Click on a thumbnail to see the full-size image
Getting Your Head Straight About Head Straightening
By Brendan Baker
Along with new-style aluminum cylinder heads came a few inherent characteristics – thinner castings, making it difficult or impossible to resurface a cylinder head; multi-layered head gaskets that require ultra smooth surfaces to achieve proper clamping force; and overhead cams (OHC), making a warped head more difficult to straighten than non-OHC heads because now you have to align two surfaces instead of one.
When engine builders first started seeing these new-style cylinder heads about 15 years ago many discovered that they couldn’t straighten them the same way as the old cast iron heads. With OHC heads both ends must be straightened, primarily focusing on cam bore alignment.
"When these aluminum cylinder heads started hitting the market several years ago, engine builders treated them like Chevrolet heads at the time," says David Monyhan, Goodson Shop Supplies, Winona, MN. "Many rebuilders thought that all you had to do was resurface it, put the cam back in and bolt it back on the block – then they found that the cam wouldn’t turn over. That’s when they discovered that they needed to straighten the heads first and get them close before they did any machining."
Today, the most widely used method for straightening aluminum heads involves a straightening plate and an oven to heat the aluminum head enough to bend it back to its original form. While this method has proved to be a successful way to get the warp out, it can be a very time-consuming endeavor. There are shortcuts but they’re not for everyone. Faster methods have been known to destroy otherwise rebuildable cores.
How Hot Is Too Hot?
As a cylinder head is heated to high temperatures it softens and becomes annealed. If an aluminum head is subjected to too much heat for too long it will not be as strong as it once was.
"Most aluminum heads are hardened after the casting has been done," says Bill McKnight, instructor, Dana/Clevite Engine Parts, Ann Arbor, MI. "The longer the heads soak in an oven at high temperature the more hardness they lose. We talked to our metallurgist and he said that at 425° F the heads were pretty safe in that range. The aluminum heads showed only a 5-7 percent reduction in hardness, which isn’t bad. But as you raise the temperature to higher levels you take the hardness out of it."
Most of our experts agreed that baking heads at temperatures on the lower end of the 400° - 500° F range is preferable today for several reasons. For one reason, manufacturers are using a variety of metals and composites in their castings. Subsequently, some heads anneal at different temperatures than others.
"You have to look at each cylinder head separately because some have composite fillers in them," says Tom Vondem Kamp, Peterson Machine Tool, Council Grove, KS. "These fillers can be affected by high temperatures, which is why you have to be careful when heating them. If you bake it too much, then the filler goes out of the head and you’ll have a leaker."
Ric Havel of Moto Tec Engineering, West Salem, OH (he is also a welding and machine shop consultant to Sunnen and Lincoln Electric) notes that one of the problems with heating aluminum heads to 500° F is, "at that temperature it might as well be chewing gum, it’s as soft as that." According to Havel, the softened aluminum can face overbending problems as well as allow fasteners to sink into the head, which can add more repair work and frustration.
In fact, according to the Engine Rebuilders Association (AERA), the typical aluminum head, made of 356 alloy and hardened to T-6, will anneal when subjected to temperatures above 400° F for two hours or 600° F for as little as one minute. Annealing will subsequently lead to all kinds of problems from guides and seats falling out to improper torque values.
The Time Factor
If you were to see an instruction manual for straightening heads it would likely read like this: “Measure the warp on the bottom and split the difference. If it’s .020˝ out put a .010˝ shim on either end of the head and bolt it down to the plate. Put it in the oven and heat it to 500° F. Two hours into the process, pull it out of the oven, retorque the bolts and push it back into the oven. Let it go for another few hours then let it slow cool. Remove it and check the result. If you didn’t remove enough warp, reshim it and do it again."
"There is no black and white method of determining when a head is going to take too much time to repair as opposed to finding a good core. It just takes a lot of experience to know where that line is," says Tom Zelinsky, Hasek Auto Service & Supply, Cleveland, OH. "Nissan heads we were doing took a lot more time than what people were willing to pay for them to be repaired. So we decided it wasn’t worth repairing and had to look for good cores instead – to keep the costs down."
Your liability on straightened heads can also be an issue, say some rebuilders. If an installer torques the straightened head down incorrectly it may bind the cam again and you may get it back. Therefore, you have to be careful with certain heads that may have cam binding problems.
"The time involved with straightening heads is the biggest issue," says Ric Havel. "What one prominent rebuilder told me after an article I wrote (March 2003, Engine Builder, page 28) was that he didn’t know you could avoid the process of bringing the head slowly back down to temperature in an oven (The process requires slowly, incrementally easing the piece back down to ambient temperature.). He said that this process added an extra day to the job because it couldn’t be started until night. It tied up their cleaning ovens, too, so they couldn’t clean at night either. Now he says (since stopping the slow cool procedure) they’re saving six hours per head!"
"One of the difficulties with following the standard process for straightening is that unless you have a bank of ovens like a Pizza Hut, you can only do one head at a time. And the worst thing is, if you don’t get the results the first time you have to go back and do it all over again," says Havel.
The oven method for straightening cylinder heads is, according to industry experts, the safest straightening method but not necessarily the fastest. There are some other methods that are being used by rebuilders successfully, however, most of these methods require special training and sometimes more expensive equipment to implement properly.
One alternative straightening method is what is called "torch" or "flame" straightening. There are a few
variations of this method but in essence you follow the first couple of steps of the standard procedure – identifying the warp, shimming half the distance of the warp and bolting the head to the straightening plate.
The biggest advantage to flame straightening is that it is much faster than the standard method of heating and cooling it in an oven for hours. However, only through experience will you be able to do this method as you will probably destroy the first few heads you straighten this way.
"You heat it just to the melting point where the tower or largest mass of metal is on the head," says Federal-Mogul’s Kearns. "There is usually a pillar of metal in every head to support cams and to add strength to the casting. You’re heating at the points where there is the largest mass of metal. You don’t want to heat it in areas that are very thin. You heat it in one spot just to where the metal starts to melt. Start from the center of the head and work your way out. You may have to heat it in several spots on both sides of the head to get it straight. But if you burn through the head," he says, "you’ve got a boat anchor."
Some rebuilders believe that the torch method is very harmful to the integrity of the cylinder head, however, rebuilders that use the torch method successfully say that the temperatures do not go above 500° F and don’t harm the heads at all.
"Most cylinder heads should be able to be straightened this way," says Kearns. "But it takes experience to be able to do it without ruining the head, so it’s a good idea to practice on bad cores first."
Shai Dhanani, production manager, Yamato Engines, Bellingham, WA, says that they use a slightly different variation of the torch method to straighten its warped heads.
"We used to put them on plates and bake them in the oven but that took a long time," says Dhanani. "We found a guy in Australia who taught us a technique to relieve the bend using just a torch in certain spots on the cylinder head. And that’s the system we are using today."
The method that Yamato uses doesn’t require any shimming or straightening plates. The warp is first identified and then it is placed on a table with both ends of the head elevated slightly. You’re essentially heating the areas that are the most stressed and relaxing them.
"You heat it until it just starts to sweat," says Dhanani. "The problem with putting heads in the oven is that you are annealing the aluminum no matter how you look at it. With this torch system you are not changing the integrity of the aluminum at all. When you heat the area and it starts to sweat you back off so you’re not melting it, just sweating it in certain spots. We’re only talking about small little circles of heat and in areas that have thick channels of aluminum."
"There are certain areas on the head where if you heat it up close to the melting point it actually straightens the head. We don’t use any plates or ovens – you can straighten a head in five minutes using this method," explains Dhanani. However, he notes that this method may not be for everyone because you can easily ruin a head if you don’t do it correctly.
"If you want to know how easy this method was to learn, I learned how to do it in five minutes," says Dhanani. "But to do it well it takes a lot of experience. Our guy who does this just knows where to put the torch because he’s been doing it for so long."
Yamato also align bores about 85 percent of its OHC heads, Dhanani says. Yamato has an oversize spray welding system in-house that is used to build up the cam journals.
"We machine the cam tunnels oversize to accept the oversize cams," says Dhanani. "You have to remember that align boring speeds the process of straightening because if the head is off a bit after it’s straightened it doesn’t matter because we’re going to align bore it anyway.
"The reason we do the oversize cams is that if you measure the wear on most of what you’re rebuilding you shouldn’t be able to use them because the [cam bores] are too worn. You run the risk of oil pressure problems, lifter problems and so on if you don’t align bore. If you want to do it right you should be align boring anyway," Dhanini says.
Moto Tec’s Ric Havel says that his method of straightening is combined with bake cleaning, so you can be paid for cleaning and straightening in one efficient process.
Havel’s method is an evolution of the plate/oven process. He says that the way some shops are straightening heads today could lead to potential problems down the road. The issue of annealing is high on Havel’s list of concerns.
According to Havel, who has been straightening cylinder heads since the late ’80s, there is too much variation in how one shop straightens a cylinder head compared to another. He says that it is very possible to straighten a cylinder head without having to line bore. With his unique method he can turn around a warped head in about 1-1/2 hours.
"First of all, stop and consider this, how long did it take the head to warp? My thinking on this was very fundamental: If it only took a few minutes to overheat then why should it take a lifetime to fix it? I call it barnyard technology."
The main area of concentration with Havel’s method is the cam bore area. "The shim and gasket companies have made it easy to take care of the deck," he says. "The most elusive area is the cam bores."
There are three keys to straightening heads Havel’s way:
1) No contact – the cylinder head should not touch the straightening plate because "aluminum is the thief of heat." The steel plate transfers too much heat to the deck and embrittles the head. Havel shims the head high enough so when it is torqued down in the middle it won’t touch the plate.
2) Less oven time – annealing is the issue here. Havel says that using a bake cleaning oven, which heats up rapidly, and only exposing the casting to heat for 20-30 minutes preserves the integrity of the casting.
3) Use a deadstop – a deadstop keeps the head from overbending and from coming in contact with the steel straightening plate. Havel shims the head higher and torques it down 25-30 ft.lbs, then he slides a piece of shim stock under the center of the head.
Havel has a lot of theories as to the whys and why nots of other methods but his biggest concern is the integrity of the casting. "The only real proof of when you’re stress relieving something is when it’s unfixtured and it absorbs the type of heat that is in the environment it operates in and it stays where it belongs – then it has been truly normalized. If it moves back to where it was before, then obviously you didn’t do much of anything."
It can be a tough call on whether or not a cylinder head is even worth straightening. An improperly straightened head is bound for trouble down the road and you may have to do it over again. That said, however, having the capability to straighten a warped head is an asset that can help set your engine building services apart from your competition. The important thing to remember is that you want a quality product no matter what.
RIC HAVEL’S STRAIGHTENING METHOD
- With the traditional method of measuring this 1.8L Brazilian GM head appeared to be flat. However, by measuring the thickness it was off .022˝ .
- There is in essence .030˝ rock left to right.
- Tighten cold to straighten and deadstop the head with shimstock.
- The discoloration from the shimstock is the only area that transferred heat to the head.
- Head being cleaned and straightened in oven, laying on three 12˝ strips of shimstock.
- The straightened head is turned upside down. The area that was the thinnest is on the right side. Now all that is left is to shim the thickest end to .014˝ and surface the head. (photos courtesy of Moto Tec Engineering)