Replace TTY head bolts?


3 replies to this topic
  • Blue4Ever

Posted July 14, 2007 - 03:42 PM

#1

The head bolts on these machines are TTY (stretch bolts). None of the manuals(yz250f,yz450f,yfz450 ect..) state to replace them?! Has anyone had a failure or got any other info on this matter? Thanks in advance!!

  • grayracer513

Posted July 14, 2007 - 04:05 PM

#2

All head bolts, or, in fact, all bolts, stretch when torqued. But you're incorrect in assuming that they "yield". If you wanted to go strictly by a theoretical ideal, you'd replace all the head bolts on any engine you rebuilt, but there is no practical need for such measures in the vast majority of cases.

The torque angle method of tightening the head bolts on the newer bikes is used to achieve a more uniform clamping pressure by eliminating the effects of bolt head and thread friction on the torque reading. The bolts don't "yield", and don't take a permanent stretch, most of the time, and are quite reusable if in good condition.

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  • Blue4Ever

Posted July 14, 2007 - 04:17 PM

#3

So you are saying these are not stretch bolts, correct? Because in general all strech type head bolts are supposed to be replace. Because they exceed there elastic range and go into the plastic stage were they do not return to the original length. What makes me question this is that the sequence they state is the same as used on strech bolts.

.





:Tech Tips:


TORQUE - TO - YIELD

CYLINDER HEAD BOLTS

Bolt Performance

Under the application of load, all bolts exhibit four main phases; the elastic phase, the plastic phase, the yield point and the shear point. In the elastic phase a bolt will stretch under tension but return to its original length when the load is removed. As we continue to apply load the bolt reaches the plastic phase from which it can no longer recover to its original length and is now permanently stretched, The point that separates the elastic phase from the plastic phase is called the yield point of the bolt. Finally, if we continue to apply load the shear point is reached and the bolt material wastes and breaks.

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.

Whilst TTY bolts are attractive to the engine manufacturer, there are disadvantages to the engine repairer. For the most of us it would be unthinkable to replace a conventional head bolt unless the bolt was damaged, i.e. stripped threads, the bolt head was rounded off, the shank was severely corroded or pitted.

Conventional head bolts simply just did not wear out. Torque to yield head bolts however, by the very nature of their design, do wear out and should NEVER be reused.

Installing TTY Bolts

TTY head bolts are also tightened in a series of stages and in sequence, however they are not tightened to a predetermined torque, they are tightened through a series of specified angles. This data is provided by the engine manufacturer and should always be adhered to. While the first step in the tightening process is normally stated as a torque figure it is done so only to provide a uniform baseline from which the true load is then applied. This is commonly referred to as a pre-load or snug torque. A typical tightening specification would look as follows:

uniformly tighten in sequence in several passes to 78Nm

tighten in sequence 90°

tighten in sequence a further 90°

This procedure ensures that friction does not cause an uneven bolt loading and that the correct high tension is achieved every time during assembly. It is essential that a quality wrench with an accurate angle gauge be used to achieve the correct angles of turn of the tightening process.

Unlike a conventional bolt, TTY bolts are tightened beyond their elastic range past their yield point from which the bolt material can recover to its original length, and into the plastic phase of the bolt material. The bolt is permanently stretched and for this reason should not be reused. The reliability of these bolts once stretched is greatly reduced. If they are reused, they are permanently stretched further a second or third time. It is also for this reason why you should never retorque a torque to yield bolt.:thumbsup:

  • grayracer513

Posted July 14, 2007 - 06:01 PM

#4

It is my opinion that any bolt that is torqued into its plastic phase should be replaced with each use, but I would expect that to be recommended by the engine maker.

It also does not follow that every bolt using an angle torque spec is torqued into its plastic phase in the process, and GM, which uses the technique on many of their newer engines, stated several times that the issue for them was one of controlling the uniformity of clamping pressure.

If you're concerned, replace them.





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