Whether it is for a head gasket replacement or the reassembly of a fresh engine rebuild, many wonder as they get to the assembly stage what to do with the old cylinder head bolts. They are critical engine components torqued very tightly and are under a lot of strain. Can I reuse my cylinder head bolts?
Head Bolts can be reused but should be inspected carefully and replaced at any signs of corrosion or wear. Although, since they are relatively cheap it is common practice to replace them. There are also certain types of head bolts that should not be reused.
Cylinder head bolts are designed to withstand the pressures of the engine combustion chamber and are a critical component. Because they are under so much pressure and can cause a lot of money and trouble if they break, it is common practice and is recommended to replace them any time the head is removed.
Many people have reused head bolts though and had no problems, but often stretch bolts are used and those should always be replaced once they have been stretched out and removed.
Cylinder Head Bolts: To Reuse or not to Reuse
Head bolts are often torque-to-yield bolts. They are designed to elongate upon torqueing to yield a greater clamping force. Hence the name “torque to yield” bolts. They are also often called stretch bolts.
If stretch bolts are not used, then conventional bolts or head studs are used.
Torque to Yield (TTY) Stretch Bolts
TTY bolts are commonly used for head bolts and this is the type of bolt that you should be concerned about reusing.
What are Torque to Yield Bolts?
These are TTY bolts (torque-to-yield) also called stretch bolts. Stretch bolts are often used as head bolts and have a special design to actually stretch out as they are installed to torque specifications.
They often have a length of the shaft of the bolt before the threads that is made to stretch once torqued to the proper specification. Torque specs will often include directions to torque the bolt to a certain force and then to turn the bolt another number of degrees. That is when the bolt stretches.
The stretched bolt provides a stronger clamping force and keeps the bolt from loosening in critical locations such as suspension or engine heads.
How do you tell TTY bolts from regular bolts?
TTY bolts also known as stretch bolts are identified by a section of the bolt shaft just above the threads dedicated to elongate. Stretched bolts are easily identified by the portion of the shaft that is a smaller diameter than the rest. Some TTY bolts are nearly impossible to distinguish unless labeled by the manufacturer.
There is a stretch zone along the shaft of the bolt. If it is deformed or if the diameter of the shaft has a step or has been reduced then it has been stretched.
Check with the manufacturer of the car or look in the repair manual for specs.
When the specifications provided for the bolt say something like, “torque to x pounds and then a quarter turn” you can bet it is a stretch bolt.
Can I Reuse Stretch Bolts?
Stretch bolts also known as TTY bolts should be replaced each installation. TTY bolts have a maximum stretch capacity and they can be reused only when they have not met this limit. Because they are readily available, and are often placed under a great amount of strain, it is recommended to replace them to avoid chance of failure.
Just go buy some new ones! It is not worth skipping on the 4 dollars for each new bolt to have a head gasket leak and have to rebuild the top of the engine all over again!
It is possible to reuse TTY bolts and some do without trouble. According to the manufacturers of the stretch bolts, there is a specification of a maximum length that the bolt can be stretched to before it becomes to week to rely on.
So, technically you may reuse the bolts after checking with the bolt manufacturer and measuring the bolt ensuring that if falls within the stretch limit specifications.
Head Studs Instead of Bolts
Head Studs are used often in performance application and are not used in most cars. Some use head studs rather than bolts for slightly more precision with the torque specs. Some think that the strength of the fastener makes a lot of difference when in reality the strength of the gasket is most important.
A note that should be made is it is not recommended to switch between bolts and studs without honing the block at the same time. Without honing the block it can distort the bore.
When is it Okay to Reuse Head Bolts?
Head bolts can be reused when the type of head bolt is not a TTY bolt and when the manufacturer specifies that they can be reused. They may also be reused when the head bolts are actually head studs which are bolts threaded on both ends.
There are these few instances where it may be acceptable to reuse head bolts, but it is a far superior choice and recommended to replace them every use!
Speaking of reusing head bolts, what if I have a head gasket leak and am considering attempting to tighten them to see if it will stop the leak before I replace the head gasket entirely.
Can I tighten head bolts to stop a leaking head gasket?
Following the appropriate tightening sequence, and it is acceptable to try to stop a leaking head gasket by tightening the head bolts. Any of the bolts having worked loose may be the cause of a head gasket leak.
Tightening the head bolts can fix your leak as it has worked in some instances, it just depends if the head gasket is torn or not. If it is torn, then tightening the bolts will not fix the leak until the gasket is replaced.
Generally, it is going to be a blown gasket that is allowing the coolant to leak through, but before you do the job, it is worth a shot!
Head Bolts-How much Force do they Withstand
All of the combustion forces that drive the crankshaft and propel your car stem from the combustion chamber. The top of the combustion chamber is the head, and that is held in place by the cylinder head bolts. So how much force does a cylinder head bolt withstand?
A cylinder head bolt is made to produce 7,125 pounds of clamping force. This is the average pressure exerted per bolt on average across the 10 most popular cars in America. Some head bolts encounter even greater pressures than this, but the common car’s head bolt has this kind of strength.
Head Bolt Pressure Calculation
At peak pressures, a cylinder in a production car normally reaches pressures of 1000psi.
To determine the force in the cylinder, we must use an average bore size for many cars.
What is the average cylinder bore size in a car in the U.S.?
The average bore diameter for the 10 most popular cars sold in the U.S. in 2020 is 3.475 inches in diameter. This is not the actual dimension of the most common bore size but rather an average that can be used as an accurate representation of the average bore size of a car.
This average comes from averaging the bore size range given for each of the most popular models and averaging the bore diameters for the 10 most sold cars in 2020. The following table provides the data.
|2020 Most Sold Cars in America||Bore Range (Inches)||Average (Inches)|
|Average Bore Size||3.475|
The data used in this table was gathered from each car manufacturers’ specifications for each car model. The range is given based upon the options of engine sizes for the sub-models.
With an average bore size of 3.475 inches, the average force in the cylinder at 1000psi is 9484 pounds.
How Many Cylinder Bolts per Cylinder
The average production car uses 4 head bolts per cylinder. More head bolts can be used than 4 for performance application as well as Hemis, Mopars, and small block Chevy engines (SBCs). The more head bolts used, the more the cylinder pressure is distributed amongst the bolts.
Given that a 6 cylinder head can regularly encounter nearly 10,000 pounds of force, each cylinder must have multiple bolts and be tightened exactly to specification.
9500 pounds of force divided over 4 bolts gives a force on each bolt of 2375 pounds.
In order to withstand this force the bolts must have a clamping force greater than this and it is not uncommon to be tightened to 3 times the forces that will be encountered.
A clamping force of 7,125 pounds for each head bolt is the average.