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Torque settings & effect of lubrication

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  • Torque settings & effect of lubrication

    Can anyone confirm if you lubricate a bolt with 5w40 engine oil you should reduce the specified torque setting by 35%

  • #2
    Some sources say 30% reduction for lubricated, some say 40%. Some differentiate between 'lightly oiled' and oiled. . Some speciality thread assembly pastes may have their own lubrication efficiency.

    The condition and material of the bolt thread can also have some affect. A brand new cadmium plated thread will already have less friction than some others, even without lubrication.

    Make sure you know if that any quoted torque is for lubricated or unlubricated.

    For some situations where torque and bolt stretch are critical (such as cylinder heads ) its recommended to fit new bolts . The makers of these bolts will often be able to tell you the correct torque for their own brand of bolts, allowing for its materials and any plating. And some specialist bolts come with thread lube already factory applied.

    For slightly less critical situations 35% reduction sound like a reasonable compromise to me bearing in mind torque wrenches are not always 100% accurate anyway. I dont know for sure but modern 5w40 oil is quite slippery so would probably exceed any 'lightly oiled' scenario.

    But some of the guys are professionals and may disagree with me and able to offer a more expert opinion.

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    • #3
      Yes Bugman thamks for the reply. As a retired Taxi driver I have driven and maintained my cars for nearly 2 million miles I have always oiled and torqued to manufacturers specks in most important places and I can not remember any instance of a bolt done like this failing after I have torqued it. but I have just become aware of this fact and on trawling bolt manufacturers site and other forums it seems a well established principle which seems to have past me by ,but there seems to be a minefield with opinions varying between 10 & 50%. The vehicle manufacturers don't usually specify oiled or not especially my Corsa one Although my 2016 Polo service manual states Transmission bolts to be done dry & most of the others are angle tightened. As I have not had problems in the past I am tending to think of going with 10 to 20% in future. especially on 8&10mm bolts I have never really torqued in the past.

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      • #4
        We could 'talk' about this subject all day and still not have a conclusive answer.
        What Bugman says there is pretty good.
        Like yourself, I have had to deal with VW units which state either 'dry' and/or angle tightening after initial torque wrench tightening.

        Although you say that you haven't used a torque wrench on less stressed bolts in the past, you have certainly developed a skill for tightening them, this is on of the basic skills of a mechanic. What I have done with students, is to give them a correctly set torque wrench to tighten certain bolts, once they are tight, I have had them make a pencil mark on the bolt head inline with another pencil line made on the fixed metal, then slacken off the bolt, followed by tightening it until the pencil lines coincide, thereby teaching them the skill of tightening to the correct torque without using torque wrench.

        The other important point to remember, is that the correct torque is reached whilst the spanned/socket is actually turning - it isn't possible to apply a torque wrench, set to the required torque, and expect the bolt to be correctly tightened - even if the bolt was under tightened in the first place, it is likely that the wrench will 'click' without the bolt actually turning.The only way is to slacken it off slightly and re-torque it whilst the bolt IS turning.

        As Bugman says, torque wrenches may well not be that well calibrated, especially if they have had a lot of use/misuse.
        If you grip the square of the wrench in the vice, measure and mark twelve inches from the centre of the square, then apply a good spring balance at the mark, you can then pull on the spring balance and the wrench should click at the correct pounds setting because the twelve inches x the pounds = foot pounds (don't ask me to explain Newton metres as that requires an apple to be dropped on your head 😄)

        Regards

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        • #5
          Yes thanks for that, I think I am going to have to do some experiments tightening up bolts dry & oiled and note the angle difference between them.

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