BC

Hey all,

Does anyone have any info on what options a 928 owner has for buying other than OEM pistons? I understand that there may be some "coatings" that need to be applied onto the pistons to make sure they can deal with the block. I don't want to have to sleave a block if I do not have to, but aftermarket pistons are really one of the few choices when you are dealing with medium boost figures on a 928 engine.

Also - doesn't sleeving a block bring all the wear characteristics of a chevy engine - rebore after a certain time?
etc?

Thanks,
brendan

Larry Parker

the pistons have some sort of tin or iron coating, and im unsure if anyone does this coating........
Another option is to nikasil coat the block and then
you can run a uncoated piston while getting all the benfits of the nikasil coating...... it must be good
they used it on the 930's and alot of there race motors......I am planing on having my 944S2 motor done....... soon

<a href="http://www.morissdampers.com" target="_blank">www.morissdampers.com</a>

dr bob

A parallel thread on the 928 list reminds us that the block is already nikasil, and the pistons need to be hard-surfaced with iron or similar to avoid scuffing.

The metalurgy of the block, and specifically the cylinder area, mas nodules of silicon in the aluminum. The bore prep exposes the silicon by removing some of the aluminum, so that you have something close to a ceramic or glass-faced bore surface. Interesting technology. Meanwhile, aluminum piston skirts are way too soft to use with that hard bore face.


Follow up is in the Wally P post on the subject.

Jim V

This is something I wonder about also and would
like to throw this info in the mix:
In the small white dimensions/tolerances books
the decribe the pistons as "Cast light alloy with
chrome or iron plated bearing surfaces"
I picked up a set of Mahle Euro-spec replacement
pistons on E-bay and the only reference on the box
is "chromering". The rings are dark,almost black
so the look like iron to me. The pistons are definatly plated and it was done pre-machining.
The plating on the pistons is bright silver in
color but dull in finish and looks very thin.
I'm assuming that the pistons are unpolished
chrome-plated which brings me to the point that
if all that's requires is a plain chrome-plate
it should be easy to get done and very reasonable
price-wise.
Very interested to hear what everyone else has
to say about this.

JimV

Jim Nowak

JimV,

Sounds easy huh? Well I can tell you its not that easy to find a coating that will stay on the piston at those temperatures and through so many heat cycles. There are plenty of coatings that would make it through the first heat cycle but will fail after multiple cycles leading to premature engine failure.

I always thought that a ceramic coating would be the way to go but I've heard the heat cycling of the aluminum kills the ceramic and causes it to chip off the piston. Please let me know if you can find a cheap source that will last. I think the last batch of 16 pistons my mechanic had done cost more than $1,500.

Jim Nowak

Larry Parker

I think there is some confusion about the factory coating of the block it is ALUMASIL and not NIKASIL. Nikasil was used on the 930 turbos and such but i am almost positive that it was not used on the 928 series. And you do NOT have to run a coated piston in a nikasiled block.

Steve Cattaneo

, “JE pistons or any custom piston, without a coating of any sort to the piston will work in a 928 cylinder bore treated with nickel/ silicon”.

Nickel/silicon is electroplating with nickel matrix and silicon carbide. It is a different process then aluminum/silicone (alicil). For aluminum/silicon the cylinder “bore is machined until the silicon protrude from the aluminum so that the piston and rings only make contact with the silicon” then the bore’s are power honed with a silicon compound completing the process, the end result is a aluminum silicon prep bore.

Us chrome will do the complete process of nickel/silicon electro plating and cylinder bore machining

Cylinder bore machined to 104 mm is $480/$60 per bore
Electroplating 8 cylinders is $920/$150 per bore

Total is $1400 to prep the block (this price is w/o the cost of pistons)

For any pistons to work in an aluminum/silicon prepared cylinder bore, the pistons have to be chrome plated or iron coated.

<a href="http://www.uschrome.com" target="_blank">www.uschrome.com</a>

Steve C.
The Great White

dr bob

Larry wrote:

&gt;&gt; I think there is some confusion about
&gt;&gt; the factory coating of the block it is
&gt;&gt; ALUMASIL and not NIKASIL. Nikasil was
&gt;&gt; used on the 930 turbos and such but i
&gt;&gt; am almost positive that it was not used
&gt;&gt; on the 928 series. And you do NOT have
&gt;&gt; to run a coated piston in a nikasiled
&gt;&gt; block.

Thanks for the correction. Larry is right. Adding nickel and chrome to the bore makes it practical to use softer pistons and rings. My mistake.

BC

Us chrome will do the complete process of nickel/silicon electro plating and cylinder bore machining

Cylinder bore machined to 104 mm is $480/$60 per bore
Electroplating 8 cylinders is $920/$150 per bore
Total is $1400 to prep the block (this price is w/o the cost of pistons)

..Has anyone with a similar block done this - that you know of Steve? I would greatly apreciate any info on this. Did you do this to your own engine?

Thanks,
Brendan Campion

Aaron Rouse

Didn't Devek a couple of months ago say how they were testing the nikasil coating in a 6.5L 16V Euro spec motor currently? I thought a long thread started up about this on the 928 mailing list and how BMW used to use this same sort of block coating. If I recall correctly, BMW had to recall those motors due to the coating not holding up well to sulfur and a lot of areas in the US have a decent amount of sulfur in their gasoline.

Above someone mentioned being able to get 16 pistons coated for $1500 or so, where were those pistons coated at? That still does not seem like a bad price when you look at how much a set of factory 968 pistons will cost you.

So far the best thing I have found is just to run 8 951 pistons in a boosted 928, but I would certainly like a different option since I am far from building a motor.

Jay Wellwood

Although cost will become a major issue - IMHO, sounds like it is better to spend the $ for the Porsche psitons if they fit the application than try to re-engineer another acceptable method (and survive a roll of the dice).

Just another useless $9.28 worth I'm sure...

Aaron Rouse

Is there any reason why the 951 piston would be a bad piston to run? I have heard they are not all that costly to buy used ones from some of the 944/951 performance shops. The only thing I have heard is when running the stock crank and those pistons, you need rods 2mm long, at least I heard this for the 32-valve heads and would assume it is the same for the US spec 16-valve heads.

Was the US spec 16v 4.7L a lower compression motor just like the 4.5L? I have been debating just running stock motors with decent amount of boost, since those stock motors can be had rather cheap. If anything just get them and some 944 heads.

2V4V

Aaron,

Do keep in mind that the 951 pistons are 100mm. The 4.5L had a 95mm piston and the 4.7L has a 97mm piston. You would have to bore the block to match the piston diameter.

Greg

2V4V

Aaron,

Do keep in mind that the 951 pistons are 100mm. The 4.5L had a 95mm piston and the 4.7L has a 97mm piston. You would have to bore the block to match the piston diameter.

Also, the majority of 944 turbos were of the 2v variety, so if you use them with 4v heads there will be clearance issues. I guess that would be what the shorter con-rods that have been mentioned fix.

Greg

Aaron Rouse

It is a 2mm longer rod actually. Marcus Hutchinson(sp?) recently built a 5.0L 32-valve motor using 951 pistons and the slightly longer rods in order to place the piston properly, I think he did have to do some clearancing on the pistons for the valves.

Yes, I know they are a 100mm piston and I know a 4.7L could be bored to 100mm but the 4.5L most likely can not. Really I had two questions though.

The first being is there really any reason why not to use a 951 piston? A lot of people seem to be searching for a way to use JE's which sometimes implies to me that there might be something wrong with using the 951 pistons.

My second question was just in reference to the stock compression ratios availible in the 4.5 and 4.7L US spec motors? I think they are both low, below 9:1, but I was not possitive.

I am just wondering if maybe it is even worth the time and effort to build up a 5.0L with 951 pistons over just using one of those stock 4.5 or 4.7L US spec motors since they can be bought for next to nothing these days. I know the 951 adds the benefit of forged pistons vs the stock cast ones found in those older US spec motors. Both would be 16V motors, I have been entertaining the possibility of 944 heads on them as well.