Stretch bolts didn't stretch?

I put in my new flywheel on Saturday and then realized that I forgot to put on the dust cover (or whatever you call it). I pulled the flywheel yesterday and know that I am supposed to replace the bolts. Just out of curiosity, I measured the bolts and they measured between 19.4 and 19.5 mm on a digital caliper. Spec on a new bolt is 19.5 mm, so if these stretched it was not very much at all.

So what happened here. Are the bolts toast? I know they designed to be single use. I know many people use them multiple times, but that's usually after many years/thousands of miles. Does it make sense that these bolts wouldn't lengthen permanently in 1 day? Thanks.

I'm not sure they are stretch, thats the hex head ones for the smf?

These are the 17mm hex head bolts. They are 22-44+90 degrees.

Seatman said:

I'm not sure they are stretch, thats the hex head ones for the smf?

Click to expand...

What is the thread pitch on the bolts? If the permanent deformation was 100% of the 90*, then at most it would be .25 x the pitch. I doubt you'd get 100% elongation, so unless you know exactly what the bolt length was prior to installation, you are still in an area of question as to whether you can tell.

I don't know what the proper answer is, but if you installed them properly, and you are now reusing them, it would probably be best to get a new set of bolts. No fun, to be sure, but in this case you'd sleep better knowing the flywheel isn't going to fly off the crankshaft.

"Stretch" bolts is a lay term for what they actually are "Torque to Yield".



http://www.keytometals.com/page.aspx?ID=Check
Article&site=ktn&NM=107

http://fp.optics.arizona.edu/optomech/references/OPTI_222/OPTI_222_W4.pdf

You want to hit the area in the strain hardening but before necking. Which could be between .3 and 3 mm. To know where you actually are on the curve you'd have to test the bolt off line. I'm sure VW's numbers are conservative so that you don't get into necking (BAD). Imagine how much fun customers would have if their chinese bolts started snapping at VW's suggested torques.

If you really wanted to get scientific about it you'd get a calibrated torque meter and 1000 bolts and start turning them and recording the torque and measuring them.

Nice I am the torque wizard at work and we apply the same torque that says the bolts are snub the we do 90 degrees 90 degrees 90 degrees; measure the bolt stretch and look at the stress/strain curve to make sure we are up as close to the maximum tensile strength.

Chris,

Do you have any tips for looking at a bolt, new or removed and identifying if it is a TTY? Several years ago IIRC I installed some new factory TTY bolts on a Dodge 2.2L cyl head and you could see the reduced diameter section of the bolt just below the head.

Corrections accepted.

Thanks,

Gary

GCBUG00 said:

Chris,

Do you have any tips for looking at a bolt, new or removed and identifying if it is a TTY? Several years ago IIRC I installed some new factory TTY bolts on a Dodge 2.2L cyl head and you could see the reduced diameter section of the bolt just below the head.

Corrections accepted.

Thanks,

Gary

Click to expand...

It depends on the design of the whole assembly. Some tty fasteners are necked to reduce spring rate, some not. Can't tell by just looking at them.

GCBUG00 said:

Chris,

Do you have any tips for looking at a bolt, new or removed and identifying if it is a TTY? Several years ago IIRC I installed some new factory TTY bolts on a Dodge 2.2L cyl head and you could see the reduced diameter section of the bolt just below the head.

Corrections accepted.

Thanks,

Gary

Click to expand...

Mercedes uses that thinned section to identify TTY head bolts...

We had some critical fasteners that we had to measure the length as we tightened them to get the optimal stretch.

Mike