This article is extract from www.enginebuildermag.com which wriiten by Larry Carley, as we feel sometime we should not just blame the gasket is bad due to the problem face after the installation.
Cylinder heads and blocks may need to be resurfaced to restore flatness or to improve the surface finish, or milled to change the deck height for a variety of reasons. The deck surface on the head or block may need to be resurfaced if the surface isn’t smooth or flat. A head may need to be resurfaced after welds or other repairs have been made, or milled to increase the compression ratio. The manifold surfaces on a head may need to be cleaned up due to corrosion or erosion, or the angle changed slightly to better align with an aftermarket intake manifold. The deck surface on the block may need to be resurfaced. Whatever the reason is for resurfacing these parts, 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.
Misalignment
Before you can resurface or mill a cylinder head or block, you have to square it up with your resurfacing equipment. You can’t just plunk a head or block onto a fixture, clamp it down and start cutting. You have to line it up so the surface is parallel with the cutter head fore-and-aft, and side-to-side. Once you’ve accomplished that you can set your depth of cut and proceed with a rough cut or finish cut.
One mistake to avoid here is moving the head after you have aligned it. With some older styles of fixtures, the act of clamping down the head to make it rigid can disturb the alignment. The fixturing on some newer machines allows you to make the part rigid, then align it before you cut it. One resurfacing machine also has an indicator gauge on the rail that makes set up quicker and easier for accurate resurfacing.
You also want the part (and the resurfacer) to be as rigid as possible, with no movement while it is being resurfaced. Any movement will affect the surface finish.
Wrong Surface Finish
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 or fail. 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.
As a rule, the recommended surface finish for a traditional composition style soft-face head gasket in an engine with cast iron heads and block is 60 to 120 microinches Ra (roughness average). But the recommended surface finish for the same type of head gasket in an engine with an aluminum head on a cast iron block is smoother, typically 20 to 50 microinches Ra. On late model engines with multi-layer steel (MLS) head gaskets, the OEM surface finish recommendations tend to be even smoother, say 20 to 30 microinches Ra or even 7 to 15 Ra. But the aftermarket also sells MLS gaskets with special coatings for many of these same applications that can handle surface finishes in the 50 to 60 microinch Ra range. So you have to know your gaskets and the surface finish recommendations for them by the gasket manufacturer, or the OEM if you are using a factory-style replacement head gasket.
Don’t assume close enough is good enough. Eye-balling the surface finish will tell you if the surface is really smooth (a mirror-like finish), really rough (like sandpaper) or somewhere in between, but it won’t tell you if you are in the recommended range. Dragging your fingernail across the surface isn’t much better, either, because a 30 Ra finish feels almost identical to a 50 Ra finish. And the smoother the finish gets, the more difficult it is to see or feel much difference.
The only way to accurately determine if the surface finish is within the correct range is to check it with a profilometer. This is an expensive electronic instrument that drags a diamond-tipped stylus across the surface to calculate its profile characteristics. The profilometer can then display various values for the surface including roughness average (Ra), average peak height (Rpk), average valley depth (Rvk), and even waviness. These numbers may not be needed for an economy Chevy 350 rebuild, but they can be critical when building high performance engines or durability engines. The mistake to avoid here is assuming the surface finish is correct when you haven’t actually measured it.
Wrong Feed Rate/Speed
The quality and smoothness of the surface finish requires using the correct feed rate and speed for the type of tool bit. This, in turn, will vary depending on the diameter of the cutter head.
To achieve the best possible finish, you should use a higher spindle speed and lower table feed rate with a very shallow cut on the final pass (less than .001").
If you are using a carbide insert to refinish a cast iron head, the spindle rpm required will typically be about 140 rpm for an 11-inch cutter, 120 rpm for a 13-inch cutter or 110 rpm for a 14-inch cutter.
With CBN (cubic boron nitride) or PCD (polycrystaline diamond) inserts, the recommended spindle speeds are much higher: 1040 rpm for a 11-inch cutter, 880 rpm for 13-inch cutter, or 720 rpm for a 14-inch cutter.
If the head or block being resurfaced is harder, high silicon content alloy, the speeds need to be slowed down a bit: 690 rpm for a 11-inch cutter, 580 rpm for a 13-inch cutter or 540 rpm for a 14-inch cutter. With a single CBN or PCD insert cutter spinning at 1,000 to 1,500 rpm, the feed rate should probably be less than two inches per minute on the final cut to achieve a surface finish in the low teens.
Removing Gaskets The Wrong Way
Engine disassembly is a dirty, greasy, time-consuming job, so any shortcut that makes the work go faster is a good idea, right? Maybe not if the short cut ends up damaging parts or creating more work for you in the long run.
The practice we’re talking about here is using an abrasive pad in a drill to grind off gasket residue that may be stuck to the heads or block. The abrasive will certainly whiz the gasket debris right off, but it can also whiz off metal leaving a shallow depression, a dig or a groove that may create a sealing problem when the engine is put back together.
Another reason not to use an abrasive disk to grind off or clean a surface is that it generates a lot of dust. Some gasket fibers may be hazardous to breathe. A dust mask can protect your lungs, but the residue can end up in other places where it may cause problems later (like in the cylinders, intake ports, oil or coolant passages).
The best way to remove gaskets is with a sharp scraper and/or a can of aerosol chemical gasket remover. Spraying the gaskets with a chemical remover eliminates hard scraping and the risk of scratching or gouging the surface, especially on soft aluminum heads and blocks. The chemical does most of the work by softening the gaskets. The residue can then be easily scraped off the surface.
One mistake to avoid here is using the wrong tool to scrape off the gaskets. An old screw driver is not a gasket remover. Nor is a putty knife. A gasket scraper is the right tool to use because it has a sharp, beveled edge that gets under and lifts the old gasket from the surface. Just make sure the scraper is sharp (it should be sharp enough to cut paper).
The trick to using a gasket scraper correctly is to scrape at an angle that is almost parallel to the surface. By keeping the angle small, the tip of the scraper will slip under the gasket and shear it away from the surface without digging in. If you try to use it like a chisel, you’ll probably end up gouging the surface and damaging the surface. Also, hold the scraper so you push it forward (away from you) as you scrape. This way, if the tool slips it won’t gouge you.
Not Checking Flatness
Never assume a head is flat. You can’t tell if a head or block is flat or not unless you measure it with a straight edge and feeler gauge. You should always check for flatness, especially in critical areas like those between the cylinders.
Flatness specifications vary depending on the application, but on most pushrod engines with cast iron heads, up to .003" (0.076 mm) out-of-flat lengthwise in V6 heads, .004" (0.102 mm) in four cylinder or V8 heads, and .006" (0.152 mm) in straight six cylinder heads is considered acceptable. Aluminum heads, on the other hand, should have no more than .002" (.05 mm) out-of-flat in any direction. On a performance engine, the flatter the better.
If the face of an aluminum head is warped, don’t assume the only way to straighten it is to grind metal off the face until it is flat again. The whole head is warped. If the head has one or two overhead camshafts, the cam bores will also be misaligned in most cases. The best fix here is to straighten the head BEFORE it is resurfaced. This can greatly reduce or possibly even eliminate the need to remove more than a couple thousandths of metal.
Aluminum heads can be straightened by countershimming the head on a heavy steel plate (place shims under either end of the head to offset the amount of distortion), clamping it down, then heating it in an oven to about 425° F for several hours, then letting it slow cool. The goal is to get the cam bores straight. Once they are in alignment, chances are the face of the head will be reasonably flat, too, and require minimal machining to refinish the surface.
Another method for straightening aluminum heads is to use a torch to head the top of the head, starting in the center and working towards the ends. The trick here is to keep the head temperature under 500° F to prevent softening the head too much.
Taking Too Much Metal
A head or block with a depression in the surface, or a surface that is out-of-flat can be made flat again by simply increasing the depth of cut when the part is resurfaced. The rule here is to always remove the LEAST amount of metal that’s necessary to restore flatness. Remove too much metal and you could end up with problems.
Excessive milling reduces the volume of the combustion chambers and increases compression, possibly to the point where detonation may become an issue even with higher octane fuel. On overhead cam engines, milling too much metal off the face of the head changes the installed height of the head and retards cam timing. On a pushrod head, it will alter the valvetrain geometry and may require corrections in the length of the pushrods. The only way to restore lost head height and combustion chamber volume is to use a copper or steel head shim with the head gasket, or replace the head.
Don’t ‘Make Do’ With Outdated Equipment
Extremely smooth finishes require high quality resurfacing equipment (typically a milling machine) to achieve really low Ra numbers. It doesn’t matter if you use carbide, CBN or PCD tool bits to resurface a head as long as 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.
For example, a converted grinder may be able to mill heads and blocks. But the spindles and table drives in many of these older machines cannot hold close enough tolerances to achieve a really smooth, flat finish. One equipment manufacturer said grinding and milling machines that are more than five years old are probably incapable of producing consistent results and should be replaced.
Most of the surfacing equipment that’s being sold to shops today has been redesigned for high speed milling with CBN and PCD. The machines have been beefed up with more powerful motors, heavier castings, electrically-driven ball screw tables, and tighter assembly tolerances. Some can hold machining tolerances to one tenth of a thousandth of an inch (.0001")!
Using The Wrong Tools
Though the experts recommend using PCD on aluminum and CBN on cast iron, many shops find CBN works fine on both types of metals and eliminates the need to change tooling when resurfacing different types of heads.
CBN may not be the best choice for milling aluminum because aluminum tends to stick to CBN and leave a smeared finish. Even so, there is a way to prevent this from happening: just spray a lubricant on the surface or the cutter. According to one source, the absolute best lubricant to use for this purpose is olive oil. Only a little is needed, and it’s non-toxic, doesn’t stink and is relatively inexpensive.
PCD works better on aluminum than CBN (and costs about the same), but PCD is not recommended for resurfacing cast iron heads or blocks because diamond gets too hot at high cutting speeds, reacts chemically with iron and breaks down. CBN can handle higher temperatures than PCD, and dissipates heat about four times faster than silicon carbide or aluminum oxide, making it a good choice for high speed resurfacing.
Something else that must be considered when using CBN to resurface heads and blocks is the depth of cut. CBN inserts typically have a honed edge, so the minimum depth of cut is usually limited to about .004" or .005" on cast iron. If too shallow a cut is attempted, the result can be edge deterioration, poor tool life or chipping of the insert (CBN is sometimes coated with titanium to improve tool life).
Trying To Get Too Much From A Bit
CBN and PCD last a lot longer than carbide, but they don’t last forever. One common mistake that’s made is trying to cut too many heads or blocks with the same edge. If you are using a CBN button for resurfacing, you should rotate the button about 5 degrees after 20 to 30 heads to maintain an optimum cutting surface.
Rotating the button just a little bit when it starts to get noisy will expose a fresh edge and reduce the risk of chipping the button or wearing it too far. Buttons with a beveled edge can be relapped to restore the edge if they are not too badly worn. But if the button has lost too much of its edge, the only option is to replace it with a new one.
Resurfacing Diesel Heads With Precombustion Chamber Cups
Here’s a job that can ruin a tool insert in seconds. The issue here is not the difference in hardness between the cylinder head and the precombustion chamber cubs, but the fact that many of these cups are slightly loose. Staking the cups won’t lock them in place because the first pass with a resurfacer will shave off the staking.
One solution is to remove the cups, resurface the head, then get the cup counterbore to the proper depth and reinstall the cups. Another solution is to make the cups rigid by applying a penetrating locking compound around the cups BEFORE you resurface the head. This saves the time and labor of removing and reinstalling the cups.
Trying To Resurface Rusty Heads
Iron oxide on a cast iron head will kill the life of a tool insert. The same goes for hard calcium deposits in water jacket openings. The cutter tool bit can also pick up this debris and drag it across the surface, leaving a groove. The mistake to avoid here is trying to resurface a dirty head that has not been properly cleaned. Remove all of the rust and calcium BEFORE the head is resurfaced. This can be done with chemical cleaners, a shot blast cleaner or a tumbler.
Not Wearing Eye Protection
Here’s another common mistake some engine builders wish they hadn’t made when resurfacing parts: not wearing eye protection. The cutter guard on a resurfacer will deflect most of the chips down, but a lot of debris still goes flying every which way while the machine is running. If you lean down to take a close look while the machine is running, you may get hit in the face with some microscopic metal shards. That’s why you want eye protection,: either a full face shield or safety glasses with side covers. You have two eyes so losing one may not leave you blind, but it will mess up your depth perception.
Lack Of Maintenance
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. But the resurfacer itself needs to be set up correctly and checked periodically to make sure it is still in proper alignment.
Resurfacers need to be leveled with an accurate level. Place the level on the ways of the machine or on the table mounting surface. Adjust the machine front to back as well as left to right until it is perfectly level in all directions.
Next, check the table to make sure it is running true. Attach a magnetic dial indicator in the cutter head and traverse the left and right to see if the table is true to the wheel head. You should see no more than .002" of variation across the entire traverse of the table. If the table is not running true, contact the equipment supplier for the correction procedure.
Also check the parallels, using a dial indicator and granite plate to make sure they are straight. If the parallels have runout, the resurfacer won’t cut straight.
The cylinder head and block rollover clamps also need to be checked to make sure everything is straight. If the cutter head has more than one tool bit, all must be set to equal height (no more than .0005" variation) to get consistent results.
1 comment:
Thank you for great posting very informative.
Daihatsu
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