928 Thrust Bearing Failure?

[sharkman]

Hi Everyone

I have an 85 Euro and my crank end play is 0.9mm and the max allowable is 0.4mm

Is my block trash or will I possibly get away with new bearings

My car is pretty tidy so really wondering if its best to part it out or repair, I dont really know what its worth repaired or what I would get for the parts if parted out

any opinions welcomed

 

Thanks John

[Buchanan Automotive]

Hi John,

You will have to take it apart to see if the crank has been damaged. Thats quite a lot of clearance you have there. super rare when run on 20w 50 oil, is it a track driven car

Regards

Sean

[dobbo]

Was it caused by later cars TT coupling creep?

[sharkman]

On 10/03/2022 at 09:30, Buchanan Automotive said:

Hi John,

You will have to take it apart to see if the crank has been damaged. Thats quite a lot of clearance you have there. super rare when run on 20w 50 oil, is it a track driven car

Regards

Sean

Hi Sean

Sorry for the late reply I have been away and just back

no it has had a pretty easy life since I have had it and the bolts were checked on my PPI and it has probably only done 5k since

the bolt wasnt as tight as it should be IMO

I will take the engine out in a few months when I have time and space and report back then

[Buchanan Automotive]

Engine thrust bearing wear , on any last century car engine is preventable , the below is just a sample of what we see on a very regular basis over the last few decades ( yes decades )

The below is a post I put up on Rennlist back in 2015 ( 7 years ago ) & it relates to a 928S4 in regards to the 928 Thrust bearing specs , but the principle is the same with a 1984/1985  4.7  928S & "yes" we have seen quite a few 4.7L 928S with excessively worn thrust bearings & worn camshaft lobes from using a " this" century low viscosity so called synthetic engine oil    }
 

First of all lets take an example of a 928S4 we have never seen before , in fact lets start with a "new to us" 928S4 that was booked in here two weeks ago for a pre purchase inspection & this car was from interstate & we have never laid eyes on this car ever

Year type = 1990
Version = 928S4 Auto
Kms = 167,434 Kms

In for pre purchase inspection

Among the normal checks , we ( like we normally do ) removed the engine pipes from their exhaust manifolds ( just 6 bolts & nuts ) , lower the exhaust a little & removed the bell housing lower cover , inspected & noticed the flex plate was pushed in a lot

Noticed the red painted ( factory paint ) front coupling inhex bolt was still quite tight

Removed the heat shields covering the rear of T Tube inspection round hole & turned the engine until the rear coupling inhex bolt head came into view ( could just make out a slight "red dust " covering the rear coupling & bolt head ) this red dust is rust from micro wear particles from the T Tube shaft male & female splines , this one , meaning the amount of rust powder was mild as compared to others we have seen , this is because of the rear coupling is not tight
Went to loosen the rear coupling inhex bolt & it was already loose "ish" , but this rear coupling bolt had not turned by itself , it was just stretched ( as bolts do ) & already knowing that the front coupling inhex bolt was no where as loose as this rear one ( all very normal ) , because no one knows to check these on a regular basis

Next step , removed both the front & rear inhex bolts & moved the engine flex plate back into position , this flex plate had been in this bent position for some time because it didn't want to straighten up probably because it had been in this bent position for years

Then we connected the dial gauge & measured the crankshaft / thrust bearing play & it measured 0.22mm , which is well within the wear limit( 0.40mm ) , but was / is in the more than the non wear specs ( 0.06 - 0.192mm ) , meaning the thrust bearing on this engine was/is worn a little , but this can be stabilized easily with high oil film strength engine oil , like we have been doing for decades

Subnote } we have had a few ( new to us ) 928S / S4's / GTS were the engine thrust bearing wear is into the .30 mm range & yes these cars were all on so called synthetic engine oils previously( low oil film strength ) and with varying degree of flex plate deflection & when we go to measure these very worn ones ( later monitoring ) the previous wear spec has completely stabilized when on a 20w-50 that is also OK in AW ZDDP

Subnote } the high the viscosity the higher the oil film strength , meaning going from say a 5w-40 to a 20w-50 & both these oils have identical AW ZDDP , the 20w-50 ( just because of viscosity ) has higher oil film strength ( all very simple stuff ) , but most 20w-50 last century oils a richer in ZDDP as opposed to this century 5w-40 or 5w-50 oils & that's why when we do a thrust bearing measurement of say a 928S4 that has been on a 15w-50 or better still a 20w-50 oil in the years before this check , the measurement is always in the non wear spec area ( 0.06 - 0.192mm )& that's only because of Oil Film Strength

This is the reason I mention oil film strength in the context of engine thrust bearings is because all thrust bearings in these engines must have excellent oil film strength ( its that simple)

And that is the reason we have see these cars with very deflected / pushed in flex plates but because like the 928S4 I mentioned above these 928's were just lucky that they were already on decent oil film strength oils ( like 15w-50 or 20w-50 ) and hence NO thrust bearing wear that was going to be an issue , or put another way , yes the flex plate was flexed , but the thrust bearing was protected by an extremely effective slippery oily bearing = The Engine Oil = who would of thought ?

Because remember , the engine thrust bearing MUST not make metal to metal contact , if it does the soft thrust bearing metal wears & then finally galls = disaster & its preventable

Now back to the pre purchase inspection }

The next thing we did ( after measuring the thrust bearing wear & writing it down ) we re-positioned the T T quill shaft back to its central position , meaning the half moon cut out in the rear of the TT shaft aligns perfectly with the coupling , meaning the new bolts will go straight in , then we install new genuine inhex bolt ( with a little grease on the threads ) so we can go to the rear coupling inhex bolt on the next major service & then the next major & so on to check its torque , this later service check only takes ten minutes , because we only have to move the heat shields sideways a bit to give access to the inspection hole to check this very important Rear Coupling inhex bolt & of course a new inhex bolt for the front coupling

No Loctite on the threads of the inhex bolt/s , because this will give you the wrong torque / feel the next time you check its torque / tightness ( say 1 or 2 years later ) because the threads will be locked but if the bolt stretches ( and stretch they do ) & the underneath of the head of the bolt will not be holding the clamp with the force required

Its all very simple to maintain & we have no issues at all

Now why do we go to this trouble on a pre purchase inspection on a 928S4 that we have never seen before ?

Answer = We / I want to know do we have an excessively worn thrust bearing or do we have a thrust bearing with a little wear but within spec & we can control this & stabilize it ( like the car mentioned above )

All simple stuff & no need to ever wear engine thrust bearings at all & yes its a team effort between the engine oil & the unneeded extra deflection / slight constant push forward of the flex plate being in this position , meaning its a combination of two things

All straight forward stuff

__________________
Bruce Buchanan
Sean Buchanan
Buchanan Automotive

[3legs]

So reading that I take it that the correct oil is the key to preventing a failure.

At the risk of starting another oil debate thread, what oil and brand do you recommend so I can make sure at the next oil change my independant Porsche specialist uses the correct oil.

BTW my 928 is an 85 4.7L

[Buchanan Automotive]

I do NOT get into recommending engine oil brands or types ( I only recommend viscosities ) , I have found over the decades that oil companies will just changed the formula of what we refer to as an AW ZDDP package in engine oils ( without warning ) , there was a famous case when a well know oil company in Australia back in the very early 2000's changed their AW ( Anti Wear ) package in their 15w-50 engine oil , they actually lowered the ZDDP but added other AW Boron additives ( this was for upcoming emissions protocols because some engines could use 15w-50 in this century ) and it was a complete disaster  when used in engines with High Loaded Flat Tappet Designed engines ( like the 928 and 944 ) , camshaft lobe wear / valve guide wear and thrust bearing wear occurred far too quickly because of it & on the air cooled 911's valve guide wear and rockers and rocker shaft wear and cam lobe wear happened very quickly  , but I am sure on other car brands engines particularly modern engines that had overhead cam with roller rockers , it was probably OK "ish "/ less of a problem 

But luckily , most 20w-50 engine oils escaped this experimental carnage & it was because of that episode I have stayed well away from 15w-50 ever since ( last 20 or so years ), I never used 15w-50 anyway , but I most certainly stayed away after watching that saga go down with other repairers ( I actually felt sorry for them )

Remember , engine thrust bearings in engines are under assault from the one way push in manual transmissions when your foot is on the clutch , the crankshaft is being shoved under a lot of pressure while spinning from the pressure of the pressure plate diaphragm , same goes for the less pressure of the Torque Tube shaft if its migrated forward ( flex plate pushed in a bit or a lot ) , but the big difference is its always there while the engine is running where as in a manual trans its only momentary when going into gear or changing gear

So the 928 with auto trans , MUST have its front flex plate ( yes there are 2 of them ) kept in the non flex ( straight ) position and maintained that way , its very easy to maintain once you know how and why and combined with a high oil film strength 20w-50 or similar there is little or no wear over the long term with normal driving 

Now we get into 928 engines that have had to suffer / run the gauntlet of being made to run on 10w-40 or even worse 5w-40 engine ( so called synthetic ) low oil film strength oils in the Australian climate 5 , 10 , 15 , 20 years ago , and it may be ( particularly with a 5w-40 viscosity ) in a 928 or a 944 or a air cooled 911 that the human finds on a summers day ( in traffic ) that the engine Oil Pressure Warning Light is glowing bright red at idle ( no oil pressure ) and the engine is very noisy ( little or no oil pressure  ) and the human thinks " gee I wonder if no oil pressure is OK ? " and it might be that this poor bloody engine only had to suffer this for ( say ) 5,000 kms or 10,000 kms , but the damage is done  . Worn but not worn out cam lobes , worn but not worn out valve guides , worn but not worn out timing chains gear teeth , heavily worn rockers and rocker shafts on the air cooled 911 etc etc etc .    .   Switching to a 20w-50 now WILL NOT put this worn metal back on , the damage is done , the best result is a stabilisation of the wear and nothing else 

All simple physics and very simple engineering 
Regards

Bruce Buchanan
Buchanan Automotive