All greasy

[Bob_Salmon]

Serious question guys, even if it is pretty basic.

I normally use Copperease or similar grease on bolt threads which I expect to be undoing again (OK., I know that all bolts are going to be undone at some stage, but I meant ones like wheel nuts). For 'no load' areas (like pedal assemblies) I use white lithium. Other areas I spread a general purpose grease, just to stop rusting.

So, my question is should I be using different types of grease for different torque loads? Obviously, high torque means the threads will be squeezed more tightly - very technical that - but do different torque loads need different types of grease?

Thanks.

Bob

[hot66]

you have to be careful when apply grease to 'torque sensitive' bolts. If the torque spec was given assuming a clean thread, then the application of grease could mean you over-torque the bolt.

[fetuhoe]

Torquing bolts can be quite complex and the torque applied is designed to allow a ceratin amount of stretch in the bolt which is where the effective clamping force is provided.

The baseline figure that you are using really needs definition but there a few basic 'rules of thumb' that can be used.

If the baseline torque figure assumes dry bolts then using an oiled bolt means that you can reduce torque by around 15-20% for the same amount of stretch. Using a Teflon or Moly dry film lubricants can mean torque reductions of between 40 to 50%.

Subsitituting plated bolys can mean torque reductions of 10-15%.

If the baseline figures assume oiled bolts then you need to take this into account.

Dry bolts can also give around 15-20% variation in stretch when torqued to specific values and I am not too keen on using dry values.

[Bob_Salmon]

Thanks for that. I was taught that the length of most spanners relates to the size of the nut/bolt they are being used on, given a 'normal' mechanics hand/arm strength. We all know that it is possible to strip threads using a little more effort than was needed and I have always assumed that use of a torque wrench eliminates the human element and delivers whatever the design engineers intended.

When I got to thinking about it, type of grease, or even if it should be used at all, must influence applied torque, yet I have never seen a service manual specify if or what grease should be used.

Discounting precision engineering, can I take it then that manuals usually assume something like CopperEase is being used, or is that a dangerous assumption?

Bob

[Sebring]

From my experience on suspension components almost all are specified dry, although the OEM will take lots of care specifying torques and even the coating on the bolt will make a big difference. Some coatings have PTFE in. You can imagine the problems since chrome 6 has been banned, nearly every bolt on production cars has had to be re-tested in every application.

Engines can be a different situation and the manual should spec if they are wet or dry. Arp are very specific with their rod bolts etc and give different torques for their lube versus engine oil!!

Cheers

Mark

[Gilbert '71 T]

Not such a basic question after all then Bob

Great info !