Thread: 300TDi Head Torque Question

Originally Posted by Tank

Has anyone here ever worked out what the torque figures in Nm or Ft./Lbs. would be after torquing down a 300TDi head.
That is after initial 40Nm, then 60 degrees, then 60 degrees and then 20 degrees on the inner bolts.
Any idea what each torquing sequence would read (after the initial 40Nm) if each sequence was done in torque figures and not degrees, what would be the final amount of torque applied in Nm's or Ft./Lbs., Regards Frank.

Frank - have a read of Bush65's detailed comments on why angle torquing is superior.

AFAIK what you propose cannot be done accurately.

Originally Posted by isuzurover

Frank - have a read of Bush65's detailed comments on why angle torquing is superior.

AFAIK what you propose cannot be done accurately.

Ben, I know that friction has a lot to do with getting a perfect reading when torquing to a number and stretch or twisting a certain number of degrees might be more accurate.
Reason I'm asking is, if for instance you knew what the final torque figure was you could check against the degree method, or if you missed a sequence on a bolt, have you a link to Bush65's post, Regards Frank.

Originally Posted by Bush65

In most cases with bolted joints, the thing which is important is the pre-tension in the bolt.

Tightening torque is one method for achieving a required pre-tension, but because the tightening torque is resisted by friction between the mating threads, and between the underside of the bolt head (or nut) and the joint/washer, only a fraction of the torque contributes to pre-tension.

The friction increases as pre-tension increases and also varies greatly from one bolt to another.

Research has shown that the accuracy of achieving desired pre-tension is in the order of +/- 25% when torque control is used. In many cases this is not acceptable and other methods are used, such as part turn method. For critical applications, one of the best methods is direct measurement of bolt stretch.

Australian Standard AS4100, Steel Structures Code, Clause 15.2.5.3 does not permit torque control to be used for tightening bolts in structural joints. With steel structures (e.g. buildings), bolts may be either snug tightened of fully tensioned depending upon the design requirements.

Machinery is usually intended to be disassembled and reassembled, so bolted joints are usually designed for bolts to be tightened to approximately 65% of their proof load. In some instances when loads are high and larger bolts can not be used the design will require higher pre-tension up to 100% of the proof load - these bolts should not be re-used.

The fatigue strength of bolts subjected to cyclic tensile loads, is increased when the pre-tension is considerably higher than the maximum applied tensile load.

In the case of the flywheel connection bolts, the torque load is carried by the dowel in shear and the friction between the flywheel and the mounting face on the crankshaft. Note the bolts are not loaded in shear - they can only be loaded in shear if the dowel and friction joint fail first (in which case the bolts will also fail).

To transmit the necessary torque through the bolted connection, the required friction is achieved through the pretension in the bolts clamping the mounting surfaces together.

There are a number of factors (including: shank diameter, thread pitch, thickness of plies, size of bolt head and depth of bolt engagement into the crankshaft) that determine the part turn angle to use for the required pre-tension. Also a number of factors for the relation between tightening torque and pre-tension.

So it is difficult to determine resulting pre-tension from those published figures, but having confirmed that Isuzu published these, and mistakes in interpretation were not made, then I would use the part turn angle method.

I know from experience and investigating many cases of bolt failure, that insufficient bolt pre-load leads to more bolt failures than over tightening. Failures due to over tightening usually causes bolts to fail while they are being tightened (in which case the bolt can be replaced immediately). - BTW, the last revision to the Australian Standard for nuts was changed to be inline with ISO and revised some nut thicknesses to ensure that bolts would break, before threads would strip, so that failure due to over tightening would be obvious to the person tightening the bolt/nut.

Taking into consideration the normal practice of specifying tensions of 65% proof load for machinery, I like to err on the side of over tensioning.

Originally Posted by Bush65

Torque values are too unreliable when tightening bolts to relatively high tensions. At high tensions, torque to overcome friction increases dramatically and variations in friction result in large variation in tension.

Angle of rotation is easy. I use a cheap 1/2" square drive angle indicator. I have seen someone use a piece of cardboard with the angles drawn on it, and it worked reasonably well.

Torque values for 300 tdi cylinder head

nope cant do it....

I run tech angle torque wrenches and they give the final torque after tensioning....

theres more than a 50 NM varience at the first stage of torquing and after hte final the variation is pushing 100 NM some of the bolts go off the end of the readout limit for the half inch wrench and the othersare well inside.