Torque wrench.....

[Bill Edwards]

There is a little misunderstanding here of the 'newer' kind of torquing which is measured in degrees.

These bolts are stretch bolts, as engines get smaller and lighter it's desirable to use smaller bolts but use them to their limit rather than just say 'let's make them thick and use loads of them'.
To get the ultimate strength/clamping force they are tightened to a set amount with a torque wrench then turned further. This stretches the bolt and takes it close to the point where it will snap, and this can only be done accurately with angles. You can feel a bolt become easier to turn just before it snaps - well that's exactly what this technique is for.
For this reason, you must never reuse stretch bolts.

The other thing is there is a big difference between the force needed to keep turning a fastener and the force required to start one turning, as noticed by WorkshopChris.
When you get near to the final torque you must give yourself room to keep turning in one movement until it clicks, you cannot stop to ratchet back a little as doing so may cause the wrench to click without it moving.
Also, on older engines you are often supposed to re-torque the cylinder head bolts when the engine has been run up to operating temperature. When doing this it's critical to back off the fastener first then apply the torque otherwise it often won't move at all despite being loose.

 

[keithski122]

I've come across head bolts before that are torque by angle that as long as they haven't exceded a certain length then you can reuse them.Mazda diesel head bolts come to mind, did loads of heads on these and never had a bolt I couldn't reuse.

 

[ms08]

Saab also - 200 series engines can be re-used at least once and have a degree finish tightening. I am aware of stretch bolts as Saab use them on certain areas - rear dampers lower fixing and stipulate in their Workshop Information system to re-new when removed. However, the subframes bolts i referred to earlier which have a degree finish can be re-used according to same Workshop information.

 

[migcurious]

I have a torque wrench that's 10% out of calibration its not a known make so in the opinion of the experts here is it worth getting it calibrated or would I be better off buying a new one ? its mostly used for wheel nuts and not ultra important applications , last time I looked it was more to calibrate than buy new ?

 

[gordon stephenson]

Years ago, i worked on some Rolls Royce engines that powered shunting locomotives at the steel works, The big end keep bolts were lovely and had to be tightened until they had stretched 0.011". Checked before and after with a micrometer,

 

[Bill Edwards]

I've come across head bolts before that are torque by angle that as long as they haven't exceded a certain length then you can reuse them.Mazda diesel head bolts come to mind, did loads of heads on these and never had a bolt I couldn't reuse.

Saab also - 200 series engines can be re-used at least once and have a degree finish tightening. I am aware of stretch bolts as Saab use them on certain areas - rear dampers lower fixing and stipulate in their Workshop Information system to re-new when removed. However, the subframes bolts i referred to earlier which have a degree finish can be re-used according to same Workshop information.

Fair enough, if you're provided with specs to measure the length. But there's plenty out there where you aren't supposed to reuse them at all, whether this is because they're stretched further or whether it's to save the hassle of failures if people get it wrong or to make you buy new I don't know! But it's often a no for anyway.

 

[tinman123]

Fair enough, if you're provided with specs to measure the length. But there's plenty out there where you aren't supposed to reuse them at all, whether this is because they're stretched further or whether it's to save the hassle of failures if people get it wrong or to make you buy new I don't know! But it's often a no for anyway.

there would be certain engines known to give hassle if a new set of bolts arent used but they would be rare enough too, id be basing that on older engines and not the up to date tier 4 ones, i know feck all about them.
most of the manuals will state to use new bolts or as you say there is a max lth on some head bolts too.
but most of the time id be doing any work on engines here id use the bolts again, imo unless there was a known problem reusing them i see no reason not to.
im sure if we all wanted to go by the book replacing them each time id be the pc way of doing it but when my mum took the merc for its third service they replaced the discs on the car.
she landed home with a invoice for just over 1k.
i rang them back when i seen it and asked why the discs needed replacing with only 60k odd on the clock and they said it was merc policy that the discs be changed every third service, it might of been fourth either mind but it was wild early, i have a jeep here that only got discs at 150k.
it keeps the sales up i suppose...

 

[addjunkie]

Assurance and insurance. If they tell you to replace them, and they fail, they tried their best. If the didint tell you to replace and they fail, they the maker could be held accountable.

Calliper bolts on old fords spring to mid here, books says to use new each time, but who did.

 

[Bill Edwards]

There are a lot of nuts and bolts where the manual says you should replace them.
But stretch bolts are done up tighter that most to the point where they're stretching giving peak holding power, and this stretch is permanent. So try doing the same again and it may snap, which is why they often require replacement. Not like a typical suspension bolt that's done tight but not stupidly so, they generally only need replacing if there's damage to them. Of course, the reason manuals sometimes specify replacement may be because of the serious consequences should somebody put one together with damaged threads, or reused a nylock nut or didn't replace the threadlocker that's sometimes on new bolts.