Joined
·
3,126 Posts
I think, my article below could help to point out some difficulties with head machining. I had BHG on my 79 Supra 2 times because of them.
Machining challenges, or why the gasket fail
In less then 24 hours after the rough draft of this term paper was completed, the car which I was writing about was totalled in rear end collision.
Note: illustrations are omitted due to file size
The purpose of this report is to analyze each commonly used cylinder head refinishing processes and answer the question, what caused the failure of the recently replaced cylinder head gasket on 1980 Toyota Supra 2.6 liter engine. Despite huge number of the “expert’s” suggestions, relating this failure to the car’s age, hot spots, poor head design and overheating (which never happened), the author of this effort suspected, that gasket failed due to the poor finish of a head sealing surface and to the excessive out-of-flatness of it (Figures 1 and 2). In order to find, what exactly did happened, the author inspected the head surface and conducted research about the common mistakes in methods, used by today’s machine shops to resurface the head prior the reinstallation.
Based on Toyota specifications, the Ra number for a head gasket sealing surface is
40 to 60 micro inches and the surface warpage is .002 inches maximum on the length of 27 inches (1-5.3). In order to obtain such surface geometry, highly trained personnel and precision machinery must be available. The shops, however, are not interested to spend extra time to control machine accuracy and often do not inspect the finished product, they just sell the substandard work to the customer, because many engine rebuilders cannot inspect the head either (the accurate digital hand held profilometer costs more then $450 (2-1)). When the author of this report made a visit to the head shop and asked to take a look on the machining and inspection procedures, he was escorted out from there. So, he investigated the challenges associated with milling, grinding and belt sanding – the most common head resurfacing methods.
As written in the several articles, in the past it was possible to get a satisfactory surface finish with almost anything that could remove metal whether it was a grinder, carbide fly-cutter, broach or belt sander (3-12).
In some cases, older grinding equipment can be upgraded and converted to milling by changing the head. But some equipment suppliers say it is not possible to get the same results using a cutter head in a modified grinder. It is better to go with a dedicated milling machine. Yet there are combination grinding/milling machines that can do both equally well and provide the flexibility to use whichever method works best in a given situation. The examples of milling machines are presented on Figures 3 and 4.
Dry milling of heads require surfacing equipment that can deliver a smooth, flat, high-quality finish. The OEMs and many production engine builders use surfacers that have high speed multi-cutter heads. The more inserts there are in the cutter, the faster it can transverse the surface. Faster cutting saves time and boosts productivity in a high volume shop. But multiple inserts also increase the cost of the equipment and maintenance, and require more skill and effort to set up than a simple single insert cutter head (3-12).
Many shops like the simplicity of a single insert cutter because it is so much easier to set up and adjust. If a cutter head has two or more inserts, all the inserts must be set to the same height or else it will leave an uneven cut on the surface. So that is why many shops prefer the single insert setup.
Figure 1. The auto repair experts blamed the car’s age for head gasket leak
To achieve the best possible finish, the operator should use a higher rpm and lower feed rate with a very shallow cut on the final pass (less than .001”). With a single insert cutter spinning at 1,000 to 1,500 rpm, the feed rate should probably be less than two inches per minute to achieve a surface finish in the low teens. If operator bumps up the cutter rpm, he can also increase the feed rate while still maintaining the same surface finish. Using a surfacer that has an adjustable rpm and feed rate, therefore, gives flexibility to play around with the setting, so it is possible to find the optimum combination that gives the best surface finish (3-14).
To achieve today’s flatness and smoothness requirements, a surfacer must be a very rigid machine. The work table, cutting head, shaft and motor must all be very strong and constructed to extremely tight tolerances. Any flexing or movement in these parts will affect the quality of the surface finish by creating waviness.
Motors should have zero tolerance bearings. Play in the bearings may allow the motor shaft to move up and down when the motor changes speed or when the cutter encounters resistance. There should be no vertical movement in the motor or cutter head while the milling process is taking place.
The type of feed mechanism the surfacer uses is also important. Hydraulic surfacers have been around for many years and offer infinitely variable feed speeds. A hydraulic feed uses pressure to push the table along on slides. At low feed rates, though, the table may stick momentarily as it moves, creating a “stick and slip” motion that may leave an uneven finish on the head. That is why many surfacers now have electric drive ball screw feed mechanisms. A motor driven ball screw mechanism provides a higher degree of control and consistency, which helps the machine achieve ultra-low Ra surface finish numbers. Some machines with ball screw mechanisms can hold tolerances to within .0001”.
Ease of setup includes being able to mount a head on the surfacer quickly, level it and align the cutter head. The fixture should be capable of accommodating a wide variety of different cylinder head configurations as well as different lengths. The Winfield fixture is very popular and can accommodate variety of heads. It usually takes only 5 to 8 minutes to set up, which can really save a lot of time in a busy shop.
Setup accuracy is absolutely essential because there is so little margin for error on most engines today. In most cases, the machinist will be removing very little metal and just skimming across the surface to clean it up. Some surfacers have a built-in dial indicator that makes it easier to set up the machine. Time is money and the less time it takes to set up a cylinder head to be resurfaced, the more time can be spent resurfacing metal. Faster turn around means more production and hopefully more profit at the end of the day. Milling machines can use carbide inserts, specially coated carbide, CBN (cubic boron nitride) or PCD (polycrystalline diamond) inserts.
Carbide can be used on aluminum or cast iron, is the least expensive type of insert and gives a high quality finish. Coated carbide inserts provide improved cutting action and deliver even better results than plain carbide. But carbide does not last as long as the more expensive super abrasives such as PCD or CBN.
PCD is generally recommended for milling aluminum. CBN works great on cast iron and can be successfully used to mill aluminum provided some type of lubricant is used. A light coating of soap, wax or WD-40 can prevent the aluminum chips from sticking to the CBN tooling and scratching the surface.
Though most shops that mill heads do not use any coolant, milling with a coolant has a number of advantages. Coolant helps to wash away chips for a cleaner more consistent cut. It also keeps the tooling and work piece cooler, which reduces heat buildup, distortion and tool wear. Consequently, with a coolant generally improves the overall process and improves the surface finish.
When resurfacing an aluminum head that has a lot of lime built up in the water jackets, the hard calcium deposits around the water jacket openings can sometimes be picked up by the tooling and drug across the surface leaving a groove. Removing the deposits before the head is resurfaced can eliminate this particular problem.
On the other hand, the older, less equipped shops prefer a different process, called wet grinding. It is used by many rebuilders because grinding can produce a very smooth finish. Silicon carbide grinding wheels and segments are generally recommended for both cast iron and aluminum. Grinding aluminum can be tricky because the stone tends to load up with metal, causing it to overheat and score the surface (Figure 6). Pre-coating the aluminum with lubricant and using plenty of coolant should prevent clogging. A faster transverse speed should also be used, and the depth of cut limited to no more than .0005 to .001 in. It is also important to dress the grinding wheel often to keep the grain open --- but not on the final pass, so the stone will leave a smoother finish (3-14). Surface grinding machines are shown on Figure 5.
Figure 6. Poor grinding wheel condition resulted in scoring and scratches
Next, the worst shops use belt sanders, just because it can be a real time saver. The heads 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. The amount of downward pressure exerted by the operator, how the head is positioned on the sander and the condition of the belt can all affect the results. Consequently, some say belt sanding is better suited for clean up work or resurfacing hard-to-fixture parts like manifolds and timing covers. Typical belt sander, that damaged Toyota Supra’s head, is shown on Figure 7.
For resurfacing aluminum, silicon carbide belts are generally recommended. Either #40 or #80 grit belts can be used with aluminum, but #80 grit is the preferred choice and should be used with no downward pressure on the head.
It is also important to replace the belt regularly because a dull belt can overheat the head causing warpage and an uneven surface finish (3-15).
Figure 7. Peterson belt sander
Ideally, as the street racing specialist suggests, the head should be milled first, and then lapped on the plate (Figure 8). This will assure great flatness and low Ra – those two main requirements to reliably seal combustion on bimetal engine with long cylinder head (4-6).
Figure 8. Lapped surface
The author of this report concludes, that the gasket on test car failed due to poor condition of resurfacing machine, inexperienced operator, or both. The gasket failure was not caused by engine design or old age of the car, but by lack of knowledge about the process, lack of education and disrespect to the customer. Today, the car repair or engine machining is no longer a general thing but an engineering science.
REFERENCES:
Figure 1 from Toyota Supra registry page,
Machining challenges, or why the gasket fail
In less then 24 hours after the rough draft of this term paper was completed, the car which I was writing about was totalled in rear end collision.
Note: illustrations are omitted due to file size
The purpose of this report is to analyze each commonly used cylinder head refinishing processes and answer the question, what caused the failure of the recently replaced cylinder head gasket on 1980 Toyota Supra 2.6 liter engine. Despite huge number of the “expert’s” suggestions, relating this failure to the car’s age, hot spots, poor head design and overheating (which never happened), the author of this effort suspected, that gasket failed due to the poor finish of a head sealing surface and to the excessive out-of-flatness of it (Figures 1 and 2). In order to find, what exactly did happened, the author inspected the head surface and conducted research about the common mistakes in methods, used by today’s machine shops to resurface the head prior the reinstallation.
Based on Toyota specifications, the Ra number for a head gasket sealing surface is
40 to 60 micro inches and the surface warpage is .002 inches maximum on the length of 27 inches (1-5.3). In order to obtain such surface geometry, highly trained personnel and precision machinery must be available. The shops, however, are not interested to spend extra time to control machine accuracy and often do not inspect the finished product, they just sell the substandard work to the customer, because many engine rebuilders cannot inspect the head either (the accurate digital hand held profilometer costs more then $450 (2-1)). When the author of this report made a visit to the head shop and asked to take a look on the machining and inspection procedures, he was escorted out from there. So, he investigated the challenges associated with milling, grinding and belt sanding – the most common head resurfacing methods.
As written in the several articles, in the past it was possible to get a satisfactory surface finish with almost anything that could remove metal whether it was a grinder, carbide fly-cutter, broach or belt sander (3-12).
In some cases, older grinding equipment can be upgraded and converted to milling by changing the head. But some equipment suppliers say it is not possible to get the same results using a cutter head in a modified grinder. It is better to go with a dedicated milling machine. Yet there are combination grinding/milling machines that can do both equally well and provide the flexibility to use whichever method works best in a given situation. The examples of milling machines are presented on Figures 3 and 4.
Dry milling of heads require surfacing equipment that can deliver a smooth, flat, high-quality finish. The OEMs and many production engine builders use surfacers that have high speed multi-cutter heads. The more inserts there are in the cutter, the faster it can transverse the surface. Faster cutting saves time and boosts productivity in a high volume shop. But multiple inserts also increase the cost of the equipment and maintenance, and require more skill and effort to set up than a simple single insert cutter head (3-12).
Many shops like the simplicity of a single insert cutter because it is so much easier to set up and adjust. If a cutter head has two or more inserts, all the inserts must be set to the same height or else it will leave an uneven cut on the surface. So that is why many shops prefer the single insert setup.
Figure 1. The auto repair experts blamed the car’s age for head gasket leak
Figure 2. Uneven surface profile and finish resulted in rapid gasket failure
Figure 3. Modern milling machine
Figure 4. Older Storm Vulcan milling machine
Figure 3. Modern milling machine
Figure 4. Older Storm Vulcan milling machine
To achieve the best possible finish, the operator should use a higher rpm and lower feed rate with a very shallow cut on the final pass (less than .001”). With a single insert cutter spinning at 1,000 to 1,500 rpm, the feed rate should probably be less than two inches per minute to achieve a surface finish in the low teens. If operator bumps up the cutter rpm, he can also increase the feed rate while still maintaining the same surface finish. Using a surfacer that has an adjustable rpm and feed rate, therefore, gives flexibility to play around with the setting, so it is possible to find the optimum combination that gives the best surface finish (3-14).
To achieve today’s flatness and smoothness requirements, a surfacer must be a very rigid machine. The work table, cutting head, shaft and motor must all be very strong and constructed to extremely tight tolerances. Any flexing or movement in these parts will affect the quality of the surface finish by creating waviness.
Motors should have zero tolerance bearings. Play in the bearings may allow the motor shaft to move up and down when the motor changes speed or when the cutter encounters resistance. There should be no vertical movement in the motor or cutter head while the milling process is taking place.
The type of feed mechanism the surfacer uses is also important. Hydraulic surfacers have been around for many years and offer infinitely variable feed speeds. A hydraulic feed uses pressure to push the table along on slides. At low feed rates, though, the table may stick momentarily as it moves, creating a “stick and slip” motion that may leave an uneven finish on the head. That is why many surfacers now have electric drive ball screw feed mechanisms. A motor driven ball screw mechanism provides a higher degree of control and consistency, which helps the machine achieve ultra-low Ra surface finish numbers. Some machines with ball screw mechanisms can hold tolerances to within .0001”.
Ease of setup includes being able to mount a head on the surfacer quickly, level it and align the cutter head. The fixture should be capable of accommodating a wide variety of different cylinder head configurations as well as different lengths. The Winfield fixture is very popular and can accommodate variety of heads. It usually takes only 5 to 8 minutes to set up, which can really save a lot of time in a busy shop.
Setup accuracy is absolutely essential because there is so little margin for error on most engines today. In most cases, the machinist will be removing very little metal and just skimming across the surface to clean it up. Some surfacers have a built-in dial indicator that makes it easier to set up the machine. Time is money and the less time it takes to set up a cylinder head to be resurfaced, the more time can be spent resurfacing metal. Faster turn around means more production and hopefully more profit at the end of the day. Milling machines can use carbide inserts, specially coated carbide, CBN (cubic boron nitride) or PCD (polycrystalline diamond) inserts.
Carbide can be used on aluminum or cast iron, is the least expensive type of insert and gives a high quality finish. Coated carbide inserts provide improved cutting action and deliver even better results than plain carbide. But carbide does not last as long as the more expensive super abrasives such as PCD or CBN.
PCD is generally recommended for milling aluminum. CBN works great on cast iron and can be successfully used to mill aluminum provided some type of lubricant is used. A light coating of soap, wax or WD-40 can prevent the aluminum chips from sticking to the CBN tooling and scratching the surface.
Though most shops that mill heads do not use any coolant, milling with a coolant has a number of advantages. Coolant helps to wash away chips for a cleaner more consistent cut. It also keeps the tooling and work piece cooler, which reduces heat buildup, distortion and tool wear. Consequently, with a coolant generally improves the overall process and improves the surface finish.
When resurfacing an aluminum head that has a lot of lime built up in the water jackets, the hard calcium deposits around the water jacket openings can sometimes be picked up by the tooling and drug across the surface leaving a groove. Removing the deposits before the head is resurfaced can eliminate this particular problem.
Figure 5. Van Norman surface grinding machines
On the other hand, the older, less equipped shops prefer a different process, called wet grinding. It is used by many rebuilders because grinding can produce a very smooth finish. Silicon carbide grinding wheels and segments are generally recommended for both cast iron and aluminum. Grinding aluminum can be tricky because the stone tends to load up with metal, causing it to overheat and score the surface (Figure 6). Pre-coating the aluminum with lubricant and using plenty of coolant should prevent clogging. A faster transverse speed should also be used, and the depth of cut limited to no more than .0005 to .001 in. It is also important to dress the grinding wheel often to keep the grain open --- but not on the final pass, so the stone will leave a smoother finish (3-14). Surface grinding machines are shown on Figure 5.
Figure 6. Poor grinding wheel condition resulted in scoring and scratches
Next, the worst shops use belt sanders, just because it can be a real time saver. The heads 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. The amount of downward pressure exerted by the operator, how the head is positioned on the sander and the condition of the belt can all affect the results. Consequently, some say belt sanding is better suited for clean up work or resurfacing hard-to-fixture parts like manifolds and timing covers. Typical belt sander, that damaged Toyota Supra’s head, is shown on Figure 7.
For resurfacing aluminum, silicon carbide belts are generally recommended. Either #40 or #80 grit belts can be used with aluminum, but #80 grit is the preferred choice and should be used with no downward pressure on the head.
It is also important to replace the belt regularly because a dull belt can overheat the head causing warpage and an uneven surface finish (3-15).
Figure 7. Peterson belt sander
Ideally, as the street racing specialist suggests, the head should be milled first, and then lapped on the plate (Figure 8). This will assure great flatness and low Ra – those two main requirements to reliably seal combustion on bimetal engine with long cylinder head (4-6).
Figure 8. Lapped surface
The author of this report concludes, that the gasket on test car failed due to poor condition of resurfacing machine, inexperienced operator, or both. The gasket failure was not caused by engine design or old age of the car, but by lack of knowledge about the process, lack of education and disrespect to the customer. Today, the car repair or engine machining is no longer a general thing but an engineering science.
REFERENCES:
- Goms, Gary, Sealing the combustion, 2005
- Toyota service specifications, Toyota Motor Corporation, 1980
- Carley, Larry, Surface finish is more important then ever, 2004
- Greddy head gaskets, Technical page, 2001
ILLUSTRATIONS:
Figure 1 from Toyota Supra registry page,
Figures 2, 5, 8 from Speed shop machining service,
Figures 3 and 4 from Century machine shop,
Figures 6 and 7 from Used equipment sales.
Sept, 16, 1979 – May, 3, 2005
Figures 3 and 4 from Century machine shop,
Figures 6 and 7 from Used equipment sales.
Sept, 16, 1979 – May, 3, 2005
