Re-torquing K head bolts

[TorAtle]

After running the engine properly warm after a rebuild, should one tighten the head bolts once more? If so, how much?

[Peter Carmichael]

NO! Absolutely not. The through bolts are angle tightened and a torque setting would be very unreliable.

[Paul D Jones]

If you used the Haynes book for a Rover car look very carefully at it the pictures of the engine the side the exhaust comes out and make sure it's the Rover engine as there is also a Honda engined Rover book for the Rover cars.

 

The head bolt torque settings are different.

 

Paul.

 

Paul.

See My Car Here

[TorAtle]

I actually meant turning all of them say 30 degrees more to be sure, but all the same...got my answer (and peace of mind). Thanks.

[Sheds Moderator]

NO NO NO, I second absolutely what Peter says. Follow the the letter the instructions in the book, if not you will break the bolts at best and ruin the head/engine at worst.

 

You need a good torque wrench and the special (E12) socket, they are REALLY tight.

[V7 SLR]

I went into a lot of detail in my research into stretch bolts recently. Essentially you can't overtighten them to the detriment of the head because the plastic region that they are in when tightened properly is fairly consistent in it's tension. You will therefore gain nothing from tightening them for they will simply stretch further into the plastic region. You will, however, have stretched your bolts further, meaning you will have less re-use of them.

 

Of course, if you snap one, then you will have an instant "no-tension" bolt which may bugger the head up.

 

Summary, as per other advice. Don't.

 

Worcs L7 club joint AO.//Membership No. 4379//Azure Blue SLR No. 0077//Se7ens List Tours

 

[Sheds Moderator]

They are in the PLASTIC region? *eek* No wonder they are so bloody tight, and need measuring before replacement.

Why is this necessary? I would have thought that the point of tightening a bolt was to give the tension in the elastic region, once it begins to permanently deform there is no more pressure available

[R2D2]

When a bolt yields the force that caused it deform elastically is still present so the bolt will not come undone because it yields.

 

When yielding does occur the stress/strain curve becomes parabolic rather than linear and the material will "work harden". The rate of this work hardening depends on the material chemistry and heat treatment but the change in strength caiused is relatively small.

 

This really means that once a bolt has yielded it doesn't really apply to much more axial load to the structure that it is clamping together and the further along the "flow curve" the smaller the effect.

 

"Stretch" bolts have been used quite commonly (The first time I saw them was in a FIAT Uno fitted with a FIRE engine) and they allow bolts to be tightened quite quickly on automated lines without worrying too much about torque settings.

 

I think that the variation in clamping loads is much lower with this type of bolt than with the older designs that used elastic loading and needed very consistent torque measurement an lubrication tro give even tightness. Some experiments have been carried out using high quality bolts, good quality torque wrenches and variations in axial stretch of around 30% have been measured.

 

There is one other phenomenon that occurs when a steel is used in its plastic region and this is known as stress relaxation.

 

The peak stresses that are produced when a bolt is taken beyond its yield point will gradually relax with time and temperature and so clamping forces will reduce slightly. They will never reduce to a value that is lower than the yield strength of the material at the temperatures experienced in car engines so the bbolt will always stay tight.

[Roger King]

Interesting contribution from Chris Flavell there.

 

'Strectch' bolts do provide a more reliable means of head fixing on production lines. The biggest problem that they have for hand built engines is probably psychological - they feel horrible! Not very scientific I know, but true nevertheless.

 

I can confirm the comments about the difference in bolt stretch with the same torque settings. We prefer to fix big end bolts by stretch rather than torque if we can get the correct stretch figures and it is quite worrying to find how far from the correct stretch a bolt can be when torqued, depending on lubrication etc. I should point out that I am talking here about the stretch on a bolt in it's elastic region. Once undone, the bolt should return to it's original length. This provides quite a useful way of checking if a bolt is past it's sell by date, assuming of course that the builder thought to measure each bolt individually before assembly.

 

With a bit of experience you can actually feel when a bolt is stretched the correct amount, although I stress that we don't use this method!