Crackshaft Thrust Problems!!!

-
sir-quackers

sir-quackers

"One lug nut short"
Joined
Oct 19, 2008
Messages
2
Reaction score
0
Location
Alberta, Canada
I need some help please.
I took my engine apart to find the #3 main bearing & thrust surface has been mashed and chewed to the point that the crank thrust cheek is badly damged as well.
I am trying to find the root cause of this, can anyone offer suggestions/advice to remedy/prevent this from happening again.
Here is my related car/powertrain info:
  • 67' Dodge Dart 270
  • 70' 340 block w/ stock crank internally balanced
  • MP flexplate w/ 7/16" bolt holes
  • TCS 2700 stall converter w/ 5/16" flexplate bolts
  • A-727 auto w/ stage II shift kit
  • Amsoil full synthetic fluid
  • B&M Star Shifter (ratchet action)
My long block was built approx. 10 years ago by a machine shop that has gone out of buisness since that time. I only put about 1000 miles on the engine/trans since I put the car together 8 years ago. During the time I drove it, it always was running very hot, (on a hot day in traffic, it would bury the temp gauge to 250 F easily at the lights). I run a big alum. Griffin rad w/ a 19" flex fan & custom SS shroud, MP 8 blade water pump, etc. but the engine still ran hot.

This is was prompted me to tear the motor down, which is when I found the chewed bearings (thrust bearing surface & scored journals/ crank cheek). This has lead me to believe I have a trans/torque conv. problem but I would like a second opinion from anyone offering. Here are some of my theories as to what caused the excess forward pressure on the crank:
  • improper crank end-play when engine was built (should be .002" - .007")
  • balloned torqure conv.
  • improper pressures/blockages, internal problems within the trans caused by poor rebuild or bad driving habits (although, I have never raced the car, only street driven & never revved past 5000 rpm)
  • wrong flexplate used (I currently have been using a MP 7/16" bolt hole FP on a 5/16" bolt Tq Conv.)
  • the back of the crank flange where the recess is for the torque conv. nose/hub has a tapered and filleted inside profile that does not match the hub/nose profile of the torque conv., thus causing the torque conv. to have no room for movement back and forth cancelling out the purpose of the flexplate in the first place.
I am currently leaning towards my last theory, as I have measured, impression tested, and paint transfered the gap (or lack there of) the inner profile of the crank flange recess to the outer profile of the torque conv. hub and have found it to be too tight. In fact, the torque conv. actually rocks/pivots in the middle back and forth when I place the crank on end between two level tables w/ the flexplate and torque conv. in place. This tells me that there is contact being made in this location rather than contact being made from the flexplate to the mounting bosses on the Tq. Conv.

Is it a good idea to machine/bore out the back pilot recess of the crankshaft flange to better accomodate the TQ. Conv. nose/hub? I am also thinking about getting a diff. converter (3000 - 3200 stall) anyways because I think it is the wrong stall for my application:
  • 8 3/4" rear w/ 3.9:1 gears
  • Speed-pro flat tappet hyd. cam w/ 300 dur. 230 @ .050", .480" lift
  • 6-pack carb system (makes for further sluggish acc. until i get it tuned just right)
Am I on the right path folks? Thanks.
 
Lets start with the crank problem. In 10 years after it's rebuild, how often does this see agressive driving and or raced? A problem like this is odd for a grandma driver and would show up very quickly after a rebuild.

I'd just rebuild it and be aware of everything as I put it toghter.
 
Like Rumblefish said rebuild it.You should start with a 4 bbl. try it out,then work your way to the 6 pack set up.You may not approve, but you want driveability right??Take steps and eventually the outcome will be worth it..
 
Your problem is the convertor. Period.
get a flexplate that fits whatever convertor you're going to be using. bolt it to the crank. See if convertor will slide into the recess on end of crank. If not you have a problem. Mike them both and see where the problem lies. It might be possible to clear out any tight areas with a grinder/dremel. Do you have any pictures of the end of your crank and/or convertor?
 
What was the thrust measured at when it went together? (I'm guessing you don't know?) Many things can hurt the thrust. The hub indexes the snout of the convertor. They do nut sit tightly in place. The flexplate is the hard connection, the hub simply aligns the convertor and there should be some play there. How hard was it to bolt the convertor to the flex?
If the crank or rods were not machined correctly, they can cause the crank to walk forward or back in the block. Enough taper accross a few journals will make a wedge that "leans" the crank onto the thrust as the engine turns. So can not enough side clearance in the rods. What does the wear surface of the main bearings look like? What do the rod bearings and he sides of the rods look like? Any signs of heat?
 
I put a 340 in my Dart in 2003 and it ran hot in traffic just like that. I was disappointed with the track times also hoping for a 12 second car it would run 13.30. I pulled the motor and found the exact same problem you did. The thrust face was wiped out and was scraping the block. I didn't have any difficulty installing the convertor and when I removed it from the car it spun freely. I sent a e-mail to Rick from Mopar Action and he mentioned it was probably to much end play to begin with. The next short block I purchased was from Indy Cylinder Head 415 small block. This was its 4th summer. It has well over 100 passes on it now at the track and plenty of street driving. No problems with anything on the new engine. Jysnflem

milan1.jpg
 
I just disassembled my 408 and found the same problem. My thrust bearing surface on my Eagle crank is fried as well as the bearing. I had no problem with the flexplate or converter fitting in the hub of the crank. Although you can see where the hub of the converter has bottomed out in the cranksaft female end. Any ideas? I'm afraid my crank is fried. Can it be machined and run an oversize thrust bearing?
 
Is anyone looking at the pump in the trans to see whats going on in there? Might be a clue as to whats going on with the thrust bearing.
 
Linear push of the torque converter is nature of the beast. Sole purpose of a flex plate is to protect the thrust bearing.
OEM flexplates vary in thickness / strength , and get weaker with age like everything else. Aftermarket performance torque converters are the common culprits.
 
Hi all, here is an update to my first post initiating this thread.

So far I have had my crankshaft sent away to Gopher Machine in MN. to have the mains ground down .010" and the conn journals ground down .005" to clean up the surfaces. When I got it back I was a little dissapointed to see the finish of the surfaces was not a 'mirror' finish but rather a little dull. After talking to Gopher Machine they assured me that it is how they finish their grind jobs, so I have decided to go with it.

I also had my crank machined out in the back recess in the middle of the flange to clean it up too and give extra room for the torque converter nose/hub, as it was definatly making contact with the crank recess, eliminating any job the flexplate might be doing.

As to the cause of this issue, and any issues you folks may also experience regarding this, I am in the process of disassembling my 727 to see if there was anything funny going on with it as well. I suspect it will be fine and not the root cause of too much pressure put on the thrust bearing, but I live by being safe rather than sorry.

I am also currently looking into getting a new torque conv. (TCI, Hughes, or J.W. Perf.) in a 2400 - 2900 stall.

I had the engine built by a business that turned out to be quite un-reputable (found out after the fact) so I can imagine the end play of the crank was out of spec. For my new rebuild I hope to have a crank end-play of about .005" to .007", and bearing clearance of about .002" to .003".

For those of you with similar problems I suggest:
  • Research torque conv. flexplates until you are blue in the face, find out which combos work together and which do not
  • Measure the clearance between the torque conv. hub nose and the recess at the back of the crank flange. There needs to be at least .050" between these components to allow for forward movement of the torque conv. and for the flexplate to 'flex' under this load, thus allowing it to do its job it was intended for
  • Make sure you set your crankshaft endplay, it should be no less than .003" and no more than .010"
  • Double check to make sure your torque conv. is fully positioned and seated onto the input shaft and into the pump of the auto transmission
  • With a dial indicator, measure the input shaft of the transmission endplay, it should be between .026" and .053"
  • Another good point that someone brought up is mains and conn. rod journal taper that could cause forward or rearward walking of the crank.
  • And one more issue I thought may play a part in #3 Thrust Bearing damage might be not having enough clearance for your driveshaft yoke to slide in and out of the tail housing/output shaft of the tranny... but this would lead to other major problems besides thrust bearing issues.
I hope this helps anyone, feel free to add to this or blast me for anything that may be incorrect.

I will be starting the rebuild this weekend and hope all will be well this time around.
 
You must log in or register to reply here.
-

Back
Top