Torque to stretch bolts
Thread Starter
Racer
Joined: Feb 2003
Posts: 262
Likes: 0
From: Lumberton TX
Torque to stretch bolts
Hey Guys,
What is the purpose and reason of torque to stretch bolts. I guess it has something to do with aluminum block and heads. Why not regular bolts and a torque wrench.? Thanks for educating me. Have a great week. Frank
What is the purpose and reason of torque to stretch bolts. I guess it has something to do with aluminum block and heads. Why not regular bolts and a torque wrench.? Thanks for educating me. Have a great week. Frank
Team Owner
Joined: Sep 2001
Posts: 23,283
Likes: 906
From: Lake Elsinore, CA
TTY head bolts have a significant advantage over conventional bolts. I've found some goodinfo on a couple of web sites that says it better than I can.
Engine design has reduced the number of head bolts.
Engine design has increased the length of head bolts.
Higher performance outputs require higher clamping loads.
Lighter castings require more consistent clamping loads.
So torque to yield bolts and angled tightening achieve almost all the requirements for a modern engine.
A simple explanation of the difference between "torque bolts" and "torque to yield" head bolts.
Traditional head bolts are tensioned to a predetermined torque measured by a torque wrench. This torque reading is not an accurate measurement of the downward clamping load but rather an accumulated measurement of friction resistance between the two threads and the spot face and bolt head and the clamping pressure. With this system the accuracy and consistency of clamping loads are very unreliable. Each head bolt could have varied amounts of friction created due to differing contact surfaces, thread conditions and bolt contact surface finishes. This method often results in inconsistent and inadequate gasket clamping load.
Torque to yield head bolts
Torque to yield (TTY) bolts, also commonly referred to as angle torque or stretch bolts, are used in many of today's modern engines predominantly for cylinder head bolts but also main bearing and big end caps.
Compared to conventional type bolts, TTY bolts offer the engine manufacturer a number of advantages including greater flexibility of design, reductions in component costs, more accurate assembly and reliability of seal. Engines designed utilising TTY head bolts require fewer head bolts to achieve the desired clamping loads then those using conventional bolts. With fewer bolts the engine manufacturer has more flexibility in cylinder head and block design as well as reducing the cost of the engine.
Engine design has reduced the number of head bolts.
Engine design has increased the length of head bolts.
Higher performance outputs require higher clamping loads.
Lighter castings require more consistent clamping loads.
So torque to yield bolts and angled tightening achieve almost all the requirements for a modern engine.
A simple explanation of the difference between "torque bolts" and "torque to yield" head bolts.
Traditional head bolts are tensioned to a predetermined torque measured by a torque wrench. This torque reading is not an accurate measurement of the downward clamping load but rather an accumulated measurement of friction resistance between the two threads and the spot face and bolt head and the clamping pressure. With this system the accuracy and consistency of clamping loads are very unreliable. Each head bolt could have varied amounts of friction created due to differing contact surfaces, thread conditions and bolt contact surface finishes. This method often results in inconsistent and inadequate gasket clamping load.
Torque to yield head bolts
Torque to yield (TTY) bolts, also commonly referred to as angle torque or stretch bolts, are used in many of today's modern engines predominantly for cylinder head bolts but also main bearing and big end caps.
Compared to conventional type bolts, TTY bolts offer the engine manufacturer a number of advantages including greater flexibility of design, reductions in component costs, more accurate assembly and reliability of seal. Engines designed utilising TTY head bolts require fewer head bolts to achieve the desired clamping loads then those using conventional bolts. With fewer bolts the engine manufacturer has more flexibility in cylinder head and block design as well as reducing the cost of the engine.
Last edited by Patches; Oct 10, 2006 at 09:22 PM.
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
Just to add to this, ARP specifies a torque value based on their moly-lube, which in essence assures them some level of control over the friction of the threaded assembly and as a result bolt preload.
