Torque wrench

I know that this is a very basic question to some. But not to a spanner monkey

I have to connect the saildrive gearbox of my boat to the engine. There are about 8 No M8 or 10 bolts (no nut) to go around the perimeter. It is in a very confined space.

I am aware that this should be done in a diagonal sequence. I will be borrowing a torque wrench ( click type not flexible rod to a dial) of the range to suit the bolts. I currently do not know the correct torque, but will look that up from the manual.

Can someone please describe the correct proceedure for using the wrench.

I am told that if a bolt is set, then trying to torque it again, gets an incorrect reading. If that is so then how does one tighten the bolts evenly around the perimeter of the gearbox.

I just wonder how an engineer does it properly- Assuming that there is a "proper" way

torque all to 50% then all to75% then all to 100%

The method used for cylinder heads is to "sneak up" on the final torque in stages.

First "snug up" all the bolts by hand, then tighten to the first torque setting (llower than final setting) in the required sequence using the torque wrench. When all bolts are tightened to the first setting, reset the torque wrench to the next higher stage & repeat the tightening sequence. Then increase the torque wrench setting for subesquent stages & repeat. Some head bolts add another final stage of rotating the nut by a set number of degrees after the final torque is achieved, for which special degree discs are available

For example : my wife's BMW R65LS motorcycle cylinder head bolts torque sequence is stated in the factory manual to be achieved in 3 stages - 15Nm first, then 35NM & finally 40NM (+/-2 NM). The tightening sequence is also shown.

I try to apply a continuous, smoothly increasing, pressure while tightening & stop as soon as the wrench clicks. I don't do the tyre shop method of repeatedly pulling the wrench to click point after the initial click.

Difficult to know how accurate the wrench is without at ability to have it calibrated, but a rough check can be made with a spring balance & a bit of calculation according to the wrench handle length. The wrench setting screw should be backed off after use to prevent "setting" the internal spring during storage.

After setting the wrench, I try it on a bolt in the vice before actual use to ensure that it does actually click at what seems like a sensible figure - stripped a bolt once when it turned out the wrench mechanism had siezed in storage & would never have clicked !

HTH

Nigel B.

Edited By mgnbuk on 01/02/2023 12:31:06

You are correct !

Depending on the setting you either do it in one go straight to the setting or you do it in steps. There is the starting torque and the moving torque, so if you stop before the setting , back off and start again. Steps need to be greater than the difference between the two values as mentioned above. Another point is the value given in data, there may be 2, dry or lubricated. The bolt size is not the only value you will need, it's pitch is also important, as is it's tensile strength. For metric this last value may be stamped on the head, 8.8 , 10.9 or 12.9. Imperial used letters, R, S, T, X, or stamped patterns of lines on the head, this latter often American. There are no doubt other ways as well ! Then there is angle tightening and strech bolts, but thats another story ! Noel.

I like reference to " the tyre shop method " creates a bit of drama for the uninitiated but a COMPLETE waste of time - see above for the reasoning. N

Edited By noel shelley on 01/02/2023 12:38:25

An important point with the “click” torque wrenches is the the nut (or bolt) should be turning when the wrench clicks. If you stop tightening, then restart and the wrench clicks before the nut starts to turn again, you have probably under-tightened it.

8 torque wrenches and a friendly octopus ?

Octopus ! At least with 1 torque wrench any error would be consistant. Noel

The answer is it's complicated. The materials, any plating and lubrication used change the torque values required. If the threaded hole is a soft alloy, for instance, it will be stripped of its thread by the torque used for the same screw going into steel. The value we had at work for non-critical M8 fasteners were as follows for 8.8, 10.9 and 12.9 screws respectively: 25Nm, 35Nm and 42Nm. There is an additional caveat that these figures, as well as only being for non-critical fasteners, are for natural finish or light oil lubricated only, for permanent joints using nuts and bolts or studs. When the joint was to be dismantled again then the torque was to be reduced by 17.5%.

Some major connections we did we were not allowed to used hot dipped galvanized nuts, bolts and washers because over time the zinc could squeeze out like toothpaste. Sometimes we measured the resulting stretch rather than using torque to achieve a good connection. Sometimes we torqued to a value that was going to give good metal to metal contact then turned the nut through a given rotation to give the correct stretch, much as used on some modern cars.

Frequently when people use a low friction moly based type of lubricant, thinking it can't hurt, the torques used were easily able to snap bolts or strip threads. Use of these types of lubricant require a big reduction in applied torque.

Martin C

As already said, tighten smoothly, so that bthe wrench clicks when the socet is moving,. If you stop, it might click before myou mexceed the "Stiction"

As said, back moff and retighten.

Don't denigraten bthe "Bendy bar" torque wrench, I have two that are accurate whenever checked.

Some "Click" type wrenches will vary according to how they are used A "snatch" or jerk to click will not apply the same torques as a steady pull. (I've seen some absolute shockers in Industry! )

As Noel said, if the fixings are M8 or M10, (Which imoplies Metric coarse ) the correct torque will depend on the bolt material garde. What is correct for a 12.9 might yield or shear a 8.8 fastener.

Ideally the threads, male and female should be clean and either dry, or lightly oiled. (The oil tends to give more consistent results; but note what the instruction manual says.the tor condition, torques, and tightening sequence.

HTH

Howard,

Thanks everybody for taking the time to post. I have learned something today.

Obviously things like this need practice & I only have one gearbox to assemble. However, I will have a starting point to work from.

I think you're going to be surprised by just how easy it is to torque a bolt to 30NM. Most people are overtightening small(and M8 are small) bolts by a considerable amount.