928 engine crank bearing question

[5l32v4c]

Hi Everyone,

Been a while - too many other things (like kids) demanding my money and attention.

I pulled the engine on my 1985 32v automatic some months back and tore it down. I took the block (pistons removed) into a machine shop to have it honed and line-bored; they also polished the crank and I have fitted new main bearings.

In the workshop manual, there's an exploded view of the block and it shows what is tagged a "grooved ball bearing" on the rear end of the crank.

I can't recall if mine was fitted with one of these when it went into the shop, but it ain't there now. Do I need one of these bearings? What's it for?

Going to try to attach a picture.

[Pete R]

Pretty sure that's the pilot bearing for a manual clutch. What's the description?

[Sapientoni]

Pete R is correct. It's available cheap from any industrial supply house. Not needed if yours is automatic.

[Mrmerlin]

since you got the crank polished did you clean out the throws with atleast 3 cans of brake cleaner and a bore brush?
Then wash the crank oil ways atleast 3 times with hot water and simple green? Then blow dry with compressed air?
If you failed to do these steps then your engine is in grave danger of seizing within 20 mins of run time.

What happens is that the polishing compound fills the oil ways and then sits in the sludge traps, this will stay here till the engine is installed and hot oil is pumped through at 60 PSI.

NOTE the grinding compound will neatly wear the new bearings and fill the cam followers with abrasive compound and the engine will quickly grind itself up

[Danglerb]

I stopped at the word "honed", which is not done on Alusil blocks, but maybe that's just what they called it and actually used the proper treatment.

[1980-928]

What do you mean by "the proper treatment"?
Please I need to know for my engine-

I stopped at the word "honed", which is not done on Alusil blocks, but maybe that's just what they called it and actually used the proper treatment.

[Mrmerlin]

Just for grins how many miles are on this engine?
Why was it taken apart?

more than likely you have just turned a good engine into a coffee table.

If any of the bores were damaged then work would be needed to refinish them. Like a gouge.
BUT bore damage isnt a usual issue.

If the refinishing was not done by an experienced shop thats very familiar with silicon impregnated bores,
then by honing or boring they would have turned your block into a coffee table.

NOTE usually nothing should be done to the bores of a 928 engine,
as most equipment isnt the right type to refinish the bores.

As far as proper treatment you will need a copy of the WSM (work shop manual) to find out how the bore surfaces are treated

[Danglerb]

Proper treatment is with a polishing paste of some special kind and a tool with felt pads. The polish wears away some of the aluminum leaving a surface of exposed silicon crystals.

[jim devine]

This is an article on the proper way to finish the bores:

Honing Options for Hypereutectic Aluminum Cylinder Bores - Sunnen

also check the workshop manual for any variances in the procedure.

If it was just done with a conventional hone you may be looking for another block.

[stepson]

Originally Posted by jim devine

This is an article on the proper way to finish the bores:

Honing Options for Hypereutectic Aluminum Cylinder Bores - Sunnen

also check the workshop manual for any variances in the procedure.

If it was just done with a conventional hone you WILL be looking for another block.

There. I fixed that for you Jim.

[jim devine]

The block MIGHT be savable if oversize pistons could be found and it properly honed to fit. Why was it lined bored & honed in the first place?
Probably cheaper to find another short block and follow the factory workshop manuals.

[5l32v4c]

Originally Posted by Mrmerlin

since you got the crank polished did you clean out the throws with atleast 3 cans of brake cleaner and a bore brush?
Then wash the crank oil ways atleast 3 times with hot water and simple green? Then blow dry with compressed air?
If you failed to do these steps then your engine is in grave danger of seizing within 20 mins of run time.

What happens is that the polishing compound fills the oil ways and then sits in the sludge traps, this will stay here till the engine is installed and hot oil is pumped through at 60 PSI.

NOTE the grinding compound will neatly wear the new bearings and fill the cam followers with abrasive compound and the engine will quickly grind itself up

Wow! This makes complete sense but I never even thought of it - I had assumed the machine shop would have done this but I'm not going to risk it.

[5l32v4c]

Originally Posted by Mrmerlin

Just for grins how many miles are on this engine?
Why was it taken apart?

more than likely you have just turned a good engine into a coffee table.

If any of the bores were damaged then work would be needed to refinish them. Like a gouge.
BUT bore damage isnt a usual issue.

If the refinishing was not done by an experienced shop thats very familiar with silicon impregnated bores,
then by honing or boring they would have turned your block into a coffee table.

NOTE usually nothing should be done to the bores of a 928 engine,
as most equipment isnt the right type to refinish the bores.

As far as proper treatment you will need a copy of the WSM (work shop manual) to find out how the bore surfaces are treated

It's all good - the work was done by a specialist who is familiar with these engines. Looking back, I have done way more to this engine than was absolutely necessary, but the previous owner passed away and very little was known about its history. It only has 65k miles according to the odo., but who knows until you get in there.

The "honing" was done correctly and the line-bore was not an aggressive one - one of the main bearings was showing a little uneven wear. All the tolerances are right on the money with new bearings.

"More than likely you just turned a good engine into a coffee table" - where the hell does that sort of patronizing comment come from?

[stepson]

Originally Posted by 5l32v4c

"More than likely you just turned a good engine into a coffee table" - where the hell does that sort of patronizing comment come from?

From the experiences of dealing with new 928 owners that believe they know all there is to know since they have been driving (you pick)Chevys, Fords, Mopar products their whole life.

I salute you for having the manual and taking the time to read it.

[Danglerb]

Originally Posted by 5l32v4c

"More than likely you just turned a good engine into a coffee table" - where the hell does that sort of patronizing comment come from?

We hear about half a dozen cases of shops not that familiar with a 928 screwing the pooch every year. Many owners drive their cars so little even massive problems fail to show up immediately. Most of the things normally done in a rebuild are rarely done on a 928. The cranks are super hard and rarely have any wear, pistons are almost never replaced, many times not even rings.

Rennlist had a LONG discussion about torquing heads and various studs and bolts and it did not reach consensus or fully agree with WSM. There are literally dozens of odd mistakes to make rebuilding a motor, tricks that work, some that don't.

It tends to make a few of us pessimistic, maybe snarky, but no gentle way to say you need to carefully review all the work done.

[Mrmerlin]

Please forgive me if I did not recognize your mechanical talents.
I am only speaking from experience,
and the fact that your doing major machine work to any part of a 928 block with as it turns out less than 250K miles leads me to believe that your search FU is poor.

That said the 928 block is a very well built part of the power plant assembly , and it has been engineered for a very long service life and to provide a hi level of power during this service life.

Most mechanics that set out to rebuild one of these parts has not taken the time to understand exactly how different these machines are and to what extent they must be treated for proper service life.
Quite a few owners have gotten the heads off then honed the cylinders as they would do to a iron block from a USA built machine, the results are usually the same catastrophic engine failure.

Anyway if you personally have not checked the machinists work on the crank polishing I would urge you to investigate what I posted about cleaning before you start the engine.

Goodluck on your engine refresh,
I hope it runs perfectly.

[5l32v4c]

Thanks guys for the input - I sincerely appreciate it. I guess nobody intentionally does the wrong thing while rebuilding one of these engines, so it's invaluable to be able to benefit from other's mistakes / experience.

I trust that the work done so far has been done right; I took the block to a trusted Porsche service shop who commissioned the work from a machine shop. The person who worked on my block and crank is known to the service shop so, fingers crossed, they knew what they were doing.

I guess the only way I will know is if the engine fails prematurely.

Have I gone too deep? With hindsight, I can say that I probably have. So little was known about the car and the engine that, had I not torn it down, it could have blown up anyway.

So, I hope the build goes OK and the engine lasts. Due to your comments, though, the doubt are creeping in.