Without having the proper torque specifications, assembling an engine, or any threaded fastener as far as that goes, is just asking for failure. This is especially true on today’s lightweight engines that use mostly aluminum castings.
Most people do not realize that a bolt or a stud acts like a big spring to hold parts together. It has to be stretched a certain amount in order to do its job. Even though there are tools to measure bolt stretch, most people do not own them and really do not need them. But almost anyone who does mechanical work will have a torque wrench.
Manufacturers determine the amount of torque that is needed to stretch a bolt or stud the correct amount and they include that torque specification in their service documentation.
It is important to note that most engine torque specs require 30w engine oil to get the correct stretch on the bolt. Some specialty fasteners require a specific lubricant. It is important to know which lube, if any, the bolt requires and to use it when tightening it.
A torque wrench really only senses friction. For example, take a bolt with a torque specification of 50 lb ft. If you were to tighten it with no lubrication at all and mark it so that you know where it stopped turning, then loosen the bolt and apply some engine oil to the threads and under the head of the bolt, when you torque it again to 50 lb ft, it will most certainly turn considerably further, which means it has stretched more. This is why it is so important to know what, if any, lubricant the manufacturer specifies for each fastener.
It is also important to pull on the torque wrench slowly and evenly without any type of jerking motions.
Torque Plus Angle Method Of Tightening
This method of tightening fasteners in engines is relatively new. The reason for it is to get a more consistent stretch in the bolts. Since a torque wrench senses friction, there are a lot of variables that can affect proper tightening, especially as the torque spec increases.
By only using the torque wrench for a small initial torque to seat the bolt, and then turning the bolt a certain number of degrees of rotation, the manufacturer can be pretty sure that the correct stretch will apply to the bolt. An example would be to tighten the bolt to 20 lb ft and then turn it an additional 90 degrees (1/4 turn) There are torque angle tools available that will work with a standard ratchet and enable you to accurately measure the number of degrees you are turning the bolt. Fel Pro makes a very inexpensive plastic version that is great for the home mechanic.
What About Torque To Yield ?
Torque to yield fasteners are stretched to the point that they are just about to “yield” or lose their springiness. These types of fasteners can only be used once and then must be replaced. They have mostly been used as cylinder head bolts but are starting to be seen in other parts of engines as well.
Torque Plus Angle Is Not The Same As Torque To Yield !!
A lot of people, including professional technicians do not understand that just because a bolt uses the “torque plus angle” method of tightening, it is not necessarily a “torque to yield” (TTY) bolt. You absolutely have to refer to the service documentation to determine if a bolt must be replaced or not. For example, Honda has used the “torque plus angle” method of tightening head bolts for many years, but I do not think that any of their head bolts require replacing so they are not “torque to yield” bolts.
The most important thing is to have the correct torque specifications for the engine you are working on.