Help
I have an online manual which specifies a procedure for tightening the cylinder head to first tighten the bolts to a pre-set value then further tighten by a certain angle. Looking online a higher torque only is provided, which is the right way?
Considering the manual also tells you to measure the bolt length to check for stretch before using them - I would be using the low torque + angle tightening method.
Tightening torque
Stage 1. 3-11 N.m
Stage 2. 13-17 N.m
Stage 3. 43-47 N.m
Stage 4. 88°-92°
Stage 5. 88°-92°
There is an engineering reason for angle tightening rather than full torque tightening.
As you increase torque - the friction on the threads and under the head of the bolt rises very quickly.
This friction can distort the true tension you have applied to the bolt.
By using torque to seat the head and pre load the head gasket - you don’t need to deal with too much friction.
Then by tightening the bolts to a set angle - you ignore any friction factor and the gasket is clamped by a set amount ( 90 deg = 1/4 of the pitch of the bolt in clamp force )
No problem I have just goggled it.
Thanks again John
Also consider if lubricant is used on threads / under bolt head.
Not sure if it’s required but any lubricant used will also alter the torque setting applied.
Agree
This is why for critical joints - low torque is only used for seating the joint.
Then angle tightening is used.
In the factory - they sometimes tighten to yield with the specialized tooling they have available to them.
If you are tightening to the yield point of a fastener, you need a bigger fastener.
Agree mate, 20 years in the oil industry using torque and tensioning tooling etc etc etc 
When hydrotesting Risers and flowlines up to16,000psi working pressure leaks were a deffo no go 
Tell the aerospace and aircraft guys that.
It is common practice with the correct equipment to take a fastener to its elastic yield point for optimum clamp force.
Agreed - once you pass the elastic yield point and permanently stretch the bolt - it is scrap.
But that is why the equipment to do it costs so much.
But you are mainly talking Titanium bolts and assumed clever people and where the weight of the fastner is somewhat critical and that you can make them single use items. We tested enough of them for HiShear to crash a plane.
I’ve seen enough blown gaskets where they “have felt the yield” If you reach plastic deformation, it’s too late.
Agree - once you have reached PLASTIC deformation limit - you have gone too far.
But ideal tension is achieved within the ELASTIC limit of a fastener.
@PaleRider @CBRDEANO @NickD As you are all pointing out, it depends what has been designed for. Mechanical engineering practices vary between tension, torque and angle of turn tightening in my experience. My team generally kept to the former two because fatigue was a designing characteristic in most joints. Angle of turn and yield tightening are more or less synonymous.
@johnmilli It looks like Mazda designed for both yield/plastic tightening and elastic. Did you find out the relevance of the bolt stem length and hence the method of tightening?
Yield tightened bolts are only “trash” when replaced.
It’s not the plastic deformation limit, plastic deformation starts at the yield point. You don’t want to get to that point. Depending on material properties, that point can result in an amount of stretch that will immediately result in a loss of tension. Most people will have experienced this with soft, ductile, fasteners when the torque required suddenly reduces followed the the material reaching it’s tensile limit and snapping. Other have far less “give” as far as we can feel, and just snap.
As you are inducing the stress in the fastner by the twisting of the thread, (nut or bolt) lubrication can have a great effect on the strain. Usually torque figures are for none lubricated mating surfaces. As lubrication has far less affect at low torques, that is why rotation figures are then used after the initial seating, as it negates torque figure errors. However, always go by what the manufacture has said.
OK cool - I will bow out here and keep theses discussions for work purposes.
Two turns from finger tight anyone ?
Great discussion guys
I have an online manual which give torque +angular values for tightening these bolts, I then have one torque value from forums, after posting this query there has been an avalanche of information (much appreciated) I have decided to go with the torque +angular method. Note also bought a Haynes manual which also advised the torque +angular method although didn’t give torque or angular values. Thanks for all your comments.
Has the technical term “FT” become defunct?
For the sake of clarity, this was a poor attempt at a humour.
In the modern society they use three ugga duggas - haha