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transmission TH400 not efficient


cquisuila
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before MY TRANSMISSION WAS GOOD and the gears were passing

 

but now nO !!

 

i heard noise as a clac clac....

 

AND when i removed the cover that covers the flywheel and the converter i found a bolt broken

see photography please

 

image.png.8a0fb2714957ca7b7e825b7af064cf92.png

what's the problem please ? IS it serious ? Give me your advise please ?

 

Philippe from France

 

 

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i think diameter 3/8 - head 11/16

 

it moved before but

i heard noise as a clac clac....(i thought that it was the engine)

and after a big noise

and now nothing

: R D L1 L2 nothing

the rear wheels don't move

 

 

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My guess is something bad happened to the flex plate that attaches the torque converter to the crankshaft.  Either the flex plate failed (cracked) and sheared the bolt(s) or one or more bolts came loose and damaged the flex plate.  Can you rotate the engine by hand through 360* to inspect the flex plate and torque converter?  If you are lucky, the parts will be OK and only the bolts will be broken.  Even then, getting the remainder of the bolts out may require pulling the transmission...

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2 hours ago, EmTee said:

My guess is something bad happened to the flex plate that attaches the torque converter to the crankshaft. 

EmTre is on target here...  I'm not sure but I believe 4 bolts connect the torque converter housing to the flexplate.  In order for the converter housing to not turn, all 4 "connection points" must be severed. 

 

As was mentioned, in order to have sufficient clearance at the back of the flexplate, the transmission will probably have to come out.  

 

I'm sure those bolts are grade 8... 

 

Keep us posted.

 

Paul

Edited by pfloro (see edit history)
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2 hours ago, EmTee said:

Can you rotate the engine by hand through 360* to inspect the flex plate and torque converter?

Doing this in increments will tell if the torque converter is rotating as well. If not, all 3 torque converter bolts may have sheared off and it moved aft out of the crankshaft pilot hole (disengaged).

The engine cranks on start and runs?

Flexplates usually crack around the 6 crankshaft bolts producing an awful rattle. The transmission would still function however.

I believe Flexplates can be changed with the torque converter disengaged. But, transmission removal is recommended for inspection & FOD. (foreign object damage)

For 1966, should still have the 13" Switch Pitch Torque Converter, a plus!

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12 hours ago, cquisuila said:

i think diameter 3/8 - head 11/16

 

it moved before but

i heard noise as a clac clac....(i thought that it was the engine)

and after a big noise

and now nothing

: R D L1 L2 nothing

the rear wheels don't move

 

 

Definitely a torque converter bolt. The trans will need pulled to see what's going on and assess the extent of damage. Thats not a common way for a ST400 to fail. Did anyone recently work on the transmission? See if you can get a picture of the torque converter and flex plate and post. 

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20 minutes ago, JZRIV said:

Did anyone recently work on the transmission?

Or was the engine removed recently?  Removing the transmission bolts around the bellhousing before unbolting the torque converter from the flexplate could have damaged the flexplate.

 

7 hours ago, pfloro said:

I'm sure those bolts are grade 8... 

Yes, they should be GR8.  The bolt head in the pan looks like a GR5...

 

Hex Cap Screws | stainless steel hex socket cap set screw

Edited by EmTee (see edit history)
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There had to be some odd noises happening BEFORE this happened & it was dis-regarded or maybe not knowing anything was wrong.

IF the other two bolts are still in there, remove them & slide the converter back. It should give you enough room to try & remove the remainder of the bolt as they don't usually seize in place .

IF the flexplate is not damaged too badly you could just put in new bolts. They are, IF I remember correctly, 3/8" x 16 with an 11/16ths. head about 1/2"-5/8ths. inch long or so. 

Any decent trans. shop should have or be able to get some. Upon installation use Red Loctite.

IF the flexplate needs to be replaced it can be done WITHOUT pulling the trans.  Get two 7/16th. bolts, course thread, about 4-5" long. Remove two side lower bolts from the trans. & install the bolts. Leave the heads on the bolts so the trans. will not slide past the heads. Dis-connect the trans. mount bolts & remove. The trans. linkage mounting will now also be loose. Remove the rod shift lever from the side of the trans. Put a jack under the trans. pan with a 10"x10"x1/2" piece of plywood & jack up the trans. to get it off the mount. 

Remove the 2 center support support bolts & the bolts from the rear of the driveshaft & just let it hang there. Don't forget to mark the relationship of the driveshaft to the rear pinion.

Now remove the other 4 bolts. No need to dis-connect trans. lines as they will move back with the trans. 

You may have to loosen the dipstick tube & remove the bolt, BUT IF I remember correctly one of the bolts holding the trans. to the engine holds it in place.

Get two big screwdrivers or pry bars & move the trans. back. When it is far enough back use a 2 pieces of 2x4 to put between the trans. & the block.

Now you can remove the flexplate. Pay attention to the guide hole between the crank & the flexplate for engine balance.

 I don't think so but I may have forgotten something.

IF I remember I will add to this post.

 

Tom T.

Edited by telriv (see edit history)
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thank you very much  all !!! 🤎

i will see with your advise !!!

 

 I HAVE TO rotate the engine by hand through 360* to inspect the flex plate and torque converter;)

 

the broken bolt is grade 5

 

my converter turns easily

 

the engine was not removed recently i think

 

my question : Can only a broken bolt cause the drive shaft to no longer drive the rear wheels ?

Edited by cquisuila (see edit history)
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At least one bolt was GR5 so those were not original to the car.  If the bolts were loose I could believe the torque of the engine and inertia of the torque converter would eventually cause the flex plate to slice the bolt heads from the shank.  If the torque converter spins freely now, then there's no coupling between the engine and transmission and the car won't move.  Make a careful inspection of the area and look for the other bolt pieces.

 

Even if the driveshaft and wheels can turn, the torque converter will not.  As Tom T. said above, there must have been a lot of metallic rattling coming from that area right before the last bolt was broken...

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In the position of the converter bolts you do NOT want them to be grade 8 as they are too hard & have very little flexibility. 

Grade 5 is the CORRECT type bolt as it needs the flexibility.

Grade 5 is what they originally came from the factory with. Who are we to second guess what the factory did or why??? It has lasted for 50+ years with no problems on yours & MANY other vehicles.

 

Tom T.

Edited by telriv (see edit history)
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12 hours ago, cquisuila said:

my converter turns easily

TC rotates with the crankshaft or, independently?

 

8 hours ago, telriv said:

converter bolts you do NOT want them to be grade 8

Tom, down to details. How about Crankshaft bolts, gr. 5 as well? In case any one of 6 originals are AWOL.  Locktite on those but not on the three TC bolts?

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What I meant was if TC is rotating with the Flexplate (w/starter ring gear) - red arrows. (bolted)

 

Internet image shows TC dis-engaged from the flexplate (pushed aft) - blue arrow. (unbolted at X, 3 places for service work)

 

Maybe unbolt the remaining TC bolts, slide it aft a few CMs and inspect.

image.png.3e6bf73808624bdff9c417a0da2c6b10.png

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2 minutes ago, XframeFX said:

What I meant was if TC is rotating with the Flexplate (w/starter ring gear) - red arrows. (bolted)

 

Internet image shows TC dis-engaged from the flexplate (pushed aft) - blue arrow. (unbolted at X, 3 places for service work)

 

Maybe unbolt the remaining TC bolts, slide it aft a few CMs and inspect.

image.png.3e6bf73808624bdff9c417a0da2c6b10.png

OK i will see this point !! :)

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If the flexplate to torque converter bolts are broken or missing the crankshaft is not connected to the transmission, so the car will not move because the pump in the transmission is not spinning.  Turning the rear wheels will not spin the torque converter because the converter drives the pump which generates pressure to make the hydraulic servomechanism operate.

 

The task is to confirm that the torque converter and flex plate are disconnected and then determine whether there is any permanent damage to the flex plate or torque converter.  If those parts look OK, then you need to collect any remaining bolt pieces from the bellhousing area and reattach the torque converter to the flex plate with new bolts appropriate for that purpose.

 

Before running the engine, you should spin the torque converter by hand (or disconnect the coil and bump the starter) to ensure there is no interference between the bolt heads and anything else.

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5 hours ago, cquisuila said:

when i turn the rear wheel (left or right) the converter doesn't rotate

when i turn the converter, the wheels don't turn

 

There’s no mechanical connection between the two. The connection is hydraulic (fluid) and until the fluids are working, there’s no synchronization.  You’ll never see anything like what you’re looking for doing what you’re doing.

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On 5/28/2022 at 2:17 AM, telriv said:

IF the flexplate is not damaged too badly you could just put in new bolts. They are, IF I remember correctly, 3/8" x 16 with an 11/16ths. head about 1/2"-5/8ths. inch long or so. 

 

Tom T.

i think that you are agree with the bolt i found :)

 

i found this information in the workbook

image.png.b25d27841a7f846761d321ebedb00644.png

image.png.7cb293b535dea696766a93cb69a6e670.png

where are the specifications for the converter ?

SORRY but i'm french and i don't find the converter here ;)

 

Edited by cquisuila (see edit history)
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i sawn my converter

in fact there isn't bolt

so no liaison with the crankshaft

see photo please

 

in fact there was only one screw left

The latter unscrewed and banged (my noise CLAC CLAC....) and finally broke

 

I ordered 3/8 bolts  lenght 1/2-5/8 (i verify on my car...)

 

and i think that i will solve the problem

i hope !!

 

 

convertisseur le mien.jpg

Edited by cquisuila (see edit history)
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It looks like you will have to use a file to flatten the flexplate around the bolt holes.  I see some metal sticking up where the bolt thread was caught in the hole.  Unless the area under the bolt head is flat you will not be able to properly tighten the bolt.  Also, check that the bolt heads of the bolts that you use do not interfere with anything.  I haven't dropped the pan to look at my '67 Riviera, but sometimes the original bolts have a thinner head than standard hardware in order to provide clearance.  I second Tom's call for red Locktite.  You don't want to risk losing those bolts a second time...

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There has to be a trans. shop by you someplace???

A regular 3/8ths. bolt will ONLY have a 9/16ths. head. Not large enough to do the job required of it. 

Although it will work in a pinch don't get over confident.

 

Tom T.

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1 hour ago, RivNut said:

You should consider that if you found bolt heads in the pan, you might find the remnants of sheared bolts still in the converter.

Yes when i turn i don't hear a noise of broken bolt...

where are the broken bolts?

perhaps when i removed the covercle i haven,t noticed the broken bolts and they lost on the floor

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6 hours ago, NailheadBob said:

Here is new torque converter bolts from Corvette Central.

Small purchase to send all the way over to France but worth it. I might search out these bolts for myself.

 

It is a set of six - for a truck TH-400. Actually good. I would weigh all 6 and pick 3 most matching for weight. The TC bolts are on a 15 cm radius. Too lazy to do the math but 13gmX15cmX3200 cruising RPM = Force and would be a considerable imbalance if a bolt was missing!

 

Use of red locktite was stated. No lockwasher here because it would raise the head and cause clearance probs to the block. Perhaps in this situation, the bolt worked loose, backed out and sheared off?

 

The 3/8"-16 X 1/2" pictured is off an original TH-400 13" Variable Pitch TC from 1966. Original bolt? It has a 9/16" head.

image.png.a9b5fb426eba0db530a1df45ef0498a8.png

 

 

 

 

 

 

 

 

Edited by XframeFX (see edit history)
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3 hours ago, telriv said:

Those bolts appear to be flexplate to crankshaft bolts.

Tom, you have a point. I automatically assumed 6 bolts in the Corvette Central Set to be for a truck TC. Many TC bolts online come with nuts and might be correct for aftermarket TCs but certainly not OE.

 

Set of six 3/8"-16 X 1/2" Grade-5 TC Bolts:

https://www.corvettecentral.com/68-77-torque-converter-to-flex-plate-bolts-533078

image.png.b81ea837fcc64c831637b7aa867b994b.png

The dirty bolt image I posted yesterday is indeed off a 1966 Buick TH-400 torque converter, all 3 the same. But who knows? Previous mechanical work might've used crankshaft bolts. They are also 3/8" UNC?

 

 

 

Edited by XframeFX (see edit history)
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It was difficult to get a picture of the flex plate to crank bolts on the engine of my 64, which is on a stand. This engine has never had the crank out or the flex plate off. 914C56BC-EA27-4221-B30E-251D96D583EC.jpeg.b149bb44176f1f39e64c42821545e59e.jpeg I’ll go out tomorrow and look through the sandwich bags for the converter bolts.

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1 hour ago, RivNut said:

picture of the flex plate to crank bolts on the engine of my 64, which is on a stand.

3 ticks on 7/16"-20 X 5/8" crankshaft  bolts indicates Grade-3. Has a 5/8" Hex Head.

Yet to find 3/8"- 16 X 1/2" grade-3 TC bolts with a Hex Head other than 9/16" on any of my variable pitch stash.

 

Not intending to beat this to death, but I scrutinize all rotating fasteners especially if on a substantial radius. To keep out of trouble, I always try to re-use original bolts.

 

I will be using Russ Martin's Flexplate and pilot hole adapter on my 'Silver Nail' when swapping out my Dynaflow. These bolts will be on-the-radar at that time.

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