Piston scuff - 9.2 deck 4" stroke 6.125 rod alum SBF

[mjschneidy]

Built a 9.2 deck 4" stroke with 6.125 rods and 1.075 pistons and finding scuffed pistons and scuffed bores. Cross hatch worn off on both thrust and non-thrust sides complete length of skirt travel. Polished off really as there are no noticeable ridges with fingernail.

All good parts, Callies and Diamond in an aluminum 9.2 RDI block. No power adders. Took the heads off for another reason and noticed the scuffing. Motor has a few chassis dyno pulls and about 20 mins of easy road racing time. I don't think the rings ever seated either as cold cranking compression with all plugs out and WOT was only 110 psi across all. Rings are std tension .043 .043 3mm with lateral gas ports.

Is this common? Would coating the skirts help? Really surprised at the amount of wear with such little run time. And the low compression to boot. Thinking of having it honed again and another set of rings but that's not going to prevent the wear from reappearing. Piston clearance with an alum block? Or is this common and I should put the heads back on and run it hard.....looking for advice.

Thanks, Mike

[MadBill]

Got any pix?

[modok]

If the pistons are scuffed along the usual thrust surfaces then it's usually some grit that became embedded in the skirts. Forged pistons can be really soft and they do this worse than normal pistons. The grit comes from being dirty at assembly or the hone job was not good.
Honed with too much pressure can LOCK grit in the crosshatch surface and it gets released as the rings wear in. The best measure against it IMO is hone last 7-10 strokes with a SOFT abrasive stone, or a brush, or both. Both if they are moly rings.
A soft aluminum piston piston can't scuff iron unless it has hard stuff stuck in it.

[MadBill]

Another possibility: It's easier to suffer piston scuffing and butted rings with an aluminum block, as the cold clearance and gap is normally less than for an iron one. Romp on it too hard before it's fully warmed up and the pistons/rings heat up faster than the block. Check the ends of the rings for shiny spots.

[modok]

Good point! yes rings can scuff cylinders too

[user-23911]

Every time I see scuffed pistons it's because there had been insufficient clearance, as in the piston got too tight in the bore.
Either the piston got too hot for the clearance it was built for, maybe due to a bad tune.
Or
The clearance it was built for wasn't enough for the power output expected.

It's very common on old diesels which have worn out and started partially running on their own oil.

Don't know about aluminium SBFs though?
Must have cast iron sleeves?

[cjperformance]

What was piston material, piston to bore clearance and what did diamond reccomend for those pistons?

[Dan Timberlake]

"complete length of skirt travel"

So the upper 1 inch or so of the bore is un-worn?

[Wolfplace]

Not enough info,, what does the top of the bore & ring area look like?
All pistons I would suspect a clearance issue for the application if the top of the pistons/rings look reasonably happy
Also check bottom of bores to see if the sharp edge was broken
If not it can act like a scraper & will shear the oil off of the piston,,,

[mjschneidy]

Thanks guys...a lot of good thoughts and questions back to me.
- The piston are 2618 and were set at 0.005. I see on Diamonds site they are now recommending piston-to-wall clearance between .0065in to 0.008in with 2618 billet. What do you guys recommend for an alum block, a bit more?
- I don't see ring butting on the one piston I took out.
- No sharp edges on the bottom of the sleeves, I paid close attention and deburred them for that reason.
- Correct, the upper 1" of the bores where the skirts don't ride are clear of scuffs.
- Motor built for about 600-620 fwhp and road race endurance usage. Naturally aspirated / carb
- Cast iron sleeves
- I'm suspicious it may have been honed with high pressure as they chose to hone it to size vs bore and then hone. Possible grit lock that I wasn't able to get cleaned out.

Pix...

[Cubic_Cleveland]

I'm not so sure it's a clearance problem, usually it will scuff the length of the skirt and not have an un-scuffed section in the middle. What does the spec sheet of the pistons say for clearance?

I'm thinking you have more of a scuffing problem from piston rock at TDC and BDC. But then, I also think the scuffing goes into the ring travel slightly in the first, third and fourth photo's. Not a sever as below the ring travel but I think it's there, possibly from foreign matter, but why not the length of the skirt? What do the rings look like, and how does the piston measure at the bottom, middle and top of the skirt?

[modok]

Scotchbrite used wet works great for scrubbing the skirts clean again, just use light pressure. One time I used a little bowl of soapy water and kept dipping as I went, and when I was done I used a magnet and collected a surprising amount of iron dust in the water.

The scuffing doesn't really look all that severe, there might be more distortion than actual wear....... but It might have gotten worse if you kept running. The thrust surface on those pistons is quite small, and it's a very rigid skirt too! Coating probably a good idea, if for no other reason than to let it "wear" and form a wider contact on the thrust surfaces.

[Belgian1979]

Can piston stability issues and piston touching the edge of the cyl with that stroke, rod combe be ruled out ?

[jasonn]

subscribing

[mjschneidy]

Ok so I'm really confused about how much clearance we should have set this motor up with....

Diamond Piston Spec card that came with the custom 2618 pistons says 0.0055

Then you reference an included chart that says:
+0.000 to 0.001 more for Road Race which this is -- (Why, does the piston hold more heat and expand more due to longer power runs?)
-0.001 to 0.003 less for alum block which this is -- (Why, that much more expansion when hot even with iron liners?)

So that gets me a range of 0.0025 to 0.0055

Diamonds website now says "Usually machined from a solid piece of 2618 billet aluminum, the piston has an expansion rate slightly greater than its forged counterpart. Most users set the piston-to-wall clearance between .0065in to 0.008in. With regard to weight, the billet version is typically one to two percent lighter than a comparable forged piston for the same application."

Ugh...what would you have done for an alum block road race application with a 2618 slug ?

I'll be calling Diamond Monday to discuss....

None the less my plan is to have the skirts coated and re-honing the block but need to figure out what clearance to target. Who's the guru on alum blocks?

Thanks, Mike