Cylinder wall scuffing

[turbo54]

I recently pulled apart an engine I got in set up by a big name, with less than 5k miles on it.

Ford 4.6 dohc

It had Ross forged pistons, which have/had .0038" wall clearance. Ross recommends .0035. The thrust side of the cylinder walls had a relatively significant amount of scuffing.

What do you guys think caused this? The engine was supercharged, making about 500whp or so. The engine was only torn down to check it out. It been running Motorcraft 10w30 synthetic blend oil in it.

[shawn]

It could be a few different things, but the most common one, especially with supercharged or nitrous, is detonation. The excessive cylinder psi spike loads the piston hard against the cylinder wall, causing the scuffing that you see.
shawn

[cboggs]

what did it have for a fuel system???

was there a dark "burn" mark across the center of the pistons?

Most of the time, OK, almost every time I've seen this it's been
the tune, being lean, .. not enough injector, .. or the tuner
did it one the dyno, .. pistons got hot.

Curtis

[hotrod]

Was it a single area of scuffing directly on the thrust side, or smaller areas spaced around the piston. Was the piston itself only scuffed at the upper ring land area or down on the skirt as well?

Any indicators of ring gap butting up due to inadequate ring gap?

Any indicators of high cylinder temps like cooked spark plug electrodes, or early indications of overheated valves?

Any history of an over heat event ?



This web site has some interesting comments about small 2 cycle engine pistons and their modes of failure. I suspect some of his observations are applicable to automotive engines as well.

http://www.aati.com.au/hirth/how_to_read_pistons.htm

Is anyone aware of similar patterns to scuff marks and overtemp indications on standard automotive applications ?

It would be interesting to collect a group of reference pictures of different types of failures and known causes.

Larry (hotrod)

[Joe Mendelis]

Does it go all the way up and down the cylinder? I have seen blocks that have scuff marks from top to bottom. If the pin comes down to the bottom of the bore and the piston is rocking the skirt can surely be gouged by the cylinder edge. When the piston comes up it could be transferring the "roughness" to the cylinder wall. Just a thought.

[Grocerius Maximus]

.0035" is very tight for a Ross in that bore size in a blown/turbo application with no oil squirters. We can barely get by at .005" with an oil squirter on 3.4" bores, and often set them at .008" for high HP apps. This is measuring at the top of the skirt per Ross, not at the bottom.

[cboggs]

.0035" is very tight for a Ross in that bore size in a blown/turbo application with no oil squirters. We can barely get by at .005" with an oil squirter on 3.4" bores, and often set them at .008" for high HP apps. This is measuring at the top of the skirt per Ross, not at the bottom.

Yea I'd agree, .. I talked to Loyd, the short block builder in my shop, ..
he builds several of these. His reply, .. " the things need to be almost
rattle loose to live with a blower and NOS"

He didn't say what clearence he runs but it's looser then the piston
manf. recomends.

But again I'd also look at the tune. We've had a few come back from top
tuner shops we build for with the piston like that. They tried to
use stock injectors on a ported head, big cam, blown engine.
Seen too many burned up on the chassis dyno.

Curtis

[n2xlr8n]

It had Ross forged pistons, which have/had .0038" wall clearance. Ross recommends .0035. The thrust side of the cylinder walls had a relatively significant amount of scuffing.

What do you guys think caused this? The engine was supercharged, making about 500whp or so. The engine was only torn down to check it out. It been running Motorcraft 10w30 synthetic blend oil in it.

That would be my guess at first glance....I'm using the Ross-recommended .0055" clearance in a 3.91 bore (turbocharged application).