Stretch Bolts

Problem being that you're probably not going to know if the old bolts have stretched beyond their elastic limit until you compare them with the new bolts you will have had to order and pay for anyway!

Well, there are certain higher standards for aircraft components, military issue hardware and critical care medical equipment and that is perfectly understandable. But cars? Nawww. The most dangerous nut in a car by far is still the one behind the wheel.

Stretch bolts are always stretched beyond the elastic limit. If they were only loaded in the elastic regime they would spring back to their original length.

Yes, of course. That makes sense. Feeling my age here with modern cars!

Exactly the same applies when tightening an ordinary bolt with a torque wrench.

For a long time I've used ScotClad sealer on all bolts that go into a housing - especially alluminium. Not only does it lubricate the tread when fitting, but seals it against corrosion, leaks, and locks it. And a litre tin lasts for ever.

For anything which is critical, you should check if the old bolts have stretched anyway.

But that is the whole point of stretch bolts. They are stretched to the maximum clamping pressure they can do, which is beyond their elastic limits.

London black cabs used to have every single nut and bolt lock wired at one time. Probably dating back to before self locking nuts etc became available. Plenty makers stuck with tried and tested ways long after there were alternatives.

Quite. Hardly rocket science.

There is a confusion of what is a stretch bolt.

To my knowledge the 1.4 k series engine had what most would call stretch bolts, and indeed had 2 stages of 180deg rotation after torquing.

Most manuals say these are suitable for re-use unless they're damaged and the rotation is designed to apply a known tension to the bolt. Rather than enter the plastic region.

If it?s only being elastically stretched, why would a degree rotation beyond a set torque be specified?

Tim

My thinking too. Not that I've that much experience of stretch bolts.

But given the head gasket problems with the K series, I'd not be inclined to save money on that job.

The K series engine has long bolts that go right through the engine:

Although one article here mentioned here said bolts should be well lubricated, I recall at least one instruction where the head bolts should be fitted clean and dry.

The idea that bolts should be done to one torque I guess is to remove ant potential free movement or gap, then the further turn is to apply a known strain and hence tension according to the thread pitch. Perhaps the designers thought it might provide a more reliable tension in the bolt than a torque, but hey.

Then surely you would need *two* sets of torque tables: one for lubricated bolts/holes and another for dry bolts/holes, because you will be applying more force to the joint if the threads are lubed than if they are dry!

Is that Scotclad 776 ?

Are you talking about those Rover Gaylander engines? The ones that have the dodgy head gaskets?

I freely admit I have no idea why some bolts are done to to such high torques. I mean, I can see it makes perfect sense on bolts that hold a cylinder head to its block because of the need to contain the pressures involved; I have no issue getting my head around that. But then what about crankshaft pulley bolts? They're done up to insane torque levels for no ostensible reason (as far as I can know anyway). I just can't see the need for it.

Because you get a more precise end load, that doesn't depend on lubrication or how clean the thread is.

In the elastic (springy) regime Young's modulus E = stress / strain. (clamping load / elongation). When the limit of proportionality is exceeded the bolt goes plastic and the equation no longer applies, the bolt will have stretched and will not return to its original length.

The initial torque seats the bolt. A screw thread has a known pitch and bolt is made from carefully selected material with precise heat treatment so the Young's modulus is closely controlled. The elongation from turning the bolt a number of degrees then sets the end load.

The "K" has very long bolts that go from main bearing plate to head about 4x the length of a normal head stud/bolt.

My CA18DET has instructions. Torque all to 29Nm. Torque all to 103Nm. Release bolts. Torque all to 29Nm. Torque all to 103Nm or 85-90 degrees (suggests preferred if angle gauge available).

They are not stretch and there is nothing in shop manual about replacing them. "K" and "CA" are both M12 bolts, the pitch may be slightly different but it's a comparable angle / length so similar strain and clamping load.

Yup. My very first Rover factory manual recommended it for every bolt in the all ally engine. So I've used it ever since.

If you don't, the 1/4" bolts on the waterpump always shear when it comes time for a new one. It also seals those which go into the waterjacket.

For some reason, there's no mention of it in later factory manuals.