TTY bolt questions?

Bit of a longwinded post, what I am really asking below is if I should replace my pressure plate bolts while doing the clutch. TTY flywheel bolts are going to be replaced, but there are also six pressure plate bolts. I don't know if they are TTY, some sources say torque spec is 24 ft-pounds plus 90 degrees, some just say 24 ft-pounds.  Should I also replace these while I am in there?

After installing my brand new clutch onto the engine and tightening the bolts, 45 foot-pounds plus 90 degrees, I decided to learn myself on why these bolts have the extra amount of degrees and aren't just xxx foot pounds. From what I understand, these are torque to yield (TTY) bolts, or stretch bolts. When you tighten them extra, the material of the bolt yields slightly, causing a tighter hold. 

This is the first clutch I have ever done, and neglected to do this research before ordering parts. So I didn't realize it is necessary to replace these TTY flywheel bolts along with the clutch. Thankfully my local NAPA has me covered, ordering them in for tomorrow. This got me thinking though, what about the pressure plate bolts? Some spec sources say 24 ft-pounds, some say 24 ft-pounds plus 90 degrees. Is it worth the money (about $50!) to replace these while I'm here? I don't have an issue with not replacing them, unless someone can tell me it is a necessity like the flywheel bolts.  Thanks 

Angle torque does not mean torque to yield, it just means angle.

Really, the BEST way to torque a bolt is by measuring its stretch.  If you know how much torque it takes to get to a baseline and the thread pitch, you can fairly well approximate the desired bolt stretch with an angle measurement.  A lot of critical fasteners have an angle specification because it is more accurate and repeatable, even if the bolts are not TTY.

24 pounds plus 90 degrees sounds like a LOT for pressure plate bolts.  Usually they are in the 18-23 range.  What's the application? I can check my Official™ service info for a triple check. 

That all said, if you are ever in doubt about a bolts health, put its threads against the threads of another bolt.  The threads should all mesh with each other perfectly.  If the threads ride on one side at one end and ride on the other side at the other end, one or both of the bolts are stretched and should not be re-used.

Thanks for the reply. 

The original video I found on replacing this clutch (on my 2008 audi A4 B7), says to replace the flywheel bolts and tighten to 44 ft-pounds plus 90 degrees, then tighten the pressure plate bolts to 24 ft-pounds. Nothing about replacing them, just thought if they have an angle spec also, they might be TTY bolts like the flywheel ones? I will try to find another bolt to compare them with to see they're stretched, but I agree that 24 ft-pounds should be plenty on it's own for just a clutch pressure plate. I don't think I'm worried enough about it to calculate the angles and stretch, and will probably just reuse these if they don't look stretched against another bolt. Thanks for clearing up about the TTY bolts and angle specs :)

Agreed.  Angle is just angle.

Torque specs are kind of nebulous.  When you torque a bolt to X ft-lbs, you're not doing it to make it a certain tightness, you're doing it to effect a certain stretch of the bolt.  The problem is, it doesn't take into consideration the cleanliness of the threads, a burr on the bolt, friction under the washer/head, etc.  Torque values shoot for a range because friction increases exponentially as you torque the bolt.  Instead, they discovered a better way.  By torquing to (let's say) 20 lb-ft, that will create a far more accurate value, then you use the thread pitch to know how much stretch you'll get by going 90 degrees.  If you're torquing a metric 10mm x 1.0 pitch, you know that 90 degrees gives you 0.25mm.  More accurate than just going to 80 ft-lbs.

TTY is also kind of a misnomer.  If you were actually torquing until the bolt yields, you can expect a broken bolt soon.  TTY is more or less using a smaller bolt to get the same clamping force which puts it on the higher end of it's plasticity.  It more or less means that you have to replace it if it has been removed.  It won't sustain another torquing.

As far as replacing angle-torque or force-torque bolts, it's up to you.  I see no problem using the old ones, but if new ARP bolts are $20,  it's one of those "while you're in there" things.

I can say with certainty that every single TTY bolt in a Volvo whiteblock five will yield when tightened to spec.  All twelve head bolts. All twelve "main cap" bolts. All ten rod bolts.  

And Volvo is proud of them.

I only felt okay with buying new bolts from Volvo when I priced out how much the same hardware from ARP would cost.  Gone are the days of $115 head stud sets like when I bought studs for VWAG engines.

Speaking of which... it is not always the yielding that is the reason why VWAG specifies new bolts.  Manual trans Audis are rare as hen's teeth here so I have never had the flywheel off, but I have done plenty of cylinder heads on TFSI and 3.0 V6 engines.  Those head bolts have a very slick coating on them.  Torque to, IIRC, 37 nm, then 90 or 180 degrees.  The first time you use them, they twist in like butter, don't even feel like they are getting tighter. The second time you use them, they gritch and groan and pop and fight you.

That is that "repeatable friction" thing happening.  New bolts, for them, are a known quantity because they have some sort of moly-teflon-antientropy coating on then, that wears off after one installation.  Also very important when the bolt is ten inches long, 8mm in thickness, and you are tightening it at the opposite end from where all the friction is.

Yes, "use new bolt" and "torque to yield" are also not the same thing. 44ft-lb + 90* isn't TTY unless the bolts are either the same size as the pressure plate bolts or made out of cheese. Also I think actual TTY bolts will usually have a necked down portion where the yielding actually happens so that's a pretty good clue about what is or isn't a TTY bolt. Flywheel bolts seem important enough that you should replace them just to be safe, probably pressure plate bolts too.

Unless the new bolts cost something like $100 each I would not reuse them.  Cheap insurance.