When is it necessary to "use new screws"?

[prwood]

In many (not all) of the VIDA repair/replacement instructions, it says to "use new screws" when refitting parts. Outside of VIDA I don't see this indication very often, with the exception of an occasional YouTube tutorial that mentions it. Some kits that I order come with new screws, so I use them there obviously.

Under what conditions is it truly necessary to use new screws, rather than reusing old ones?

[Rattnalle]

In Vida the instruction applies to everything with loctite on it. Volvo's mechanics aren't trusted to apply that on their own.

In reality mostly only screws that are tightened to stretch must be replaced.

[abscate]

Any screw that takes a torque which gets into the plastic region, where the shank of the fastener starts to stretch, needs to be replaced.

Typical on Volvos..

Ball joint bolt/nut
Head Bolts
Caliper bolts (I confess to reusing these)
Subframe bolts
On 1999- the axle bolt.

[oragex]

As above, or in simpler words, any bolt that says "torque to xxx lb.ft then rotate xxx degrees". The rotation brings the bolt into plastic deformation. About 80% bolts on these cars are like this.

Metals are elastic materials (at a very small scale). On most metals, the tightening torque can be increased at a relativelty steady rate while the metal is in the elastic deformation zone. When the metal is approaching its limit of resistance, it enters the plastic deformation zone where the torque keeps increasing for a while at a much lower rate, then starts decreasing, then finally the metal ruptures. The plastic deformation zone doesn't mean the metal becomes suddenly "brittle", it rather means the metal becomes less elastic. The more into the plastic zone, the more it loses irreversibily some of it's elasticity.

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.

All this to say that when reused, such bolts can never reach again the original designed torque - if someone still tries to put the design torque on it, that bolt will go further into the deformation zone and one may see the tightening torque decreasing as he is turning the wrench. Even if the person stops turning the bolt when he sees the torque decresing, the metal resistance can be quite inferior of what it was originally designed - in other words under normal stress the bolt can fail.

In practice - I think many of us reuse several of these bolts, but I guess they use a slightly less torque. I don't want to give details as this is a matter of taking its own risk.

A very easy way to test all this is to go at the junk yard, ondo a 14mm bolt, put some grease on the threads (only for this test, grease on a bolt is the worst mistake) then start tightening with a torque wrench until the bolts snaps. Thw elastic and plastic zones will be very easy to observe while tightening.

[jimmy57]

I disagree a little.
Volvo lists replacement for head bolts on short I6, the VEA 4 cylinders (Drive-E), and diesel white engines. All of these use torque plus two angles and I think some diesels uses two torques plus two angles.
Red block and the gasoline white engines do not list head bolt replacement. These two have or had max bolt length specs. In the paper manual for white engines it was there but when I look in VIDA for a few of them it is not there. The white block head bolts have a max length of 158 or 158.5 mm flange face to bolt end.
The short I6 and VEA family use smaller diameter head bolts with threads on all but a few mm of the shank of bolt.
Torque plus angle started being used on the same engine with the same bolts in many cases, red block engines are one of these. Tq+A has two bg reasons it is used. FIrst is that it is WAY more consistent. Consistency in threads, cleanliness, finish of the component, and finish of the face of bolt (or nut) all contribute to the torque needed to rotate the fastener. Elongation of the stud or bolt due to its tensile properties is what is desired as that sets what the engineer desires: uniform clamping force holding the pieces together. Tq+A stsrts with a torque low enough that friction is negligible. Then you turn the fastener the specified degrees based on thread pitch and tensile strength of bolt or stud. If it takes 3000 pounds of force to elongate a 12mm bolt .25mm then turning a bolt with a 1.0 mm thread pitch 90 degrees you get 3000 pounds of clamping force (simplified explanation but the point is correct). If that same bolt was torqued 105 ft-lbs then some would go 90 deg, the cleaner holes with a little oil residue might go 100+ deg, and ones with coarse component surface or some dried coolant residue in the hole might go less than 75 deg and be at 105 ft-lb. The clamp force would be quite different around that engine.
Second reason is that persons or robots using automated tools need it. The tools that tighten a number of things like head bolts, subframe bolts, etc use a multiple driver head with torsion rod socket drivers (integrated socket is a torque bar) with the outside of fastener having splines. The machine drives all bolts at one time for a set time period (like .1.2 sec) that allows every fastener to get to the low torque and then the torsion bar twists w/o moving the socket. Now a sleeve with splines inside moves down over socket and engages and the socket and the machine turns every one the same angle. If there is an issue the machine notes it and that engine gets pulled. Torquing headbolts all day long was a human-wrecking job. Driving fasteners to high torques with impact drivers wears tools out too fast. The multi head machine does the tasks thousands of times before wear items need attention.

Torque plus two angles is very strange since the bolt feels like it is going to break. The first angle usually requires a lot of torque and then when you start the second turn the force goes up and then it gets easier and you just know it is about to break but then it gets harder to turn as you reach the second angle amount. These bolts are taken past elastic phase and effectively work hardened. These will definitely be longer when removed as compared to new

[prwood]

So, in my case, I'm looking at replacing the front and rear hydraulic engine mounts. I got new screws for these. However, prior to replacing the mount, you also have to remove the nut and bolt attaching the lower torque rod to the transmission, and replace them with new ones when reinstalling. Re-installation instructions say:

Install a new screw and nut on the lower torque rod. Tighten to 35 Nm. Angle-tighten 90 degrees.

I do not have a new screw and nut for this part. What would the consequence be if I re-used the old fasteners? Would I be better off waiting until I can obtain a new set?

[oragex]

Unofficially, for such non safety critical bolt, tightening the old screw to 30Nm may do the trick.

Again, this is a case by case scenario and also a self-assumed risk. Safety bolts - such as the two securing the ball joint, may be better replaced with new.

[abscate]

So, in my case, I'm looking at replacing the front and rear hydraulic engine mounts. I got new screws for these. However, prior to replacing the mount, you also have to remove the nut and bolt attaching the lower torque rod to the transmission, and replace them with new ones when reinstalling. Re-installation instructions say:

Install a new screw and nut on the lower torque rod. Tighten to 35 Nm. Angle-tighten 90 degrees.

I do not have a new screw and nut for this part. What would the consequence be if I re-used the old fasteners? Would I be better off waiting until I can obtain a new set?

Reuse that one. You can easily inspect it to see if it walks

On edit...forgot to add..I re-used mine on my P2

[xHeart]

Are lug bolts safe for reuse? If yes, then is there a limit to how many times before discarding for new set?

[jimmy57]

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.