When is it necessary to "use new screws"?

[abscate]

I double checked jimmys info on head bolt replacement ( why do I even bother) and indeed he is correct with respect to Volvo lore.

That being said I recall at least three incidents of head boltbreaking off on head gasket jobs which is a real drag

So head bolts are NOT required new, but you may want to consider replacement

[oragex]

Side note: It shocks me to work on the bulk of vehicles with hubs to find the hub to knuckle bolt/stud count at 3 or 4 when the lug bolts/studs are larger and number 5 to 10 (I work on some over one ton trucks on occasion with 10 lug studs).
I know the wheel is a more dynamic load and the hub unit is located with a slight interference fit is more approaching a static load, but still it always is a bit freaky..... I have not seen a hub break off from a knuckle unless it was a hard collision where lots of stout stuff broke so I guess it's fine.

Only a guess, but these hub to spindle bolts - and wheel lugs as well - while being stressed from the tightening torque, they are actually mostly shear stressed under load (transverse shear). Metal is quite strong when it comes to shear. So they are not actually in tension or flexion (except from the tightening torque). All these suspension/wheel related lugs and bolts are largely over-engineered, I don't have a number but I wouldn't be surprised they have a 200%-300% safety margin. I guess the car would theoretically drive even with two lugs on the wheel in 'normal conditions' only issue would be the stress cycling during wheel rotation. Suffice to thing about the huge amount of torque some wheel lugs can take when they get really stuck in there and someone has to use a 10ft extension bar to undo one.

When I'm saying metal is quite strong when it comes to shear forces, I'm talking about quality steel of course. This line made me remember this funny (or actually not so funny) video

[xHeart]

The lug nuts are oversized and if kept near the correct torque they aren't stressed enough to worry about them.
I have only ever seen them fail from multiple missing lug bolts (studs with nuts where used) and the remaining ones taking TOO much load. If you want to check them use something flat, even a business card, and hold its edge along the lug bolt threads and check to see if the threads all touch. If the bolt has been stretched the narrowing of the shank is usually apparent.

Side note: It shocks me to work on the bulk of vehicles with hubs to find the hub to knuckle bolt/stud count at 3 or 4 when the lug bolts/studs are larger and number 5 to 10 (I work on some over one ton trucks on occasion with 10 lug studs).
I know the wheel is a more dynamic load and the hub unit is located with a slight interference fit is more approaching a static load, but still it always is a bit freaky..... I have not seen a hub break off from a knuckle unless it was a hard collision where lots of stout stuff broke so I guess it's fine.

Thanks!
I frequently remove the four 14mm(?) screws over the suspension turrets, and one 17mm bolt at the engine mount to take off cross stay -- I believe it take 50ft-lb & 80ft-lb. Is the use of threadlocker a safe alternate when reusing fasteners at the cross stay. OR do I replace these with new?

[prwood]

The lug nuts are oversized and if kept near the correct torque they aren't stressed enough to worry about them.
I have only ever seen them fail from multiple missing lug bolts (studs with nuts where used) and the remaining ones taking TOO much load. If you want to check them use something flat, even a business card, and hold its edge along the lug bolt threads and check to see if the threads all touch. If the bolt has been stretched the narrowing of the shank is usually apparent.

Side note: It shocks me to work on the bulk of vehicles with hubs to find the hub to knuckle bolt/stud count at 3 or 4 when the lug bolts/studs are larger and number 5 to 10 (I work on some over one ton trucks on occasion with 10 lug studs).
I know the wheel is a more dynamic load and the hub unit is located with a slight interference fit is more approaching a static load, but still it always is a bit freaky..... I have not seen a hub break off from a knuckle unless it was a hard collision where lots of stout stuff broke so I guess it's fine.

Thanks!
I frequently remove the four 14mm(?) screws over the suspension turrets, and one 17mm bolt at the engine mount to take off cross stay -- I believe it take 50ft-lb & 80ft-lb. Is the use of threadlocker a safe alternate when reusing fasteners at the cross stay. OR do I replace these with new?

In the VIDA instructions for attaching the cross-stay fasteners, there is no mention of additional angle tightening past the specified torque, so I'm assuming these bolts are all ok to re-use (which is what I've done all along). I also don't use threadlocker on them. Someone correct me if that's wrong.

[jimmy57]

Every grade of bolt has a standard torque that is the guideline and for most uses of that bolt these are the torque used. This is not a Volvo standard, it is the engineering standards for the bolt. There bolts in higher stress and higher risk locations, like head bolts, main bearing bolts, rod bolts, and ball joint and other suspension fasteners. These fasteners are usually a higher grade of a standard bolt or they can be very specific custom made bolts.
The bolts that hold doors, hood, tailgate or trunk lid, seats, etc. etc. will just have a torque as they are bolts that are not stressed far into their strength capacity. They are re-useable and torque that Volvo would publish would be what you would find in the standardized engineering guides. Those numbers on metric bolt heads and on the face (not the hex faces) of metric bolts are the grade numbers that are the strength rating of the fastener.
So, the answer is the cross brace bolts are re-used unless you find them to be rusty or have damaged threads.

An example of the bolt spec and torque chart:

[prwood]

Delved into VIDA a bit and found this on one of the general torque tightening spec sheets:

* Screws coated with a locking fluid, sealant, or self locking nuts must replaced with new when reinstalled.
* Nuts and screws which are not self locking can be re-used provided they are in good condition.
* Always use new screws and nuts when angle-tightening. Any exceptions to this method will be noted in the method.

[oragex]

Delved into VIDA a bit and found this on one of the general torque tightening spec sheets:

* Screws coated with a locking fluid, sealant, or self locking nuts must replaced with new when reinstalled.

I may have an exception to this rule. The 4 screws that secure the sunroof glass are hard to turn and have red thread locker on them. I see no reason why replacing them, these are really not very tight, just reuse or add a bit more red tl.

[prwood]

I did a quick run through of VIDA instructions that I've downloaded, and these are the places where a new screw/bolt is specified, and/or angle tightening is specified:

Use new screw/nut:
• Exhaust manifold
• Catalytic converter to turbocharger
• Heat deflector plate
• Cylinder head
• Front engine mount
• Rear engine mount
• Right engine mount
• Lower torque rod (when disconnecting transmission bracket)
• Wheel spindle and lock nut
• Brake caliper
• Driveshaft
• Control arms
• Subframe brackets
• Steering gear
• Ball joints
• Anti-roll bar links
• Propeller shaft

Angle-tighten:
* Cylinder head screws tighten to 20 Nm, then 60 Nm, then angle-tighten to 130deg
* Wheel spindle lock nuts, tighten to 105 Nm, angle tighten +90deg
* Lower torque rod, tighten to 35 Nm, angle-tighten 90deg
• Front control arms, front screw, tighten to 65 Nm, angle-tighten 90deg; rear screw, tighten to 105 Nm, angle-tighten 90deg
* Subframe brackets, lubricate screws, then tighten to 105 Nm, angle-tighten to 120deg
* Right hand engine mount, tighten to 35 Nm, angle-tighten to 90deg
* Wheel hub, tighten to 20 Nm, then 65 Nm, then angle-tighten 60deg

[abscate]

edited last line

[PixelDust]

"If a bolt has got only, say, to 1/3 of it's plastic deformation zone, when it's fully released then tightened back, it still starts with elastic deformation, only the plastic deformation zone will arrive at a smaller torque this time. I suspect most car bolts don't go past 1/3 of the plastic deformation zone at factory, so the metal has still - let's say - 80% of its elasticity if it's fully released. This is important, because a tightened bolt has always to keep a good part of it's elasticity because that's when it can keep a proper torque amount."



Everyday I reuse P2 axle bolts on cars getting strut work done, bushings, etc... In my experience these numbers are accurate. After the 35 NM torque is reached, I set my digital angle wrench to 90°, but can only reach between 50°-70° before I feel the point of no return.

I've yet to see there be an issue with reusing the axle bolts. You can feel when they're reaching their yield point. I've broken them before too, and they're easy to extract with a LH drill bit if you know how to use it.

It's irritating that parts doesn't always include a new bolt. I guess they're tired of explaining to the customer why they need a new axle bolt when they're in for control arm bushings. I dunno.