atb

Okay, thought I'd get into a hot and heavy topic for this new board. Does anyone know the process for checking core shifting in the 928 engine block. My understanding is that the block can be measured or somehow X-Ray'd to see that the actual block was casted from the center of the casting i.e., that the cylinders holes are centered in their castings.
The concern is that if I bore the block 4mm over to accept the 968 piston, that their will be enough material left on the cylinder wall, and that it will be uniform, so as to not fail prematurely.
Any idea on the expense of such a procedure, and any opinions on what minimum cylinder thickness should be after boring out to 104mm?

-Adam Birnbaum

Jim V

Adam,

I've never had it done but heard it refered to as "sonic-testing" and "magna-checking"
A perfectly true block of course would be the best but a block with core shift to the
thrust side is acceptable too.
Somer's machine down here in Vancouver has
the proper stuff to bore a 928 block, when I
inquired about cost he told me he would charge standard rates plus hourly if it went
the normal time to bore a US engine.I would
expect it would take a bit longer but I'd think at the very worst twice the cost of an iron block.

Jim V

Tails

Adam,
The method usually used is called Ultrasonic Thickness measurement and is carried out using a small probe placed on surface to be measured and reading off the thickness via an attached meter. This is usually done by a registred "Testing" house/company and the operator is also registered as competent to undertake the measurements. Why the company is registered etc., is that the readings are accepted as correct relating to contracts etc.

Magnaflux measurement is usually used to detect cracks by applying a fluid containing magnetic particles within a fluroscent liquid that shows the crack under a special light when an electric current/magnetic field is applies the the material.

I would suggest that you try surfing the www to find a testing company in your area.

Tails 90 S4

WallyP

Block core shift is normally associated with sand-cast blocks. Since the high-silicone 928 block is pressure cast in permanent molds, I would not expect to see any core shift.

Jim V

That would explain why they are such clean
smooth castings.
So what is the max over-bore on the various
blocks? I've got a spare 4.5 that should the
97mm pistons but how about the 100mm, thats
alot of meat off those cylinder walls.

Jim Nowak

Wallyp,

Don't bet on your advice! The 928 block does suffer from core shifting. Before you do any modifications to that block you need to check the cylinder walls and their alignment with the rest of the engine. I think many 928's suffer a little and some a lot, but then again some blocks are perfect. I had an s4 block bored to 104mm with out any trouble.

Jim

Matt Dawes

Adam,

Permanent Mold Tolling still uses expendable cores (for water jackets and such) inside a cast iron box to make the part. Only the external surfaces ot the cylinder block touch the cast iron part of the mold, so core shift on the water jackets is possible.

Magnaflux testing is for ferrous material only; it uses a powerful electromagnet and metal powder to expose a crack. Aluminum is non magnetic and therefore will not lend itself to marnaflux checking.

Dye penetrates exposes a crack with a florescent dye that is visible unaided or with the use of a black light.

X-ray checking is not readily used by machine shops, it is expensive and bulky.

Ultrasound sonic checking is the way to go.