Greetings again Brains Trust!
I was wondering about the prevalence of the Torque to Yield Bolts (TTY) (specifically head bolts) on Toyotas.
Firstly, when did they become common parts, was this during the move to alloy heads?
secondly, how can you tell what they look like? Are they stamped TTY, or should it just be assumed they are "them"?
Cheers
Hammo
I believe that Toyota refers to torque to yield bolts as plastic region bolts. I do not remember exactly when they became common, but my guess is around 20 years ago.
AFAIK, those bolts do not look any different. Toyota repair manuals have a section where they explain bolt torque based on marks/patterns on the head. In the section on head removal and installation, they will specify that the old head bolts be discarded and new ones used in the case of plastic region bolts.
If in doubt, I would assume that they are them ;-)
I know about graded bolts but not these. What are TTY bolts? Are they a bolt that has a spring action in it that only tightens down to a certain torque rating?
I am not entirely sure how they work, but my understanding is that torque-to-yield, or, as Toyota calls them, plastic region, bolts will stretch at a specified torque so that they maintain tension more precisely over a longer period of time.
TTY bolts offer 2 to 3 times the clamping force as conventional bolts, and do so much more uniformly than conventional bolts because its the deformation of the bolt material, not the torque, which determines final clamping force.They're used with alloy heads and required with some advanced head gaskets.
I don't think they're labeled specifically but they look longer and thinner than "conventional" bolts. If your manual specifies a "torque plus degree" for final tightening of head bolts, chances are good that you've got TTY bolts. For example, "torque to 120 ft lbs then turn an additional 90 degrees."
In any case, head bolts are fairly cheap. Even if you have "conventional" head bolts you're more likely to get more accurate and even torque values using new ones instead of old.
-- Mike Harris Austin TX
How would they do that without more torque or a different thread pattern?
A TTY bolt it torqued to the point where it passes the elastic (recoverable) stretch point and heads into the plastic region. Once this has happened the tension in the bolt barely increases at all despite the stretching. This is handy during assembly due to the fact that anything from poor lubrication to a notch in the thread can cause the correct torque reading but incorrect clamping force on a normal non-stretch bolt (torque used to overcome friction instead of clamping the head down). By specifying that the bolt should be taken into the plastic region by a certain amount you know that the minimum clamping load has been applied to get it past elastic deformation but it cant really over-clamp because its now deforming plastically which is a more-or-less constant tension plateau until just before ultimate failure of the bolt.
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