Steve Cattaneo

There have been many excellent threads on 928 engine rebuilding and modifications, by many talented Rennlist members, none of which have addressed what I think is the weakest link in a 928 engine, the crank thrust bearing.



Below are excerpts of a crank thrust bearing failure article.

http://www.aa1car.com/library/2003/eb50331.htm



“The crankshaft can be installed with standard bearings or oversize thrust bearings. But if oversized bearings are used, the crankshaft thrust surface must be remachined to compensate for the increased thickness of the bearing. If not, the crank may not have the proper amount of end play”.



“Overloading
AERA says a number of factors may contribute to wear and overloading of a thrust bearing:
• Poor crankshaft surface finish;
• Poor crank surface geometry;
• External overloading due to:
a) Excessive torque converter pressure;
b) Improper throw out bearing adjustment;
c) Riding the clutch pedal;
d) Excessive rearward crankshaft load pressure due to a malfunctioning front-mounted accessory drive.”
I often wondered if an over sized / heavy crank pulley in super charged engines would overload the thrust bearing.

Charley B

I wasn't aware there had been any incidence of 5spd TBF.

Steve Cattaneo

Automatic transmissions cars suffer the most; the purpose of this thread is for anyone rebuilding a 928 Engine, should consider modifying there engines crank and or thrust bearing surface for better lube flow, when the Crank thrust surface pushes on the thrust bearing, engine lube is cut off on poorly designed engines. This seems to be the case in 928s. With Automatic transmissions

Larry Velk

According to Jaguar "hot rod" books, the well known old XKE 6 is subject to thrust bearing failure which is attributed to racing clutches with heavy PP's. The fear was the dreaded 'clutch change' which is a big deal on an XKE (can't be worse than a 951). Guys over did the clutch hoping to never replace it and ruined their thrust face. The big warning the Jag guys give is it is hard to get someone to grind the face good enough. This operation must be super duper and most hacks these days can't get it smooth enough or at 90 degrees to the axis.

90 S-4

Steve,
Thanks for starting this thread, I didn't know you could modify the crank for oversize thrust bearings.
I'm in the middle of a motor build right now so this is perfect timing for me -- Thanks
P.S. can the MBZ 5spd auto be retrofitted to a late model S-4 trans axel ? Is there enough info on
that to start another thread, I definatly want to keep this thread moving full speed ahead... Yes there
are some really talented engine guys in this 928 community these days, but your the only really
knowledgeable trans guy -- thanks for looking after us.

Fabio421

Is there enough material there to retrofit with a roller bearing?

IcemanG17

Very interesting read......so lets see we have Timing belt failures, thrust bearing failures & 2-6 rod bearing failures (in track cars) to worry about......so for the ultimate in 928 worrying have an automatic track car!!!

goodspeed928

Hi Steve,
RE-CHECKED mine, all good.
Getting the pich-bolt to 65lbs was a BICH!

worf928

I'm having difficulty in visualizing TBF for a 5-speed 928 due to:
The t/o bearing isn't "adjustable." I suppose that it could be improperly installed, but I would think the clutch assembly would disintegrate before TBF.

The same goes for riding the pedal. Assuming that the clutch assembly is correctly installed, when you depress the clutch pedal the release arm levers towards the rear and releases the pressure on the disc. The intermediate shaft slides freely in the guide tube, is clamped to the shaft in the TT and cannot move relative to the flywheel?

How can either of the above conditions cause forward pressure on the crank (assuming a correctly installed clutch assembly)? I know you know a hell of a lot more about this than I do....

SharkSkin

Dave, the crank is pulled rearward, wearing the front thrust face of the bearing when riding the clutch.

Fastest928

Watch for high performance after market clutches....thrust bearing killers.

worf928

Doh!
Duh.
Some days I'm decidedly half-dimensional. I'm all concerned about forward pressure on the bearing and forget that it can move the other way too.

Ok. That 'splains riding the clutch pedal and monster-strong clutch.

How about 't/o bearing adjustment' Obi-wan?

Steve Cattaneo

They’re experience high performance engine builders out there with the knowledge; you just have to find them. I have a nice XJ6, I was going to do a LT1 conversion, don’t have the time and energy. I will have to live with the 6CLY and the Lucas electrical system, the prince of darkness.

Steve Cattaneo

If you mean the 722.5 transmission, it’s just a reliable 722.3 four speed (which you have) with an unreliable and expensive electronic computer controlled, overdrive system bolted to it. Performance wise there is no benefit, it’s not worth the thousand you will spend. Install a gear vender over/under drive in series with the Trans and engine if you what more gearing ratios. Take you for the complement; I still love what I do, 40 years and still counting.

Steve Cattaneo

A Torrington needle bearing conversion will not work; you should see how often they fail in the planetary systems in transmission.