биез, амеойрйен швйщ мзен йеъш обшжм (чцб ддъфщиеъ щме бзен фй 1.7 обшжм)
ббмеч бшжм-шащ амеойрйен оъфъзйн лезеъ вгемйн мтеоъ орет лм амеойрйен щоъфщи
еоълеех баеъе чцб .
тбше мдщъощ ббшвй TTY (оъашлйн) ,щн ргшщ иеез щвйад бдйгеч +жейъ чип йеъш.
айп мй лез мрсз аж дгбчъй :
TTY :Torque-to-yield head bolts and clamp load
by Gene Hailey
The torque-to-yield bolts have been in use for quite a while and were first used in bi-metal engines, primarily engines with aluminum heads and iron blocks. The whole idea of using this bolt design is to keep a full time clamp load on the gasket. Obviously, if the engine loses the clamping force, the head gasket will not survive for very long.
There are several different causes for gasket failure, but for now we will limit the discussion to clamping load which is solely provided by the bolts and bolt threads. The clamp load is defined as the pressure that holds the components together.
A lot of us (depending on your age) remember when it was unthinkable to use new head bolts. The old short stubby bolts were something that just didn't wear out as long as the threads weren't stripped or the hex head wasn't rounded off. Sometimes they would get tossed out if the shank was seriously pitted from rust formation. Then we were rebuilding all iron engines, using asbestos-steel gaskets with steel facings and asbestos packing in the center. These usually required a second torque process after the engine ran for a few miles.
The new non-retorque gaskets made from non asbestos materials have an advantage in that they do not require re-tightening. They DO, however require a full time clamp load. Otherwise, the combustion process is not contained and starts destroying the fire ring as well as the body of the gasket. The full time clamp is accomplished by using torque-to-yield (TTY) bolts. Torque-to-yield bolts are sort of like springs, in that they have the ability to stretch when expansion takes place and retract when the engine cools down.
The bi-metal engines demanded a different type of head bolt to compensate for their increased thermal expansion rate. As the head thickness grows, the bolt must also elongate. Otherwise the clamping force on the gasket would exceed the point where the gasket could not recover.
Once the bolt is stretched to it's limit, any further expansion results in breaking the bolt. It is critical that the TTY bolts be tightened to the correct torque value recommended by the engine manufacturer. The proper load is at the point just before the bolt starts to stretch. Tightening the bolt beyond that point will ruin it because it will be stretched beyond the maximum yield. Not having enough torque obviously is also detrimental to the sealing of the gasket.
Bolts which are stretched more than .050" should be scrapped, so how do you tell how much it`s stretched? The length specification for new bolts has a tolerance of twice that amount, so if you don't know the exact length of the bolt when it was new there is no basis for comparison.
Using a thread gauge is one method but may be time consuming. Checking the threads with a nut is not a totally reliable method due to the wide tolerance in the threads of the nut. New bolts on certain engines may be the best bet for quality to cost ratio..
While the bi-metal engines have special requirements, let's not minimize the importance of proper torque on the iron-iron engines. The gaskets on these engines also require a full time proper clamp load.
To better appreciate the importance of torque load aside from the reading on your torque wrench, let's look at where that torque is distributed. The whole 80 ft/lbs does NOT apply to the clamp load as is often believed. The diagram below is approximate percentages of what uses the torque that's applied to the head of the bolt. Only TEN PERCENT of the total is used for clamping, the other 90% is spent on friction between the bolt, the threads and the bearing surface which is the underside of the bolt head and the cylinder head.
Please bear in mind that these numbers are based on assembly with CLEAN THREADS with a light application of 30W motor oil applied to the threads and the bearing surface. Dirty or damaged threads can absorb enough of the total torque value to result in very little or NO clamp load.
The situation is aggravated by only one or two bolts with insufficient load. This results in a wide load variation on different areas of the gasket, and even to no load at all in some areas. Most engines have a bare minimum of cylinder head bolts and losing the clamping force of only one bolt results in a serious leakage.
The best procedure is to run a bottoming tap in each hole. This can be done rather quickly using a reversible air drill.
Dry threads will understandably lessen the usable portion of the torque value. Just a couple of drops of oil on the bolt threads and on the underside of the head will do the trick. Do not over oil the bolt hole on blind holes, this may cause a hydraulic action between the bolt and the bottom of the hole which will result in a false torque reading.
While we are on the subject of bolt holes, do not overlook the bolt holes for the intake manifold. Even though the rebuilder does not necessarily install the intake manifold, the threads should be ready for use by the installer. Insufficient torque on the intake bolts can often cause all sorts of problems, enough for another article so I'll stop with that.
There are a few other factors which shorten the life of head gaskets but improper clamp load is certainly one of the leading causes of failure.
Thousands of gaskets are produced with the same technique and materials in each manufacturing run. The vast majority last for the life of the engine but a few never live through the warranty period.
Too may times these "failures" are blamed on the gasket that somehow was responsible for blowing or burning itself out. The real culprit may be insufficient clamp load.
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