Thread sealant

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The bolt in question, I think, is the bolt that is used to set the belt tension on the P/S pump. As such, it passes through a bracket with a slot. The bolt-head cannot make a seal there. The seal has to occur at the block end of the tunnel, where the bolt enters the block's cooling system. Theoretically the bolt torque should seal it.In practice, it doesn't always. If it is allowed to continue seeping, the coolant residue will eventually fill the tunnel and make removal of the bolt nearly impossible.
I'm pretty sure you know all this, so the info is here for others who might benefit from it.