mark kibort

Does anyone know if its possible to lightly hone the 928 4.7 block's cylinders.
I was told you want it to have a mat finish and thats the silicon that is left behind when you remove aluminum on the silicon paste process. However, say you have a perfect block and someone takes a honer to the cylinders. (just lightly as someone has suggested) what would the down side of doing this be?
someone has said that the rings will "seat" better, but Ive heard differently.

If you do a light hone and not replace the silicon, will the exposed aluminum now, cause faster wear of the rings against the cylinders? less compression, catostrophic failure?

there was one mechanic that put together a 944 turbo block and it had a minor imperfection that was fixed and then all cylinders were lightly honed. he said this 944 was still on the road today with no oil burning or power issues?

thoughts??

thanks,

Mark

Ketchmi

If it is lightly honed with the felt pads and paste that are required for Alusil, no problem. If you take a set of stones or one of those many balled hones to it you will have the same thing that happened to the last engine.

BC

I have had two blocks lightly honed with the proper machine and technique. It just removes the superficial stuff, and does nottake very much material away. A good start for new rings.

mark kibort

and what was that??????? i think i remember you talking about it.

the post from brendan, said that a light hone to remove the superficial stuff is ok and a good start for the rings. put it this way, ive seen a block with the cross hatchin lines, and the suface looks shinnier than the dull mat finish of the proper prepared silicon paste process. what did your failed blocks look like?

thanks,

MK

BC

It was described as the sunnen machine on the last lapping setting.

Ketchmi

You will not have a cross hatch pattern on a 928 block. It will be smooth as glass. The cross hatch pattern is for steel sleeves and will absolutley destroy a Alusil block.

A set of felt pads and a paste are what is used to hone Alusil. Any other way and you are tossing it onto the trash heap.

atb

Mark,

I'm no expert, but I would venture to hypothecate the following:

The cross hatch hone pattern on an iron cylinder serves a different purpose than the felt/paste last stage on an alusil cylinder.

The cross hatch pattern from a ball/cage hone is to retain lubrication on the cylinder wall while the rings are seating. No problem where you're typically running 4 thousandths piston to wall clearance with a forged piston and iron cylinders. On a 928 block, with its tighter tolerances (eight TEN-thousandths), and aluminum piston and cylinder, the cross hatch pattern would scuff off the piston coating like a cheese greater and you'd have yourself a nice little friction weld.

Hmmm.... maybe I AM an expert.

atb

Uh...., make that cheese grater.

Okay, so I'm not an expert....

FlyingDog

The honing process has been posted before. Tom Falkenberg also posted a link to a BMW site with an info sheet describing sleaves and I think honing of Alusil, Nikasil and other exotic blocks.

Tom's thread:
https://rennlist.com/forums/928-forum/214289-interesting-info-on-alusil-motors.html

mark kibort

Thanks, thats good information.

what about the 944 block story where the shop did a light hone with the stones and the engine is still running. Lucky??? or just wont end up lasting long!

Mk

Ketchmi

I'd have to see it to believe it. It's like saying that a virgin gave birth...

fst951

Hey now...what do you think that Joseph said about that? I bet it went something like this "Immaculate conception, huh????? All I know is that this better not happen twice!!!!!"

Good night.

atb

>>>>>what about the 944 block story where the shop did a light hone with the stones and the engine is still running. Lucky???

Can you do a "light" hone? I was kind of under the impression that you mounted the stone, programmed to machine to the proper specs and let it run.

If you look at the KS manual regarding boring/honing of alusil cylinders, it shows three different stones that are used before the felt lapping: roughing, finishing, and polishing. The finest stone, being the polishing stone, still takes out .01mm of material. If factory piston/wall clearance is .02mm (7.9 ten thousandths), by running the polishing stone you've just increased the p/w clearance by 33.3 percent. Will the porsche run? Sure. Will it run strong? I guess that's the question for the experts.

m21sniper

You can do a light hone with a bead type drill bit attatchment with a hand drill.

Not reccomending it, but i've seen it done a bunch of times on various V-8s.

SharkSkin

That's specifically recommended AGAINST IIRC.... There is a special etching process that is supposed to leave exposed silicon crystals for the piston/rings to ride against.