Installing a newer version of the 2.3 4ZD1 engine into my '92 pickup. The newer style head has not been drilled and tapped for the earlier EGR manifold that screws into the head at the exhaust ports. We've drilled it out and are now ready to tap the threads. A 1/2 inch tap is too small. Does anyone know the proper size? Appears to be 20 threads per inch and is tapered (ie, pipe threads)