ltoolio

I got 1 rear one and 2 front ones from Roger, and they were identical.

Imo000

You didn't measure it correctly then. You need a dial indicator mounted parallel to the crankshaft axes.

worf928

Maybe I'm reading this wrong. But,....

you can't check crank end-play in a reliable manner that way.

The front pinch bolt needs to be removed so that the crank, flywheel and flex plate assembly can move as a unit with the drive shaft moving freely in the coupler. Otherwise you are measure flex plate flex not crank end-play.

And you lever the flywheel back and forth against the bell housing (with something between to keep from marking-up the housing) not the flex plate. The goal is to move the crank with respect to the thrust bearing in the engine girdle and then measure that movement. All other sources of 'measurement noise' must be removed.

worf928

I've been to OT.

Mongo

Yep that is one tool still missing from my set. Maybe Harbor Freight's is good?

M. Requin

For one that reads in thousandths - all that is required here - that's fine. For tenths you might want something better, and I have found Enco is a good source for moderately priced measuring tools.

M. Requin

Dave, I hope you don't think I've been prying on the flex plate! And although I do appreciate the point, if the rear clamp is loose, bolt removed, I don't think much "noise" is added to the measurement the way I have done it. That kind of noise, which would mostly be caused I think by friction in the TT, would tend to result in a noticeable variation in measurements as they are repeated, which has not been the case. (One of the worst sources of noise, IMHO, is poor dial indicator setup- it has to be truly rigidly mounted to the engine for best results.) My pics-to-come should make clearer how I approached this.

Black Sea RD

Good thread on why owners should not use Red Loctite on the front splines to arrest movement of the front flex plate coupler/drive shaft union.

To check crankshaft endplay correctly, the front coupler pinch bolt should be removed and the crank/front flex plate should then be able to travel back and forth on the splines without hanging up. Then using a dial indicator the full travel of the crank shaft can be measured more precisely.

If you really want to do it right, also remove the 6 bolts of the front flex plate attaching it to the flywheel and slide the front flex plate out of the way completely to let the crank be able to be moved more precisely without any friction from the front flex plate clamp on the splines which could interfere with the readings.

If the front flex plate clamp is stuck to the drive shaft splines this measurement cannot be done easily or correctly.

Removing the rear pinch bolt will also not work well since the drive shaft will then have to move back and forth through the torque tube bearings to allow for the crank shaft movement to be measured. This is could lead to false readings and misplace the rear pinch bolt cutout in the drive shaft to become misaligned to allow the rear pinch bolt to be replaced.

There are now a few different mechanical options at different price points out there to never have to use Red Loctite to arrest the front flex plate movement.

Imo000

The Harbor Freight dial indicator with the magnetic base is perfect for this.

M. Requin

These are all good observations and useful, and based on my experience so far I might not use Loctite again for this. "Might" only because I haven't broken the adhesive bond yet, since my local gas supply gave me an empty acetylene cylinder this morning in exchange for MY empty acetylene cylinder...

And I certainly respect your solution- I met you at SITM in 2008 which was, IIRC, when you first showed the clamp and bearings to 928 owners. (You won't remember, but we had a conversation over breakfast about some ideas I had about instrument lighting, UV, etc.) When I replace the TT/shaft assembly it will likely be with your products.

But for now, I do have one question: in your clamp installation instructions, what do you recommend in terms of positioning the clamp on the shaft? should the clamp be held at its rearmost position while the pinch bolt is tightened? is that irrelevant?

Thanks for your comments, I had hoped you would chime in.

M. Requin

Yep, that will work just fine, and probably the piece screwed into the base will be threaded 8mm, so you can use Leon Speed's suggestion of unscrewing it from the base and screwing it directly into on of the bell housing holes.

Black Sea RD

Hi Martin,

I do remember that meeting at breakfast, seems like a bit longer than 7 years ago now. Funny how aging bends time that way.

As to your question, we just ask for the crank shaft and flex plate to be pulled to the rear, but not artificially holding them there by force as the six clamp bolts are tightened.

The WSMs don't mention anything about this, just to clamp the front flex plate bolt as one of the last steps in the process of replacing the torque tube.

We defaulted to directing for the assemblies to be placed to the rear knowing how critical it is for any forward pressure to be exerted on the rear of the thrust bearings.

Good luck with your project and please keep us informed!

Kind regards,

FredR

Having previously checked my end play with the coupling free [preferable] and later locked and having taken the same reading of 0.2mm both times, the only problem I noted with taking the reading with the clamp in the locked position is that it is a two man job- one to pry & the other to read.

With the flexplate connected there is some spring pre-load to overcome and the crank has to be held in that position whilst the dial gauge reading is taken- a bit of a nuisance of course but nothing unmanageable as I am aware- trust I am not missing anything here.

If the coupling did not flex it would not be a flex plate. Remember it is the crank that is moving- not the drive shaft.

Rgds

Fred

M. Requin

I do not like to work on things I don't understand, and was having a hard time understanding the conditions that lead to TBF. I believe I have read almost every post on the subject on RL, which left me the more confused, the more I read. The latest discussions on the rear pinch bolt lead me to finally inspect mine, and what I found caused me to stop working and to try to figure this out once and for all. Here is what I found, and my conclusions.

First, I will note that in May 2009, at 140K miles (I had purchased the car in March 2007 with 131K miles on it), I released the flexplate clamp and observed 3.5mm of movement, and measured, with a vernier caliper, .008" of end play. I added Loctite 270 to the splined connection, and painted a "witness" mark on the clamp to spline surfaces. In September 2009 at 151K miles I checked with a DI and measured .006 end play, and no movement, and then in August 2012 at 180,000 miles I ascertained the flex plate was flat, no movement of the clamp on the splines, and end play measured .006". All along, before and since, eyeball inspection through the port on the bottom of the bell housing continued to confirm that there was no movement of the clamp on the driveshaft.

This week, at a mileage of 201K I decided to remove the rear pinch bolt for inspection, and measure crankshaft end play and flex plate flatness. I found the measurements to be .008" end play, and max .004" bow in the flexplate. The rear pinchbolt was centered in the inspection port as it should be normally, but was somewhat difficult to remove, symptomatic of movement of the clamp. There was no rust, and a small and apparently recent "gouge" on the shaft of the bolt, confirming this. The photo below shows this mark. To me this indicates a fairly recent movement of the rear clamp - IOW I think I lucked out.




So what does it all mean? Please note that the following comments apply to an 86.5 automatic transmission car. Thinking about the geometry of the driveshaft in relationship to the forward and aft couplings, I concluded that the shaft is located in a fixed position within the drivetrain by the rear coupling, as this coupling is non-adjustable. Given that things are in order here, the front clamp is adjusted so that the flex plate is flat, i.e. in a neutral position, and can respond to axial forces transmitted by the drive shaft in both directions. The forward clearance need only be enough for an oil film to be maintained between the crankshaft and the thrust bearing, but the flex plate must have enough movement aft to accommodate the shortening of the drive shaft under acceleration induced torque. Then if both clamps are sufficiently tight, as the driveshaft shortens and returns to normal length, the flex plate accommodates all the movement, and if clearances are proper, there is no problem. These and the following observations of course are just my opinions.

However, if the front clamp is loose, the driveshaft will pull out of the flex plate when it shortens, and when it returns to normal length, will push the flex plate forward, reducing the end play of the crank shaft, and after sufficient iterations, finally push the crankshaft against the thrust bearing.

If the rear clamp is loose, the driveshaft will similarly pull out of the rear clamp, causing the rear pinch bolt to distort as the drive shaft contracts, and allowing it to ride up on the splines of the shaft.

So really, either pinch bolt can cause TBF.

Next I'll post some measurements, and pics of what I have done with my car, now that I think I understand the mechanical system I am dealing with.

Comments, snipes, rude jibes - all are welcome!

FredR

Martin,

I think you have called it spot on. You will likely get different opinions but each to his own etc.

Personally, i have no intention of disturbing my Loctited flex plate until r/r of the tube is called for which could be any time of course. At such point in time I will go for Constantine's super bearing kit and probably his clamp if I am still financially viable!

I am about to replace the rear pinch bolt because I am convinced that is a good thing to do every 10 years and seemingly your photo suggests the same- looks as though you might have just dodged a rear end bullet.

Good to see you have a historical record of previous measurements.

The only point of note from your post is that the oil film thickness between the crank and thrust bearing is largely irrelevant- that is a given for the type of oil you use. A certain amount of clearance is required to allow for expansion as things heat up and that is probably the lower end of the range [0.1mm]. Invariably the clearance appears to run at about the 0.2mm mark but even if you read something slightly higher I would not blink an eye.

Once TBF has occurred there will be no mistaking it I suspect. It seems the thrust bearing and crank rearward faces deteriorate rapidly until the crankshaft comes into contact with the casing webs and at this point the end float will be in excess of 1mm and the engine wrecked.

On my original GTS motor the float was about 1.3mm when I diagnosed the failure and had to break the news to the previous owner who as trying to sell the car. I made him a reasonable offer [all things considered] given I had my late S4 motor to put in it- he said he would consider my offer so I duly advised him to consider that my offer would reduce to the tune of $1000 per day and he could take as long as he wanted to think about it- done deal there and then!

Rgds

Fred