Adamant1971

The car sat for ten years and during this time the coolant leaked out via the rad. Engine was torn down due to partially dry head gaskets from improper storage and lack of coolant in the block.

It was a good thing as there was a huge amount of crystallized coolant around cylinder 3. And the head gaskets were shot.

Once the heads were off and engine out it just made sense to take it all the way.

slate blue

Agreed, please see response to Imo000 for more details, I was just sharing my observations really regarding this wear that occurs with highly grooved bearings.

In regard to your response stating that the bearings were designed for specific engines I accept that in principle but you need to look more closely at this 928 specific example.

N.B. I am not reducing the mains to 68 mm on a standard crank, there is no real point to that as the cost would be very high due to re-hardening the crank. So the changes are to be made to a bespoke piece.

However going back to a factory crank for a moment, the sole purpose of the groove in the main bearings is to oil the rod bearings. If I accept you assertion that the crank is drilled 170/190 degrees instead of 180 degrees it will still oil the rod bearings constantly when you have 220 degrees grooving on the main bearings.

However if you switch to 140 degree grooving the oiling is not so constant. It will be slightly interrupted but still oil the rod bearings for 280 degrees of rotation. That is a lot.

Then if we look at "Chevy or straight shot oiling" for which my research shows this 140 degree grooving is common. It means that the rods only get oil for 140 degrees of the 360 degrees of crankshaft rotation.

It doesn't seem like much when you look at the above example but that is all that is required for the rod bearings. So if a NASCAR engine or street LSX engine can live with 140 degrees of oiling I'm sure that the 928 could tolerate 280 degrees. The problem more lies in the super soft bearings used in the 928 rods. I just had my rods resized to accept the Toyota Supra race bearings which are much harder. They are going on the factory 928 crank using a 0.0015" clearance. It will run at least 20w-50 oil.

Incidentally the rod journals on that crank had some taper in them and needed regrinding, so soft rod bearings don't guarantee no wear either.

FredR

Wishing you every success in your endeavours- you were fortunate not to have severe corrosion at the top outer side of the cylinders by the sound of it.

When I acquired my current 928 GTS and transplanted my late S4 motor into the car we debated whether to strip the bottom end and eventually decided not to given the relatively low mileage and the apparently good condition of what we could see. 10 years on it seems we made a good decision.

Hopefully you can post something about how you select bearing tolerance sizes etc.

Regards

Fred