For "normal" applications (say attaching the oil pan or valve cover) are there standard or typical torque values for fasteners? I'm wondering if it is possible to use a rule of thumb for those fasteners when you don't have access to a service manual.
The answer is no. Absolutely not. Those charts are not for automotive kinds of torque.
Every "chart" or "table" about specific grades of fasteners are for achieving a condition called critical torque. This is very rarely the criterion for torquing automotive fasteners. Exceptions are head bolts, and certain suspension components.
The specific torque required for generic automotive applications will vary by the application, not the grade of fastener. In the "oil pan" example, you are liable to strip the threads out of a cast iron block before you reach chart torque for a 6mm or 8mm grade 8.8 hex head cap screw. (Besides totally squishing out the gasket to the point it no longer seals, as other posters have mentioned.)
For suspension components, the risk of using chart torque values is less, provided the female threads are of the same grade. Since this usually is not the case, or cannnot be easily determined [e.g. welded nut in unibody for control arm bolt] the only "chart" information you should refer to must come from service information.
The internet is a great place to find the kind of final torque information you need, when you don't have direct access to service manual data.
tl dr: No. There are too many variables.
There are two things at play when tightening a fastener for an oil pan:
- How much torque can the fastener actually handle?
- How much squish can the gasket handle before it doesn't seal.
Torque values for bolts will differ, due to size and hardness. An ARP bolt of any size will handle way more tensile load than its generic counterpart, because the superior material does not stretch as much.
You still have to take into account what you're fastening, though. Some gaskets have metal washers built into them, where you can tighten things down within reason and you'll not destroy the gasket.
All-in-all, the easiest way to go about it is to watch the gasket squish and not worry about a torque wrench. Sounds kind ludacris, but it's the way I've been doing it all my life ... on older engines. Newer engines it's a different story, though. A lot of the aluminum panned engines (like the LS1), use the pan as engine support. Torquing them correctly is going to keep it from causing issues with warpage, engine structural integrity, and with leakage. Older engines you just look for the gasket squish and see that you don't tighten it anymore. Make sure the squish is about the same all the way around and you're golden. Sounds very backyard, and it probably is ... I never had an issue with it though.
There are standard torque charts out there. They are practical for steel to steel applications with static loads. In these charts the torque is defined so that the strain on the bolt is in the elastic deformation range. If there are gaskets, mixed materials with high differences in thermal expansion, high heat differences or dynamic loads involved you need a better calculation of the screw connection than just assuring to be in the elastic deformation range.
Case "gasket" When you are placing a gasket in between the two materials being fastened the theoretical torque value is useless because normally the gasket is compressible or deformable.
Case "dynamic load" The torque charts are not designed for withstanding high dynamic loads, there the screw setting needs advanced calculation/simulation. Example: Head bolts, connecting rod screws.
Case "high temperature differences" Same as dynamic loads, especially if mixed materials (Example: magnesium material fastened with steel bolts)
Just some thumb rules:
- If none of the above situations apply and there is a steel on steel application, dry bolts (no lubrication, no coating): Use some torque chart.
- If there is a rubber gasket: Torque until you see the gasket deforming and the torque start becoming higher. Is the gasket is very soft and the screw is very stiff you can damage the gasket. If the gasket is too stiff and the screw to weak you damage the threads/screw.
- Every other situation: Sorry, no rule. Some people just try to "feel" it, with mixed results. Others just prefer to stay safe and use the manual.
How to learn when there is enough torque:
For training i used different screws with different materials (old car parts) and torqued the screws until they break. Over time you should get a feeling on the behavior of the material. Also a great training on how to pull out broken bolts / repairing threads.
You could look up the head markings in something like this and then use a chart such as this to determine a ballpark torque figure. It may not be perfect, but it ought to do in a pinch.
Edit: I thought it would be a good idea to separate out the SAE marking and include in this answer. The jpegs aren't perfect, but they'll do.
You can get a ballpark figure from the fastener sizes for "most" assemblies although there are plenty of exceptions and special cases. If you don't have the torque spec for a heat shield, for example, you can probably get away using the fastener size as a guide.
One thing to be careful about: any "ballpark" figure is going to assume a corrosion-free surface and usually dry threads. Working around cars a lot of bolts are rusted and/or become oil soaked and this significantly changes their clamp-up properties.