K series engines, troubles and cures.....
 

Thought some of you may find this useful.....
https://www.youtube.com/watch?v=eXfNS-kayrE

Thanks for posting, enjoyed it but oh dear where do I begin, torque to yield :duh:

No good asking me, I just drive the darn things :D

Torque to yield is where a bolt is tightened beyond the yield point of the material.


When tightened to just below the yield point it is deformed elastically, so when removed it returns to it's original length and in most cases can be re-used.


When tightened past its yield point it is deformed plastically so when removed does not return to its original length and must not be re-used.


Head and other bolts need to supply a consistent, repeatable clamping force, this can be achieved by applying a specific torque to the bolt, but depending on if the bolt is lubricated and other factors the clamping force at a specific torque setting can vary, substantially in some cases, but amazingly around 85% of the torque needed is used to over come friction in the threads and at the bolt head to cyl head interface.


So for example, for a dry bolt torqued to 100Nm, 85Nm is used to overcome friction and only 15Nm applies the desired designed clamping force, however, lubricate the threads and the bolt / head interface and you reduce friction, on applying 100Nm torque in this situation the lubrication will reduce the torque needed to overcome friction to say 60Nm, the remaining 40Nm now applies the clamping force which will exceed the design spec.


Torque to yield with torque angle tightening overcomes many of these variables, at the initial small torque say 20Nm friction isn't a factor and it gives an equal start point to all the head bolts, then tighten by angular means.


So an engine manufacturer will say to a bolt manufacturer, I need a bolt of Y dimensions to produce a clamping force of X, the tightening instructions could say 15Nm, then turn 90 degrees x 3 for the clamp force, which will be consistent day in day out and friction variables due to lubrication, or lack of, variations in thread tolerances etc won't affect the clamp force or the integrity of the assembly, which could occur by simply using a torque wrench with the same variables as above.

I know there is a you tube video where a man from Rover tells people about the K4. In the video he mentions torque to yield for the long through bolts, but that is clearly not what happens here!
Depending on which source you use, the initial torque is applied to each bolt in turn, and should be done over a number of operations. So if the initial torque is given as 20 Nm, I would do that over a couple or more operations. Say torque to 10, 15 and then 20 Nm in succession.
That will ensure that the fire rings are all loaded equally, before the angle tightening.
The pitch of the thread of the bolts is 1mm, so one full turn of the bolts will stretch them a maximum of 1mm!
Take that over the total length which must be close to 400mm, i guess, and the elongation of the bolt is a fraction of one percent!.

This is well within the elastic deformation of the bolt, and the bolt will behave according to Hooke's law. That is that the force exerted by the bolt will be linear and proportional with the stretch, and totally reversible.

Not correct.

Hooke's law only applies in the elastic region of the stress-strain curve.

A 1mm stretch of a circa 400mm steel rod (bolt) actually represents quite a large strain.

Also, plastic deformation, which is what is said to occur with these bolts, continues beyond the yield point for many metallic materials so to use the term torque-to-yield is not inaccurate in this application.

Sorry but I don’t need tuition in yielding bolts. :duh:

The K-series bolts are tensioned to an elastic state.

You will be missing out on a whole wide wold of engineering education. I rather enjoyed reading Ducati750cc's post! :}

Are you saying that, contrary to popular belief, the k-series bolts do not stretch when torqued-up and then remain at a longer length than original when removed?

Sorry. It should have been "elastic", not plastic. E has been swapped for P.

Kaiser and Lovel,

This is a genuine question. Are you saying that these are not in fact stretch-to-yield (more correctly stretch-to-beyond-yield-point) bolts used on the K-series?

I have seen some previous threads on this topic and note that they were not conclusive.

Personally, I would have thought that to use bolts which stretch into the plastic region would render the whole assembly unstable following temperate cycling that occurs in an engine and therefore the application must be within the elastic region. But this thinking may be somewhat overly simple.

I wonder if the bolts operate close to the yield point but within the elastic region during normal tightening and temperature cycling but may go into the plastic region due to overheating and then become incapable of being reused due to permanent (i.e. plastic) stretch.