stereo could drown it out....LOL

Pontiac's used a rope seal till mid-70"s? I do think there is a replacement lip seal, from Fel-Pro, I believe. Only problem is the engine has to be removed from the car to remove the oil pan. At least this is what I had to do to a 70 GTO I had.

To my defense, I did a clutch plate changeout to a THM400 and everything was right there behind the oil pump for replacement of the clutches and you don't have to disassemble the extension housing. This is why I said its easier than a THM350 and Powerglide. THM350 has those stinkin' snap rings on the end of the shaft to remove, requires two scribes and alot of patience, plus the little clip at 5 o'clock position. Powerglide has to have the rear housing come off to replace low-reverse clutches. This means the speedo gear has to be removed, mine's a press-fit, then remove the governor, and finally the shaft comes out from the front.

To each, their own. I still like my Powerglide.

[Crosley]

I'd have to say the glide is a bit easier to rebuild than a t-400 for a novice. Although any of them can be a challenge for a novice. You need to check various items for wear in the gear train of the t-400. Some of the pinion gear pins in the panetarys sometimes move out and rub things they should not rub.

C-6 bronze pump bushing , t-350 torrington pump bearing and shims make for a nice smooth back end of the t-400.

I machine a groove in the C-6 bushing to duplicate the OE t-400 case bushing for lube flow

[BillK]

Dennis,
If you dont mind an hour or so drive, call Fred Frey Sr at Fred's transmissions in Marlow Hgts, just south of Washington 301-423-4302. Fred does all his own converters and is great to deal with. He might just be able to balance yours. He can also do the transmission. If you want someone close, ATI in Baltimore is the lick, but they are really geared more towards all out race stuff.
As far as your engine goes, the Pontiac V-8's are actually externally balanced in the back only. The flywheel does not have a weight on it, but has some big holes on one side which makes the other side heavier. Sounds to me like you problem is in the converter though, not the engine.

[dennisthemenace]

Bill, your correct. I didn't see the forest for the trees. I was so busy inspecting the flexplate for cracks that I didn't notice that there are a number of 5/8" or so balance holes near the perimeter on only one side. There are no welded on weights or drilled balance holes and that is what I was looking for.

As Everett reminds me, the vibration stopped with the converter disconnected, so the converter is the problem. I was thinking a sloppy rear main would be sensitive to the extra mass of the converter. I guess if it were in bad shape things would sound lots worse even with the converter off. It actually sounds quite good with the converter disconnected. I agree the converter is outta wack so I'll focus my attention there. What are the pros and cons of having a converter opened and repaired vs buying a new one?

My mechanic looked at the oil mess inside the bell housing and believes the transmission seal is bad. There's not much to debate though since both engine and transmisison will get new seals.

Everett, I think they did use rope seals in the Pontiac engines. My first '68 Firebird was delivered new with the rear main seal leaking. I expect they have two piece lip seals that slip in place if you can get the pan slipped out of the way. With the engine already out, it should be easy.

[Everett#2390]

As BillK suggests, give Fred a call and listen to what he suggests about reworking the old t/conv or buying a new one.

Myself, I'd buy a new one for its warranty.

I looked at my GTO from underneath to see if I could slip the pan out of the way. I asked another Pontiac owner about it. Both the owner and the book suggested if the oil pan has to come off, remove the engine. Ask around, maybe times have changed.

[dennisthemenace]

I'll see what Fred says about the converter and the pump. I'm inclined to go with a new converter. I'm guessing the center hub of the pump on the inside is not supposed to be missing a tiny chunk either. A small bit about 1/8" x 3/8" is missing and it wasn't in the pan.