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I agree with Jeff.
Many manfacturers specify that you are supposed to change the head bolts. However, I have swapped quite a few heads, and never have chaged the bolts. I do not kow what the manufacturer specifies for your vehicle, but if you consult a service manual you will find out.
The reason:
The bolts are designed to stretch when assembled. _All_ bolts "stretch" when they are under tension. That is just how materials work. If the bolt was approximated as a simple solid the deflection "stretch" would equal PL/EA where P is tensile force, L is the length of the bolt and E is youngs modulus and A is the cross sectional area.
For gaskets and sealing surfaces it is important to maintain a preload under all conditions. (By preload I mean that the net force between the block and head is pulling the block towards the head) If we imagine the bolt as a tensile spring that we are using to keep the head in intimate contact with the block, you could see that if we pushed hard enough we could push the head off the block and there would be a leak.
It is a bit cumbersome to explain but remember that all material is springy, and has some spring rate. For a crude visualisation forget the fact that the bolt is a spring and the force it pushed on the head will increase if the head moves, and replace the bolt with a force. So if the combustion process is pushing the head up with X pounds of force, the bolt better be pulling down with greater than x pounds of force, or the head will move away from the block. (In reality al the deflections of all the parts Block, head, bolt need to be accounted for, and the sum better put the head in contact with the block)
The two games we can play to keep sealing surfaces sealed are the stiffness of the spring and the preload in the spring. (The spring being the bolt) We can control the stiffness of the spring by changing the geometric and material properties of the bolt. We can control the preload by controling the amount we tighten the bolt.
The whole reason that you are supposed to change head bolts when you swap some heads is that many manufacturers secify torque settings that more than "stretch" the bolt but actually yield the bolt. For the non-engineers when I say strech I am refering to elastic stretching where when the load is removed, the bolt will return to its original size. Yield implies that there was some in-elastic stretch, or that when the load is removed, the bolt will not return to its original size. (It will get smaller, but not as small as it used to be before teh load was applied.) This is to maximize the amount of preload that the bolt will generate so as to keep the block and the head in contact.
Once the bolt has been yielded its ultimate tensile strength has changed (usually it goes up) and the amount of enregy it can absorb before it breaks changes as well (usually goes down) To ensure that the bolts are performing as specified the OEMS just tell you to change them every time.
You may wonder why not just use a really stiff bolt and not have as much tightening to get the same preload force. This is possible, but it increases the sensitivity of the preload joint to deflection. Lets say we have a joint that requires 500 pounds of force from the bolt. We have a very stiff bolt, lets say 50000 pounds per inch deflection and a less stiff lets say 5000 pounds per inch of deflection bolt. The first bolt would require 0.01 inches of deflection to achiece the required clamping force and the second would require 0.1 inches. If they were the same pitch (threads per inch) the second bolt would be turned ten times more than the first for the required load. Lets say the first bolt needed 0.1 turn to reach the load the second would need 1 turn for the same load. The stiff bolt would be very sesitive to small changes in dimensions. If it backs off ever so slightly there will be a large change in preload. The "soft" bolt will have smaller changes in deflection for a given change in dimension.
This brings up the topic of "lock washers" (Basicall a series spring that will reduce the spring rate of the overall bolt) but I don't have time.
-Joe
posted by 198.208....
, Wed, 23 Jan 2002 12:53:11
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