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[Louise Martin]

hello, I’m redesigning an intake manifold joint on an IC engine where the upper and lower manifolds are plastic, and the factory setup uses M6 class 8.8 screws threading directly into plastic bosses. The plastic threads strip, so we’re planning M6 brass inserts and stainless steel bolts so the joint can be removed repeatedly. The original torque spec is 9 N·m, and I’m targeting a factor of safety of ~1.5, so about 15 N·m max. For this application, is a standard A2-70 / 304 stainless M6 bolt appropriate from a strength and torque standpoint, or am I risking bolt yielding or failure compared to the original class 8.8 fastener when torqued into brass inserts?

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[Alba Thomas]

A2-70 is usually fine at M6 for 15Nm from a pure bolt-strength standpoint, but consider the preload you’re actually generating, and what fails first – the insert pull-out or boss cracking, not the bolt. In tensile strength, A2-70 is lower than 8.8, yes, but at M6, you still have capacity. The bigger issue is torque-to-tension scatter: stainless into brass can give you weird clamp loads and galling. Stainless can still “grab” if the threads are rough or you run it dry. Specify a light assembly lube. If you lube it, 15Nm can generate a lot more preload than you think.

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