Cylinder head resurfacing is a very common process today, especially with all the leaking head gaskets.
Above all, cylinder head resurfacing is the most common machining operation done in machine shops to date.
To make matters worse many different material combinations like aluminum; cast iron; and steel all require their own specific surface finish.
So, There are many other reasons for cylinder head resurfacing besides a head gasket failure.
- Cylinder heads may need to be resurfaced; to restore flatness or to just improve the current surface finish.
- A cylinder head may need to be resurfaced; after welds or other repairs have been made; or milled to increase the compression ratio.
- The angle changed slightly to better align with an aftermarket intake manifold.
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.
To seal properly, a head gasket requires a surface finish that is within a recommended range.
The specifications vary depending on the type of head gasket. If the surface is too rough, or in some cases too smooth, the gasket may not seal properly and leak. One common mistake to avoid here is not looking up the recommended specifications; 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.
Many machine shops today have switched to dry milling.
This is because it eliminates the mess and maintenance that go with wet grinding.
Getting The Correct Surface Finish
To achieve smoother finishes required by many of today’s aluminum heads; you need to use the right combination of table feed and (rpm) when milling the cylinder 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 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
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. This is 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. Due to 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.
How Important Is Cleaning
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.
Cylinder Head Resurfacing, 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. 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.
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 the interest in belt sanding has dropped off because; it is not as precise as milling or grinding; and relies too much on the individual operator.
All Determined By:
- The amount of downward pressure exerted by the operator.
- Positioning of the head on the sander.
- The condition of the belt.
As a result, Belt sanding is better for; clean up work and resurfacing hard-to-fixture parts like, manifolds and timing covers.
So, You can’t expect to get high quality surface finishes if you’ve neglected your equipment. Dry milling doesn’t require any coolant so there’s no coolant to maintain.
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