The abrasive disc cleaner, sometimes called a "cookie" or 3M disc can be a very useful tool for cleaning engine block and head castings BUT it must be used with care in order not to ruin a casting.
The disc itself is made of an abrasive plastic material very similar to the Scotchbrite pads used to clean pots and pans at home.
When using this type of cleaning disc, you must keep the disc very flat to the surface. If you tilt it at all it can dig in and actually cut a groove in the surface you are trying to clean.
Let me show you how you can ruin a cylinder head with an abrasive disc tool !!!!
The other day, one of my repair shop customers brought a Chrysler PT Cruiser cylinder head to my shop to have a valve replaced. He was NOT working on the car, a friend of his had removed the head from the car and asked him to bring it to the machine shop for him.
As soon as I took one look at it, I knew they had been using one of those darned abrasive pads on the gasket surface. If you look real close at the picture, you can see how "rough" the surface is.
Before repairing the bad valve, we decided to try to resurface the head to see how much damage was done. The first thing I did was "paint" the head with red machinist die.
Then we started the surfacer and took a light .002" cut off the surface. As you can see, this left a lot of "red" spots where the surface had been "attacked" by the abrasive disc.
So .... we cut another 3 thousanths ( .003" ) off for a total of .005" so far. Still have plenty of low spots showing, especially around one cylinder.
OK, lets try another .002 for 7 thousanths total ? Still not enough !
How about 8 ? Not quite yet.
Well it actually took ten thousanths of an inch ( .010" ) of milling to finally get the surface cleaned up so the head gasket will seal !!!
Now hopefully you understand how much damage can be done to an aluminum cylinder head by using this disc type of cleaning tool. Please keep them away from these types of parts.
This guy was lucky and the head was still within specifications, but usually having to cut that much off will ruin the head.