johnb

hi

just a question, cos i can't find the answer.

i read somewhere that you can soft hone a newly bored 928 alusil block with a caustic soda soaked felt honing tool.

the theory being that the caustic soda removes just enough aluminium from the bore to expose the hard silicon structure. (alusil is 70% alu 30% silicon). ready for those iron clad pistons, etal to do their job.

i did chemistry tooooo many years ago to remember whether caustic soda disolves ally!

if this is complete rubbish; what is the correct compound to use to prep a newly bored 928 alusil block?

ho hum

johnb

WallyP

This isn't a home operation. The honing is a complex, multi-step process that utilizes precise heavy equipment, and not every automotive machine shop is equipped to handle it.

Best bet would be to find a shop that you trust and let them handle the entire process.

A 928 block is EXPENSIVE!

Chris Prack

I agree with Wally, don't try this at home!

The cylinders require the use of silicone paste, a specific grit stone and a specific Sunnen machine to properly hone the block. Trying this a home with some chemicals that you think you remember is a recipe for disaster.

Why are you honing (or asking about) anyhow? Do you have a problem with the block? Just curious. If you are doing a rebuild, unless there is a problem just make sure that the cyls. are in tolerance and you still have round pisons, re-ring it and go. I would not sweat the honing.

Good Luck

johnb

hi guys

i realise that the actual process of achieving a dimensionally correct bore is best left to a machine shop with the correct tools.

my question really relates to the final prep of the bore. 928 blocks, being alusil, require that the ali content be etched back in the bore to expose the hard wearing silicon content. no amount of mechanical boring will do this.

we are talking surface prep here, not dimensional correction.

so, what is the process to correctly prepare a alusil bore for those iron clad pistons?

i've read that the old vega blocks (alusil again) could be prepared with drain cleaner!!

johnb

Jay Wellwood

The Shop Manuals refer to the following procedure-

1. Rough milling to 0.1 mm of the final size.

2. Dressing the cylinder to 0.02 mm of the final size.

3. Polishing to final size.

4. Lapping with the silicon compound to expose the silicon cyrstals.

Upon completion, the upper 0.5 mm of the cylinders must be re-shaped to a 30 degree chamfer.

hth-

BC

SO would this mean (Jay, Wally), that if you have some fine scratches in a bore surface (that you can JUST getyour fingernail to catch on), that they can be "cleaned" up, and you would not have to use a larger piston (more than standard)?

I don't want to rering my block without making the bores new. Its just a big fiasco with 50% saying don't rering old bores, and the other side saying yes.

So you could get "one size over" rings? And keep the pistons stock?

Anyone have any info?

Fastest928

1 I suggest all who have commented and are interested, to actually observe the process of honing and boring Reynolds 390

2 Reynolds 390/390A is 13-16% silicon

3 the surface prep is all about cylinder sealing..some shops do it well, most do it very poorly. do it well and you will be treated to leakdowns in the 1-2% range after 100K miles.

4 Contact Sunnen for the MSDS

5 give it a try and see what happens....I doubt that ti will produce a sealable surface.

Marc
DEVEK

Erik - Denmark

Johnb
See: http://www.msi-motor-service.com/dow...ren/alu_en.pdf