doug928

I thought I'd check my Flex Plate for deflection since I'm redoing my Exhaust and have run into the following trouble: My flex plate was deflected almost a 1/4"
After losening the coupler bolt, the flex plate moved back only 1/8". There is still about an 1/8" deflection on the flexplate.

What does anyone recommend? Should I just tighten the coupler and hope for the best or does anyone know of a way to further remove the deflection in the flex plate? Would leaving the flex plate as is and torquing the coupler leave me at a higher risk for a thrust bearing failure?

It seems to me as though the flex plate has permenently deflected to this position because the coupler was completely loose.

doug928

OK, So after some thought, I put a crow bar behind the flex plate and pushed the plate towards the rear until I had no deflection on the plate. Then I tightened the coupler bolt. So, does anyone have any comments? Has anyone experienced a flex plate that hasn't moved back to no deflection after releasing the coupler bolt? Could this mean that I might have some thrust bearing problems?

perrys4

I am no expert but I can relate what precision did to mine. Almost same thing. They did use a bar to pull it back before tightening but how much I dont know. Mine was pretty flat when the released the pressure. Replacement? There was a plate on ebay couple days ago for 15.00 bucks. I think you are on the right track. 1/4 inch isnt that much.

borland

Your torque converter bearings are shot. Time for R/R of the torque tube and converter bearings.

Nows a good time to check the crankshaft end play to see if the engine thrust bearing is also shot.

jserio

Doug, My flex plate was mashed against the front. When pressure was released mine went back about 3/8". Heavy duty. I almost dropped a lung when I saw it. Greg check the thrust bearing and believe it or not but all was okay. He used a screwdriver to take the belly out of my flexplate and tightened the coupler. Check the thrust bearing by seeing if there is any play in the crank. Greg put a screwdriver behind the balancer and checked for movement I think. If the play is miniscule to none then your good. Just put the plate even and tighten the coupler up.

jserio

for 70 bucks I am checking mine once a year now.

Chris

My 91 was like that, it had a 928intl mark on the TT of 95 so I guess someone didn't do the installation correctly. Lucky thrust bearings were fine, I had the flex plated replaced.

Chris

Bernie

My gawd!!
Give the guy some hope will ya!

Also check to see that the rear pinchbolt is torqued to spec. I recently did a post on this ---- too lazy to find it though.

From what I have learned, 1/4" is not a complete diagnosis of TB failure. Remember it also matters how long it has been sitting that way. Still, check the crank endplay. If it is out of spec, button it up like a terminal cancer patient and drive the thing until it gives up the ghost ......IMHO

Garth S

Doug,
The idea behind loosening the pinch bolt on the flex plate is to eliminate residual tension: The flex plate is almost always found ( for the first time observer) to have advanced along the prop shaft towards the engine for 2-4mm: As the crank end float is of the order of 0.15-0.6mm in a healthy engine, it is obvious that the crank is under a residual tension that is pushing it ahead against the rear face of the thrust bearing.
Usually, this is of no grave concern - as long as the crank end float is as specified as above.
Therefore, it is highly advisable to do as you noted in your first post: loosen the pinch bolt, and let the flex plate relax to its neutral position: If I were concerned that it had not relaxed completely, with my own car, I would leave the bolt loose and run the engine a bit ( wheels elevated, etc). Next, I would measure the end float, attempt to center the crank within that 'float' - and torque the pinch bolt to 65 ft/lbs.
I would NOT crowbar the flex plate backwards and then clamp down the bolt - for this will serve to leave an identical, but reversed tension on the crank. Let the plate find its own center.
Regards measurement, I have no idea how you can get 1/4" =6.4mm. When I measured mine from the edge of the flex plate collar to the machined 'ring' on the beginning of the shaft taper (to the splines) - the released amount was precisely 3.15mm. After 12 months, and the loctite 290 procedure, it has not moved at all, nada, zero! I would not know that unless a precise measurement had been taken and recorded. Good luck.

doug928

Thanks to all. Although my initial thought was to let the flex plate find it's own equilibrium point, I decided to set the flex plate with no deflection. My thought was to recheck in about 6 months and see what's up. I also marked the gear teeth on the shaft as to where I set the Collar to also get an instant visual when I do a recheck. I figure that there will probably be some movement back towards the "neutral" position. I didn't have a dial indcator to take accurate measurements but there was no noticable play in the crankshaft. I still have to check to rear pinchbolt.

borland

For crankshaft end play measurement, a dial indicator and mounting hardware can be purchased from HFT for about $20. Calipers can't be used to perform this check accurately. The WSM provides a procedure and specifications for the crankshaft end play measurement.

Since the crankshaft has a tendency to want to harbor itself at either end of the end play, you really need the dial indicator to center the crankshaft before tightening the pinch bolt, however if your torque converter bearings are shot, it really doesn't matter, cause the spline won't stay in one position for very long.

The reason your flex plate became compressed is due to the spline moving forward on the drive shaft. What's not obvious, is why this happens. Elastic torsion of the shaft does not elongate the shaft length, however:

As the torque tube bearings wear out, excessive clearances allow the driveshaft to vibrate off-axis. The torque converter bearings are deep groove bearings, so when the shaft and it's bearing race hits the ball and apposing race, impact forces are transmitted axially causing the shaft to vibrate. The torque tube bearings could also create similar forces, but the rubber mounting significantly reduces there impact.

The forward spline, being attached to the flywheel by spring action has more resistance to movement in one direction than the other. The pinch bolt provides sufficient clamping to prevent relaxation of any built up pressure.