View Full Version : Help Understanding Torque??
03-03-2009, 11:47 PM
When a manual calls for 17nm initial torque and then 31nm final torque, for example, what does this mean, exactly?
If the final torque should be 31nm, why can't you just set the wrench at 31nm initially?
If you can't tell, I'm new to this whole wrenching thing, our full compliment of tools in my house growing up was a rusted pair of pliers and a role of duct tape. But I'm determined to learn.
Thanks in advance.
03-04-2009, 02:00 AM
There are two cases where you'll see two torque settings given for the same bolt.
In the case of something fastened with several bolts - such as a cylinder head, or engine cases - you may see an initial torque (to get things snugged up evenly) and then a final torque (to put the required clamping force on the joint being tightened.) If you went with the final torque on one bolt without snugging up the others - there is a strong possibility of cocking the joint - having it not fit true to each other, and the result is leaks. Normally the instructions also call for a cross-tightening pattern, where opposite bolts/nuts are tightened in a cross or star pattern.
The other case is where "angle" torque is used. In this case (such as the cylinder head nuts on a hexhead) - the fasteners are first uniformly tightened to an initial torque (again - to get the two pieces evenly tightened up) - then you turn the nut some additional "angle". In the case of the cylinder head nuts on a hexhead - it's 20nm initial torque - then 45 degees on each one, then an additional 45 degrees on each one - in a cross-pattern. If you're only loosening and tightening one of the head nuts - it's 20nm initial, followed by 90 degree final torque.
The reason for doing it this way is the angular torque is very repeatable - and easy to get all of the fasteners the same. It makes for a more accurate job.
03-04-2009, 02:05 AM
The reason you tighten everything evenly and to the lesser of the two torques is to seat the gasket and the two mating surfaces. If you tightened one fastener to the highest of the two torques initially when you proceeded to the next fastener you could start to cock the element being fastened. You would then distort the gasket or damage one of the pieces.
On the given task: #1 install pieces together. #2 install all fasteners (bolts). #3 tighten all fasteners hand tight. #4 torque evenly to 17nm (neuton/meters). #4 tighten to final torque (31 nm). Also most extensive projects will also have a torque sequence to follow.
I hope this helps.
03-04-2009, 02:08 AM
When I sent my message yours wasn't there. Sorry! I think we both said it nicely.
03-04-2009, 03:39 AM
03-04-2009, 09:03 PM
In the case of the cylinder head nuts on a hexhead - it's 20nm initial torque - then 45 degees on each one, then an additional 45 degrees on each one - in a cross-pattern. If you're only loosening and tightening one of the head nuts - it's 20nm initial, followed by 90 degree final torque.
On my hexhead, the total cylinder head bolt angle torque is 180 degrees, not 90.
I find the torque spec for the crankcase oil drain plug kinda unique. 23 nm initially and then a final torque of 32 nm. What's the purpose of this exercise, seating a new crush washer?
Or, do I have a faulty service manual? :dunno
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