Cracked Flex Plate

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J

JWB

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Hi, I'm new to the forum. I have a 1969 Dart, 340, 727, that started making a racket underneath. I was able to crawl underneath and look at the flex plate and it is cracked. I thought that was the only problem, so I pulled the tranny and replaced the flex plate, along with the converter, along with a new front seal. Now, I have delayed engagement when going from park to any gear. Once I rev the engine it is fine and there are no other problems. I did not have to do this before. I did drain and replace all the ATF as part of this, and the dipstick reads full on a hot engine while in neutral. It appears to me that the front pump may have also been damaged and I wasted my time just replacing the flex plate, converter and seal? If it is the pump, what else do I need to replace so I don't do this again? The tranny has about 2000 miles on it after a rebuild.
 
I am beginning to wonder if the reason the flex plate cracked is because the transmission builder failed to install the bushing in the front pump to hold the converter centered. Otherwise I am unsure of what would cause the flex plate to crack in the first place. Are they that weak behind a little 340 that is not built? If that bushing was not installed, then the converter is only centered by the flex plate and it is going to flop around until the flex plate cracks, and then it really starts to move. The neck of the converter I removed was a little scored, but not that much.
 
69 is past the 68 large bore changeover. If the trans is a pre 68 small snout, then 340 may need a bushing. I broke one too before I was aware of the snout size difference. Was the convertor fully seated when you installed the trans, about 1/4 deep in the bell? Perhaps you damaged the pump if it didnt drop in twice.
 
pishta said:
69 is past the 68 large bore changeover. If the trans is a pre 68 small snout, then 340 may need a bushing. I broke one too before I was aware of the snout size difference. Was the convertor fully seated when you installed the trans, about 1/4 deep in the bell? Perhaps you damaged the pump if it didnt drop in twice.
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I do not think that is the issue. I do own the HP book on 727s and it states the converter should sit, when fully seated, about 1/4 to 1/2 past the bell housing, just like you say. Mine sat about 1/2 in. Then when I bolted the converter to the flex plate there was about 1/4 gap, and the book said only about 1/8 is appropriate, so I added two washers to each pad so the converter only moved forward about 1/8. So I am sure it was seated correctly, and I'm sure I did not pull it out too far to unseat it. The prior converter had two washers between the converter and flex plate as well.
 
Hmm. Are you saying the later transmissions (after 1969) did not use a bushing to hold the converter snout centered? If that is the case, then only the flex plate holds the converter centered, which may be the case, I just don't know.

I'm sure this is not the original transmission, because when I had the converter rebuilt the shop said it was a later style. I suspect it is a later model, like 1970s. I would think that they all used bushings to hold the snout centered, but maybe not.
 
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The flex plate only transfers rotational energy, the crank supports the convertor via the snout into the crank register.

The later cranks (post 67) used a larger crank pilot bore that the later convertors snout nested in snugly. Same as the pre 68 cranks: Smaller crank bore and smaller convertor snout. If you put a early trans onto a later motor, your snout will be swimming in the larger crank bore and it will not naturally center. Like a Mopar rim, it needs a center register, 'hubcentric'. It will eventually run out and put stresses on the flexplate ears that will crack them. The bushing is 1/8 walled to fit the early convertors onto the later cranks. 'Tflite Paddy' sells them as well as others.
 
Make sure the block has the line up dowl pins that go into the bell housing. Had one crack flex plates missing a pin. I have never seen washers used on converters.
 
Some converters have thicker pads than others. U should never have to use washers between the pads and flexplate. I have never once used washers in over 40 years. Kim
 
pishta said:
The flex plate only transfers rotational energy, the crank supports the convertor via the snout into the crank register.

The later cranks (post 67) used a larger crank pilot bore that the later convertors snout nested in snugly. Same as the pre 68 cranks: Smaller crank bore and smaller convertor snout. If you put a early trans onto a later motor, your snout will be swimming in the larger crank bore and it will not naturally center. Like a Mopar rim, it needs a center register, 'hubcentric'. It will eventually run out and put stresses on the flexplate ears that will crack them. The bushing is 1/8 walled to fit the early convertors onto the later cranks. 'Tflite Paddy' sells them as well as others.
Click to expand...

So the crank pilot bore holds the converter centered; it is not a brass bushing (in the pump housing) that centers the converter, correct? I'm sorry, but I have been reading about this on various forums and some of the threads (dodge only) state there is a bushing in the pump housing that holds the converter snout centered. Maybe that is on early years and when going from an early trans to a late engine? It may be that the converter snout (on the opposite end of the converter that goes into the pump) is undersize. This may have been the issue with why the first plate cracked, but not necessarily why I lack engagement when I go from park to gear now. If the first converter did not fit correctly into the crank, and ran out as a result and cracked the flex plate, then would that shaking also damage the pump and cause the delayed engagement now that I have a new flex plate and converter? I guess it may be possible? I'm just trying to narrow down the causes with my limited information.
So often when I work on something and after I put it together, I have a new problem, the reason more often than not is something I did while taking it apart or putting it back together. In this situation, I did not remove the pan or the valve body, all I did was drain, remove and replace the failed components. There is not a whole lot left other than some issue with the converter, although this could be an exception to my rule.
 
First off, the converter snout fits in the crank register. The converter hub fits in the pump. It seems like the pump is damaged. Did u install the tranny and converter the first time after rebuild? Kim
 
oldkimmer said:
Yes, move the converter ahead. But it may be too late now. Kim
Click to expand...
Okay, so it looks like the book was wrong and that is the reason why I cracked the flex plate, and also why I need a new pump. So incredibly frustrating that a person cannot even rely on a published book to have accurate, reliable information. Here is a picture of the page that I am talking about. It clearly says the gap cannot exceed 1/8.

thumbnail_IMG_1474.jpg
 
oldkimmer said:
First off, the converter snout fits in the crank register. The converter hub fits in the pump. It seems like the pump is damaged. Did u install the tranny and converter the first time after rebuild? Kim
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Yes, I installed everything the first time and the second time. It will be a third time now after I buy a new pump. Is there anything else I need to replace, or just the pump and the seal? Just to confirm what I think is true, once I bolt everything up, just pull the converter all the way up to the plate, which is about 1/4 inch distance, with no spacers. This will not pull the converter hub back out of the pump, but will allow the snout to be properly seated into the crank.
 
Your converter pilot should measure 1.81", and the 340 crank will have the a hole roughly the same. Measure both. According to my converter catalog in front of me, all 727s had the same size converter pilot. Different splines on the input shaft for sure in the earlier(pre-67) transmissions for sure. The 904 had two different size pilots, 1967-back 1.550", 68 and up 1.81".
Here's another shot, all be it a long one, was the engine originally in front of a standard transmission? I can't speak for Mopar, but I know when I worked for a GM dealer back in the day, GM had a crank for the automatic, and a different crank if the car had a standard transmission.
I would also think that if the transmission builder didn't install a pump bushing, it would have been leaking transmission fluid real quickly.
There is another issue that might be raising it's ugly head is the run-out of the converter hub. The shop I previously worked for aimed for about 5 to 10 thousands of an inch runout, while typical factory run out would be between 20 and 25 thousands. Just a thought.
 
I can't believe nobody recommended to do a pressure test at the low/reverse servo. You're looking for ~250psi or more at 1600rpm, in reverse. The spec is 230 to 300.
Reverse gear pressure is unregulated and shows max pump pressure. No sense pulling the trans swapping in a new pump, and then finding out nothing changed.
If the pressure is up at reverse, then your forward clutch may be hemorrhaging so off with the pan, off with the VB and air-pressure test is next. While the pan is off you can adjust the bands.

Did you re-connect your KD linkage?

But you know what; delayed engagement can also be low fluid lever or aerated fluid. Since you say it didn't do this before the flex-plate swap,and you say the fluid is properly checked and correct; I'd recommend to drive the car around the block a couple of times, to purge the air out of all the circuits, then recheck the fluid level. Top up as required, and recheck the engagement. If still delayed, I personally, would add a half a liter and see if anything changes. Why would I do this? Because 50 years is a long time for the dipstick to still be original, and all your clues point to aerated or a low fluid level.
If still delayed, THEN I would do the pressure tests.

BTW, I have never seen a convertor pull-ahead remotely close to 1/8. More often than not, it is over 1/4". IMO,There is plenty of machined flat on the pump-drive to accommodate that and I have never shimmed it down nor had a problem. Maybe I have always been lucky, IDK.
 
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transman said:
Your converter pilot should measure 1.81", and the 340 crank will have the a hole roughly the same. Measure both. According to my converter catalog in front of me, all 727s had the same size converter pilot. Different splines on the input shaft for sure in the earlier(pre-67) transmissions for sure. The 904 had two different size pilots, 1967-back 1.550", 68 and up 1.81".
Here's another shot, all be it a long one, was the engine originally in front of a standard transmission? I can't speak for Mopar, but I know when I worked for a GM dealer back in the day, GM had a crank for the automatic, and a different crank if the car had a standard transmission.
I would also think that if the transmission builder didn't install a pump bushing, it would have been leaking transmission fluid real quickly.
There is another issue that might be raising it's ugly head is the run-out of the converter hub. The shop I previously worked for aimed for about 5 to 10 thousands of an inch runout, while typical factory run out would be between 20 and 25 thousands. Just a thought.
Click to expand...
Thanks transman. I will measure the various diameters and make sure it is not sloppy.
 
AJ/FormS said:
I can't believe nobody recommended to do a pressure test at the low/reverse servo. You're looking for ~250psi or more at 1600rpm, in reverse. The spec is 230 to 300.
Reverse gear pressure is unregulated and shows max pump pressure. No sense pulling the trans swapping in a new pump, and then finding out nothing changed.
If the pressure is up at reverse, then your forward clutch may be hemorrhaging so off with the pan, off with the VB and air-pressure test is next. While the pan is off you can adjust the bands.

Did you re-connect your KD linkage?

But you know what; delayed engagement can also be low fluid lever or aerated fluid. Since you say it didn't do this before the flex-plate swap,and you say the fluid is properly checked and correct; I'd recommend to drive the car around the block a couple of times, to purge the air out of all the circuits, then recheck the fluid level. Top up as required, and recheck the engagement. If still delayed, I personally, would add a half a liter and see if anything changes. Why would I do this? Because 50 years is a long time for the dipstick to still be original, and all your clues point to aerated or a low fluid level.
If still delayed, THEN I would do the pressure tests.

BTW, I have never seen a convertor pull-ahead remotely close to 1/8. More often than not, it is over 1/4". IMO,There is plenty of machined flat on the pump-drive to accommodate that and I have never shimmed it down nor had a problem. Maybe I have always been lucky, IDK.
Click to expand...
Ok, I'll add an extra liter or so and see if that helps. If not, I was planning on doing a pressure test. Yes, all the linkage is hooked up. I won't worry about spacers anymore. It appears I was led astray on that item.
 
Half a liter. If it gets better then another half at most,; too high a level, and the spinning parts inside will just aerate the fluid even more.
BTW, what fluid are you using?
 
Has the tranny builder even rebuilt a 727 b 4? Do u know anyone else that had a 727 done by him and was he recommended by someone? If he is a trusted builder let him change the pump and install the converter. Did he measure the hub on the converter to make sure it isn’t too small? I bet a sealing ring is broken on the stator or on the input shaft. Kim
 
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