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C6 ruined by 393C

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Here is a problem most people never have. I installed a 393 C stoker in my 71 mach and after about 10000 miles noticed abnormal wear and noise from the timing chain and gears  which I removed for closer inspection. At that time I noticed by accident excessive motion in the crank shaft. Clunk clunk back and forth so I measured the motion with dial indicator at .055 in.  WTF Waayy too much motion back and forth free play. What is causing that! I could only guess it was a problem with the thrust bearing.  So off came the oil pan and the main cap at the thrust bearing journal. The thrust bearing wore a groove into the side of the crank thro next to the  main journal which caused excessive space on the side of the thrust bearing and allowed excessive  front to back motion at the crank. My $ 600  forged crank was toast  and the motor had  to be rebuilt. I am no mechanic but could not understand how this could happen. Talked to a few people who had no idea and a few who said there was a problem with the C6.   Talked to  a middle aged ford transmission  mechanic and asked if a C6 could have a problem that would allow continuous pressure on the back of the crank which would cause abnormal wear in the thrust bearing after 10000 miles and his answer was……yes. My damn rotten luck . When  I built the car I had the C6 rebuilt at a trans shop and ordered a mild shift kit installed. Something got fu____d up. How would you know if you had this problem  from the start.

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When you bolt the transmission to the engine you need to make sure the converter is not in a bind. You should be able to move the converter back and forth at least 1/8" to 3/16" before bolting it to the flexplate. If the converter does not have free play you will either damage the front pump on the trans or, as you have experienced, destroy the thrust bearing.


This is not the only reason for thrust bearing failure although it is a common one so I wouldn't assume anything until you get it apart.

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73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.

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Went through something similar on my rebuilt 429 and C-6. I know I had the correct play in the converter when I bolted them together. I didn’t get 10 miles albeit, over 6 months time frame on the car. Had the car at my engine builder for unrelated reason, he noticed a leaky trans pan and decided to fix it while there. He found small shavings in the bottom of the pan... As I mentioned, less then 10 miles on the car just from moving it around over months.


Trans had to be tore down, faulty front pump... that had been originally replaced by the trans shop in the original rebuild. Trans guy would not honour warranty as he said it was installed wrong. My engine builder confirmed that the trans/converter had been installed correctly and the issue was with the rebuilt trans... At the end of the day, the trans guy only charged me his time to rebuild, about $700. Hard to prove after the fact who/what was responsible.


I changed out the original converter 2nd time around for a higher stall unit. So far, so good, although less then 50mi over two year’s on the car since the second rebuild of the trans.


That really sucks, about your engine... Mine cost me $18k to rebuild, don’t want to think about having to redo it because of a faulty transmission. Good luck in the rebuild.

"Yes dear", has kept me in the hobby a long time...

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I have heard of this from the torque converter ballooning. If the trans is making too much pressure from the pump, it puts the excessive pressure into the converter and it pushes forward and expands. This puts pressure on the thrust bearings of the motor. Eats the bearings away, ruins crank journals. Most torque converts have anti-balloon plates to combat this. If the trans rebuilder had way too much pressure it could easily wipe out your crank. Do some googling of torque converter ballooning. There’s lots of info out there about it. Your symptoms seem like ballooning of the converter.

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1971 Mach 1

408C Stroker - C4 w/3,000 stall - 8.8" Rear w/3.73's - Disc brakes all way around.



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Sorry to hear of your problem. One which I have gone through before and currently now for the second time.


As others have stated it is probably trans/converter related although don't rule out the engine yet

Important that the trans is flush with the block before tightening bolts and you have at least 3/16 converter movement before tightening converter nuts

Another thing with the converter binding is to check the converter snout where it enters the crank spigot, should be 0.10 clearance

Some aftermarket cranks and/or converters can have a mismatch causing binding


The other thing is although not likely is your new motor had a spigot bush installed for a manual trans but doubt you would even be able to bolt the trans up


Would be a good idea to test your trans line pressure and make sure your cooler lines are not blocked or flowing freely

Here's an article on this subject - https://www.motor.com/magazine-summary/thrust-bearing-failure-prevention-analysis/


Keep us posted with your progress

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