Used TTY (stretch) cyl head bolts- how much do they stretch?

[regcheeseman]

Isn't the fact of what you are actually doing to the mechanical properties of the bolt far more relevant than how much/where they stretch.

The idea is that you take the bolt just beyond it's elastic limit and therefore permanently alter the particle (dendrite?) structure of the material, having done this the bolt is weakened, by how much is unknown. It can't be measured with a ruler though may be proportional to the % increase in length.

[745 turbogreasel]

Is that the final one after 300+ miles?

Usually the last  cold, sometimes the first warm.

[Mark(The Miser)UK]

Isn't the fact of what you are actually doing to the mechanical properties of the bolt far more relevant than how much/where they stretch.

The idea is that you take the bolt just beyond it's elastic limit and therefore permanently alter the particle (dendrite?) structure of the material, having done this the bolt is weakened, by how much is unknown. It can't be measured with a ruler though may be proportional to the % increase in length.

Clearly at any time you are in the plastic there is no clamping gain, and I'd suspect it just gets weaker as cross sectional area gets smaller. However there is reason to believe [I've read it in one of my old engineering books somewhere] that a bolt if allowed to relax a bit back out of it's plastic range  can regain some of it's strength, and even get stronger.  Presumably a form of work hardening... 

My somewhat limited testing in situ on my VW's give me figures of about 115 to 125 lbft initial venture into plastic range. Then reduces to 95 to 105 with the engine warm. I always back the bolts off to give me smooth entry into the torque. [I hate that snatching that can happpen otherwise  ]

 For whoever asked I do indeed have a lovely USA built dial gauged Snap on torque wrench. Way beyond my price range, but a gift from a tenant

[regcheeseman]

Presumably a form of work hardening...

Yep that's the theory, all well and good whilst it remains under tension at the very peak of elastic limit, the problem occurs when the fastener is undone and allowed to relax fully and further stresses induced into the material.

[92EcoDiesel Jetta]

I had a bunch of cheap aftermarket ones break on the final 1/4 turn.

Where do the head bolts usually break? If at the head, it's easier to drill and use EasyOut. What do you do if they break near the block? Pull the head? I haven't seen EasyOuts that long. Maybe put an extension on it?

[Mark(The Miser)UK]

I had a bunch of cheap aftermarket ones break on the final 1/4 turn.

Where do the head bolts usually break? If at the head, it's easier to drill and use EasyOut. What do you do if they break near the block? Pull the head? I haven't seen EasyOuts that long. Maybe put an extension on it?

I would expect that a bolt that has 'bottomed out' would be flush with the block and one that has merely 'over stretched', it would be in the middle of the exposed part of the  threads. Head off certainly on the cards unless you can drill a pilot hole, then the bolt that was merely 'overstretched' would be 'loose'

[Mark(The Miser)UK]

Presumably a form of work hardening...

Yep that's the theory, all well and good whilst it remains under tension at the very peak of elastic limit, the problem occurs when the fastener is undone and allowed to relax fully and further stresses induced into the material.

I've been thinking about  this, and I cannot see why there would be further stresses induced and even if there is some new stress, why it's a problem if you reverse the process by re-tensioning again?
I think it is time for me to do some research

[745 turbogreasel]

I think either way they  usually break near the top because it is a weakest point, but they always  came right out  using a pencil eraser to unthread them.  I think you'd need like 1/4" stretch to hit bottom.

[Mark(The Miser)UK]

I think either way they  usually break near the top because it is a weakest point, but they always  came right out  using a pencil eraser to unthread them.  I think you'd need like 1/4" stretch to hit bottom.

I vaguely remember about 14 or 15 threads to the bottom, but final torquing put it down to about 12 threads. Maybe I wrote it on a scrap of paper somewhere.

[92EcoDiesel Jetta]

I think either way they  usually break near the top because it is a weakest point, but they always  came right out  using a pencil eraser to unthread them.  I think you'd need like 1/4" stretch to hit bottom.

Good to know they come right out. I measured the following on my 1V engine:

cylinder head bolt hole depth       = 81.3 mm
block head bolt depth                  = 43.7 mm
head gasket thickness(estimated) = 1.5 mm
Sum of above                              = 126.5 mm


cylinder head bolt length (shoulder to tip)  = 115.8 mm
washer thickness                                       = 6.7 mm
subtract washer from bolt length                =109.1 mm

126.5-109.1= 17.4 mm (0.685") bolt clearance before bottoming.

That's a lot of clearance so bottoming is unlikely unless you left a lot of junk in the block hole.