View Full Version : Specs for surface finishes?

08-10-2004, 09:54 AM
Anyone know the specs or where to find the factory specifications for surface finishes (decks, heads, cranks, etc) for our engines?

08-23-2004, 06:36 PM
I'm willing to bet the crank was nitrided past that :confused: .

09-13-2004, 10:50 PM
I found this in Steve Dove's "Guide to Buick Performance Engines" in the chapter titled "Turbo V6 Engine Building Tips" (page 123 in my edition):

"...Although the spec for a crank journal is 8 to 12 RMS, the spec for a seal surface is 10 to 20 RMS with 15 RMS a good target..."

RMS means "Root Mean Square"

I'll keep digging.

10-06-2004, 04:49 PM
I found this on the Federal-Mogul web site. Doesn't speak to our motors specifically.

"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.

To 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."

10-06-2004, 07:19 PM
Wow, thats getting pretty deep. Good stuff! :)

10-11-2004, 02:08 PM
Yeah, but realistically, how many machine shops are willing to spend $2-5K on a surface roughness gage?

10-11-2004, 05:53 PM
> how many machine shops are willing to spend $2-5K on a
> surface roughness gage?

Yep, I wouldn't spend it on that tool. But I'd wanna be prepared for the geek like me that brings in his parts and states the finish he wants on them. :D

As I see it, shops know their tools and know what finishes they produce. I'm not sure I wanna work with a shop that doesn't, so I gotta ask. And if I gotta ask, I gotta know.

I'm really after an understanding what it's about and what it is supposed to be. At this point I speculate that the finish is specified by manufacturer of the contact materials, i.e. the gasket, ring or bearing manufacturers. :o