Coolant leak from crossover plug

[brtnstrns]

Has anyone ever dealt with this before? The coolant leaked from the crossover plug when I first put the engine back in the car so I replaced the o-ring with a new one. Now it seems it's leaking as well after a couple weeks. The only problem is that I don't know if it's leaking due to the bore of the crossover plug boss being too large for some unknown reason or the aftermarket plug, which have a very good reputation but perhaps mine isn't up to par.

Any ideas?