Extremely smooth finishes require Cylinder head resurfacing- (Dry Milling) to achieve really low Ra numbers. I like to use CBN or PCD tool bits to do a resurfacing job. You have to make sure you use the correct feed rate and speed – and the equipment is rigid enough to hold the cutter steady so the tool bit doesn’t lift or chatter when it makes in interrupted cut. ( having a machine that is not rigid enough is the most common issue )
Many machine shops today have switched to dry milling because it eliminates the mess and maintenance that go with wet grinding. Milling allows very precise control over stock removal, and it is faster than grinding because more metal can be removed in a single pass, eliminating the need for multiple cuts.
To achieve smoother finishes required by many of today’s aluminum heads is using the right combination of table feed and rpm when milling the head. 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 produces a smoother finish. We recommend a feed rate of two inches per minute at 1,000 rpm on a milling machine with a two-bladed cutter ( harder to index but better ) to achieve a surface finish of 12 RA. Carbide or PCD (polycrystaline diamond) tooling is usually recommended for milling aluminum, while carbide or CBN (cubic boron nitride) is recommended for milling cast iron.
How smooth is too smooth
Can the surface finish ever be too smooth? Although 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 sealability, there is a limit. Most gasket manufacturers say the surface should not be smoother than about 30 RA for most non-asbestos or graphite head gaskets because of these gaskets’ lateral support from the head and block.
When the head is bolted to the block, the metal on both sides bites into the gasket to help hold it in place. You don’t want too much bite when the head is aluminum and the block is cast iron because of the sideways shearing forces that result from the expansion and contraction of the aluminum head. Yet, a certain amount of support is necessary to keep the combustion gases in the cylinders from distorting the gasket and blowing past it.
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 to get a good, long lasting seal? 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.
One thing to watch out for here is the use of cleaning solvents or gasket remover chemicals that leave residue on the surface. The residue may interact with the coating on or materials in a head gasket leading to premature gasket failure. So be sure to wash any chemical residue off the surface after the parts have been cleaned. There is no such thing as, It should be ok.
Torque-To-Yield head bolts
T-T-Y head bolts are engineered to stretch within a controlled yield zone. Once they reach this zone, they are designed to spring back to provide a more precise level of clamping force. This stretches the bolts into their elastic range, and in some cases, the stretching approaches the bolts’ elastic limit, permanently stretching it. Once the yield zone is reached, the clamping force will be more consistent. Getting the to the precise yield zone (for maximum clamping force) is accomplished by tightening bolts to a certain torque spec, then turning the bolts an additional number of degrees.
What not to do
Some machine shops and engine rebuilders use an abrasive pad in an air drill or buffer to buff off residual gasket material on head, block and manifold surfaces. It’s a quick and easy way to remove old gaskets, but it’s also risky because there’s a risk of removing metal and leaving a depression that will be hard to seal. For this reason, some experts say this approach should not be used to remove gaskets. Instead, hand scraping (carefully) or thermal or chemical cleaning should be employed.