Plastic throttle body to plastic intake manifold bolts

Question

Throttle body and intake manifold are both made of plastic in my car. Every few years, I remove throttle body to clean Idle Air Control Valve passageway because it is inaccessible without removing the throttle body.

My throttle body is attached to intake manifold through 3 bolts (They are M6 Allen bolts). I know that proper torque values for such bolts is around 10 - 11 nm (7.37 - 8.11 lb-ft).

However, since the parts are made of plastic (which is not strong), I always tighten the bolts with a simple screwdriver only by hand without using any leverage because I'm afraid of breaking or cracking the plastic parts which are quite expensive.

Recently, I made a simple experiment to know how much torque I'm applying to the bolts when I'm tightening them with a screwdriver and the result was somewhere between 4.5 and 5 nm (3.31 - 3.68 lb-ft). Do you think this amount of torque is enough for throttle body gasket to seal against intake manifold? (the green gasket in the picture). Should I tighten them a little bit more?

As can be seen in the following pictures, the thread housing of the bolts are made of metal but I'm still not sure how much torque they can take before any deformation or crack happens.

Answer 1

As long as the TB seal is in good condition and pliable, it should seal the TB without issue.

There are a couple of considerations. First, there should be equal torque on all fasteners. If there isn't equal torque, it could cause the TB to be cocked slightly, which may introduce a place where air could leak. Secondly, your concerns overall are mostly misplaced. While you definitely would not want to over-torque the fasteners (same as with any fastener), you'd have to apply way more torque than what you're doing right now to damage the TB or manifold. The reason is, the metal parts there press up against each other when you torque down the fasteners. This does two things: 1) You can't put excess pressure directly on the plastic parts; 2) Won't allow you to over "squish" the gasket/seal between the two parts. Obviously if you torque these two tight, something would give and that something would be the plastic parts. However, if you tighten these down to snug, you can tighten them down just a little more and call it a day. Personally I probably would use anything past a 1/4" drive ratchet to tighten them down, but I'd be putting more torque on them then you're suggesting you're using. If you got really froggy, you could probably look up the spec on these and figure it out that way. While not vitally necessary to do so, would give you peace of mind you're not going to (or shouldn't) break something.