Cylinder Head Resurfacing – Getting The Correct Surface Finish
Above all, cylinder head resurfacing, is the most common machining operation, done in machine shops to date. However, many different material combinations like aluminum, cast iron, and steel, all require their own specific surface finish.
To seal properly, a head gasket requires a surface finish, that is within a recommended range.
So, very smooth surfaces are required for (MLS) head gaskets. And, smoother is generally better (up to a point) for most gaskets. Because, it improves cold sealing, but there is a limit. Most gasket manufacturers say, the surface should not be smoother than 30 RA for, non-asbestos or graphite head gaskets. This is because of, these gaskets’ lateral support from the head and block. Another very important fact, when doing any cylinder head resurfacing.
As for belt sanding, it can be a real time-saver. Because, heads and other parts, do not have to be mounted in a fixture. But, cylinder head resurfacing, with a belt has dropped off. Because, it is not as precise as milling or grinding and relies too much on the individual operator.
We recommend a feed rate of two inches per minute, at 1,000 rpm, on a milling machine. And, with a two-bladed cutter ( harder to index but better ), to achieve a surface finish of 12 RA. Carbide or PCD tooling, is usually recommended for, milling aluminum. While, carbide or CBN is recommended for, milling cast iron.
Can The Surface Finish Ever Be, Too Smooth
When the head is bolted to the block, the metal on both sides, bites into the gasket. And, this is what helps hold it in place. But, you don’t want too much bite, when the head is aluminum, and the block is cast iron. Due to, the sideways shearing forces that result, from the expansion and contraction, of the aluminum head. Yet, support is necessary, to keep the combustion gases in the cylinders, from distorting the gasket. As a result, blowing the head gasket.
How Important Is Cleaning
So, belt sanding is better for, clean up work and resurfacing hard-to-fixture parts, like manifolds and timing covers.
So, when doing any, cylinder head resurfacing, it needs to be perfect. Because, you can’t expect to get, high quality surface finishes, if you’ve ignored your equipment. Dry milling doesn’t require any coolant, so there’s no coolant to maintain.
One thing to watch out for here is, the use of cleaning solvents or gasket remover chemicals. Mostly because, they leave residue on the surface. The residue may interact with the coating on, or materials in a head gasket. Consequently, leading to premature gasket failure.
Cylinder Head Resurfacing, What Not To Do
Today, you need smoother finishes required by many of today’s, aluminum heads and gaskets. Consequently, you need to use the right combination of table feed, and (rpm) when cylinder head resurfacing. This requires a variable speed table and/or multi-speed or variable speed milling head. Increasing the (rpm) of the cutting head and/or slowing down the feed rate, makes a smoother finish.
The specs vary, depending on the type of head gasket. Also, if the surface is too rough, or in some cases too smooth, the gasket may not seal properly. Above all, look up the recommended specs, for a particular engine and/or a particular type of head gasket.
CBN or PCD Tool Bits, Can Give The Best Ra Numbers But:
You have to make sure you use, the correct feed rate and speed.
The equipment has to be rigid enough, to hold the cutter steady.
This is so the tool bit, doesn’t lift or chatter, when it makes in interrupted cut.
That’s why, many machine shops today, have switched to dry milling, for cylinder head resurfacing.
This is because, it eliminates the mess and maintenance, that go with wet grinding.
Getting The Correct Surface Finish
Clean, flat and smooth. These three words describe the surface, in any machining job, not just the head and block. But, how clean, how flat and how smooth, do the surfaces really have to be? It depends on the application.
Clean means no dirt, oil, grease or residual gasket material, on either mating surface. Both surfaces must, be spotlessly clean and dry, to assure a good cold seal.
So, there are many other reasons for cylinder head resurfacing, besides a head gasket leak:
To restore flatness or to just improve the current surface finish.
After welds or other repairs, or milled to increase the compression ratio.
The angle changed slightly to better align, with an different intake manifold.
After removing the cylinder head, for a valve job.
So, whatever the reason is for cylinder head resurfacing, you want to do it quickly, efficiently and correctly. Mistakes here can be very expensive, because once metal has been removed there is no putting it back.
Cylinder head resurfacing, is a very common process today, especially with all the leaking head gaskets.
Some shops and rebuilders use, an abrasive pad in an air drill or buffer, to buff off residual gasket material. It’s a quick and easy way, to remove old gaskets. It is also risky, because there’s a risk of removing metal. And, leaving a depression that will be, hard to seal.
Instead, hand scraping (carefully), thermal or chemical cleaning should be employed.