Re: Reflections in a Disc

[JJK1204@domain.elided]

In a message dated 6/4/99 8:42:43 AM Central Daylight Time, bgiller@domain.elided 
writes:

<<  Anyone know what 'RMS' stands for? >>

Yes, RMS = Root Mean Square is a statistical method, which in this case is a 
measure of surface roughness. 80 RMS is pretty smooth. New or resurfaced 
rotors will be fine. Following is a snip from a Fel-Pro (head gasket maker) 
discussion of head gasket sealing, the second paragraph is the definition, 
the rest is for use in context:

Surface Finish
The surface finish on the face of the head and block is also important for 
proper sealing. The surface finish for most engines should be 54 to 113 RA 
microinches (60 to 125 RMS), with a recommended range of 80 to 100 RA (90 to 
110 RMS).

Surface finishes are measured in "microinches" (millionths of an inch). RA 
stands for Roughness Average, and is a simple average of the heights of the 
peaks and depths of the valleys on the surface. RMS stands for Root Mean 
Square, and is a mathematical technique for describing the amount of 
variation across the surface. The difference between RA and RMS is about 10%.

The surface finish should be fairly uniform across the entire face of the 
head and block. In other words, the RA finish should not vary more than 20% 
from one area to another.

As a rule, the smoother the surface finish the better. When the surface is 
rougher than about 113 RA (125 RMS), there are too many peaks and valleys on 
the metal's surface to seal properly. The gasket may not cold seal and could 
leak coolant and/or compression.

Deep scratches on the surface can also create leak paths for gases and 
coolant that can lead to premature gasket failure. Using a thicker gasket 
that has increased conformability and/or a thicker soft facing can compensate 
somewhat for a rougher surface, but such gaskets don't retain torque well and 
are less durable.

Too rough a surface finish can also be hard on the gasket itself. A rough 
finish has more "bite" and digs into the gasket more aggressively. This can 
increase the scuffing and shearing the gasket undergoes as the engine heats 
up and cools down. In bimetal engines with aluminum cylinder heads, this can 
be especially hard on the gasket because of the difference in expansion rates 
between aluminum and iron.

Too smooth less than 54 RA (60 RMS) may also create sealing problems 
depending on the application. If the surface finish is too smooth, it may not 
provide enough bite to hold and seal a head gasket securely. There can also 
be movement between the gasket and metal, causing the gasket to abrade and 
leak. Though conventional head gaskets require a small amount of roughness to 
grip and seal the gasket, a new type of laminated steel or Rubber Coated 
Embossed (RCE) style head gasket that is used as original equipment on 
certain late model domestic and Japanese engines requires an almost polished 
surface finish. The recommended surface finish for some of these applications 
is as smooth as 20 RA (22 RMS). So follow the manufacturer's surface 
recommendations for the application.

John Katos

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