It depends on the Honda. The Fit (and the CR-V with which I'm most familiar) both use an aluminum pan. I don't really know much about the stamped steel pan. Since the OP was talking about a Honda Fit though, let's talk Honda's equipped w/an aluminum pan. Note: I don't know the specs for the Fit, so these assumptions are for a CR-V:
-the bolt is mid-grade steel bolt, 14mmx1.5tpi, threaded into a cast aluminum pan (don't know the grade, so I'll assume a mid-grade T6).
-the pan threads on the CR-V are roughly 6-7mm deep based on my observation. I decided to err on the conservative side and assume 5.8mm thread depth
-the aluminum crush washer is .8mm x 18mm O.D>
So, the question is "how precise do I need to be for this joint not to fail".
There are two ways a drain plug fails: it fall out (or leaks), or it goes past the yield limits of the fasteners or threads (in this case, the pan threads will yield first since they're softer). To the first point, it's almost impossible to put it in "too loose" WRT catastrophic failure. Pretty much the only way it's going to fall out is if you forget to tighten it. There forces on a drain plug are exceedingly low. OTOH, you could potentially have a leak--but that's the purpose of the sealing washer. As long as the washer is tightened to it's yield point, you won't have an issue unless there's significant damage to the threads.
OTOH, tightening the joint past the yield point of the threads is a potential issue. That's when you're going to strip the threads and run into trouble.
Rather than express this in MPa (which won't be relevant to most folks), let's pretend that all conditions are perfect and that your friction-torque wrench is 100% accurate in it's representation of joint tension. The joint shouldn't begin to yield until you reach roughly 77 N-m, or 57 ft lb. The minimum torque required involves a more seat-of-the-pants calculation, since I don't have measurements of tension between the bolt and the pan...but since they move as a system, I can guarantee it's very low. That's why drain plugs aren't falling out left and right. I'd guess it's in the 8-10 N-m range (6-8 ft lb).
Bottom line: there's a huge margin of safety build into the joint. Honda could increase this margin of safety by making the threads deeper (10mm threads would increase acceptable torque to 140 N-m!), but obviously they feel this is "good enough". The problem of course is that neither torque wrenches or operator feel are perfect, and if you look at the scatter plot on fastened joints, it's certainly possible to strip threads. That's why it happens; but since end users *typically* err on the low side, and the torque specification for a drain plug is given on the high side (torque specs typically are, for a number of reasons), problems with hand-tightened drain plugs are rare. Yes, they can happen, but it's out of the norm. If you use an appropriate sized wrench for the job, you aren't purposely trying to over-tighten the joint and you don't just plain forget to tighten the bolt, you don't need to take any special precautions to tighten a Honda drain plug in an aluminum pan.
-the bolt is mid-grade steel bolt, 14mmx1.5tpi, threaded into a cast aluminum pan (don't know the grade, so I'll assume a mid-grade T6).
-the pan threads on the CR-V are roughly 6-7mm deep based on my observation. I decided to err on the conservative side and assume 5.8mm thread depth
-the aluminum crush washer is .8mm x 18mm O.D>
So, the question is "how precise do I need to be for this joint not to fail".
There are two ways a drain plug fails: it fall out (or leaks), or it goes past the yield limits of the fasteners or threads (in this case, the pan threads will yield first since they're softer). To the first point, it's almost impossible to put it in "too loose" WRT catastrophic failure. Pretty much the only way it's going to fall out is if you forget to tighten it. There forces on a drain plug are exceedingly low. OTOH, you could potentially have a leak--but that's the purpose of the sealing washer. As long as the washer is tightened to it's yield point, you won't have an issue unless there's significant damage to the threads.
OTOH, tightening the joint past the yield point of the threads is a potential issue. That's when you're going to strip the threads and run into trouble.
Rather than express this in MPa (which won't be relevant to most folks), let's pretend that all conditions are perfect and that your friction-torque wrench is 100% accurate in it's representation of joint tension. The joint shouldn't begin to yield until you reach roughly 77 N-m, or 57 ft lb. The minimum torque required involves a more seat-of-the-pants calculation, since I don't have measurements of tension between the bolt and the pan...but since they move as a system, I can guarantee it's very low. That's why drain plugs aren't falling out left and right. I'd guess it's in the 8-10 N-m range (6-8 ft lb).
Bottom line: there's a huge margin of safety build into the joint. Honda could increase this margin of safety by making the threads deeper (10mm threads would increase acceptable torque to 140 N-m!), but obviously they feel this is "good enough". The problem of course is that neither torque wrenches or operator feel are perfect, and if you look at the scatter plot on fastened joints, it's certainly possible to strip threads. That's why it happens; but since end users *typically* err on the low side, and the torque specification for a drain plug is given on the high side (torque specs typically are, for a number of reasons), problems with hand-tightened drain plugs are rare. Yes, they can happen, but it's out of the norm. If you use an appropriate sized wrench for the job, you aren't purposely trying to over-tighten the joint and you don't just plain forget to tighten the bolt, you don't need to take any special precautions to tighten a Honda drain plug in an aluminum pan.
