TURBO TALK – HEAD STUDS

[KMS]

TURBO TALK – HEAD STUDS

This is a weekly discussion on "grey" areas in regards to turbocharging. I answer many of the same questions everyday...one at a time on many confusing subjects that really are misunderstood. I want to help, so, we will open up 1 new topic per week to provide accurate information and explain how and why stuff works. This weeks topic is head studs.

When you turbocharge any engine, peak combustion pressures can rise to 2200 psi. The factory head hardware from most manufactures was only designed for 1500-1700 psi. This additional stress causes the oe bolt to momentarily flex or stretch. This flexing is never detectable until the head gasket fails. What happens is as you are increasing rpm and the average combustion pressure increases, the head actually lifts. This reduces the clamping force on the head gasket, pressures over come the head gasket material structure, and then head gasket failure...

What is the fix? Better Hardware! The use of studs is a start. Studs provide more potential clamping force than bolts. Second is the style of stud used. Most OE head bolts use a reduced center section that allows the bolt to stretch or yield during the torquing process.



Not a bad idea because it can self align and achieve high torque values with small thread pitch diameters....however the down side is that when the pressures exceed intended design, they continue to yield or stretch. This again leading back to a blown head gasket. So, when choosing the right stud for the job, you want to use one that has little or no center reduction. That way once the stud is torqued, it will stretch no further and locking the head gasket down under extremely high pressures.

So what your saying is: If I turbo my bike, I have to replace the head gasket and install studs, right? No. If you choose a stud that has a broached end (an end that accepts a driver like allen or socket), you can remove one head bolt at a time and install one head stud at a time....saving you time and money. The name of the game is that the head gasket never moves from its original compressed form and location.

Thanks
Kelly

 

[swedenturbo]

Thank you Kelly for your useful info about studs.
When using stronger studs, should I tighten the bolts with the same torque as the OEM bolts or increase the torque? In that case, how much?

 

[KMS]

Nice to hear from you. Every bike will be a little different (length of stud, case thread pitch, diameter, etc.)...I believe you are on the KTM500, for that bike here is what we do:

TORQUE HEAD STUDS IN 3 STEPS USING A CRISS-CROSS PATTERN:
21 FT/LBS
35 FT/LBS
48 FT/LBS- FINAL TORQUE

 

[J&L Snowhawk]

Just to clarify for everyone then is this a higher than stock torque value in most cases?

 

[KMS]

Yes. Most stud applications will have a higher torque value than stock head bolts.

 

[Cpt.Ron]

Total loss

Yes. Most stud applications will have a higher torque value than stock head bolts.

Kelly, what is your opinion of some of the OEM's that use "Total Loss" head bolt torquing versus what you recommended with three stage torquing with a torque wrench. Is it also mandatory to replace total loss head bolts after each use? I personally much prefer to use a torque wrench with set values as it seems more accurate in my mind.

Thanks
CR

 

[KMS]

Just to be clear we're talking about the method to where you torque to a low torque value and then rotate say like 180 degrees or something like that?

 

[Mike@KMS]

Kelly, what is your opinion of some of the OEM's that use "Total Loss" head bolt torquing versus what you recommended with three stage torquing with a torque wrench. Is it also mandatory to replace total loss head bolts after each use? I personally much prefer to use a torque wrench with set values as it seems more accurate in my mind.

Thanks
CR

They are just two different animals. OE manufacturers have mostly moved to "Torque to Yield", also known as "Torque Angle" for most of their bolts, which torques to a given spec, and then finishes with a given angle of rotation. This many times is also done in 2-3 steps such as torque to 20 ft lbs, then 35 ft lbs, then 45 degrees. This pre-stretches the bolt to a given specification, allowing a smaller bolt to be used and still achieve a desired strength. Due to that process, these need to be replaced after every use because once stretched, they do not go back to their original form, meaning the next time it will fatigue and likely break.
When you go to an ARP style stud, these dont use the same method because they are a much stronger heat treated steel. Typically you break everything around the stud long before it is ever fatigued enough, and since they really dont stretch, its just the standard 2-3 step torque sequence to achieve the desired load. Because these dont stretch, they can be reused over and over without worry.