yet another headscratcher, waterpump leaker?

[BillFz1]

Ok guys and girls, here is a strange one. A friend gifted a new Bosch waterpump to our cause. Brand new item from IAP. I wanted to check mine due to overheating mentioned in another thread. Here is the rub. Pulled the old pump. Didn't look bad except the last guy to install it put so much silicone sealer on that it plugged the vent hole. I figured I would install the new pump. Well the new pump leaked, a lot,1 when just filling with coolant not pressureized. Thought it could be me, (I am competent but everyone screws up once in a while), so I pulled the pumpand checked it carefully. I found nothing but a good pattern on the gasket. Tried twice more same result. Pulled and compared to the original and it looked identical. Replaced the original pump, and NO LEAKS.
Has anybody on here had a mystery leak with a new Bosch pump? I looked at the front seal and it appeared dry. Of course these don't have a seep hole like some pumps so it is hard to tell where the leak is. This wasn't tiny either. I could see a stream of coolant on the block! Any similar experiences?
Thanks,
Bill

[bradartigue]

Are you certain the block mating surface is clean? The old pump may have some residual material that mates with the block surface. Or all of that silicone was there because something is dented/warped/etc. However that spot in the block would be an odd place for a warp - if nothing else put a straightedge on the water pump mating surfaces and see if there is deviation, if so you have a bad pump.

[BillFz1]

Are you certain the block mating surface is clean? The old pump may have some residual material that mates with the block surface. Or all of that silicone was there because something is dented/warped/etc. However that spot in the block would be an odd place for a warp - if nothing else put a straightedge on the water pump mating surfaces and see if there is deviation, if so you have a bad pump.

Brad,
I cleaned the surface to a near polish. I used a 3" machinists square. Totally clean and flat. Must be a bad unit but I couldn't even find the point it was leaking from! I used new copper seal washers and some sealer on the threads of the 3 bolts that are through to the water jacket in the block. When I compared the two pumps, (old and new) I couldn't see any obviously bad spots on the engine or the pumps. When I filled the water to a point above pump level and it streams out. weird.
I went as far as to measure the space under the bolts to be sure they weren't bottoming out in the block. I guess we quit worrying about it and just send it back.
Thanks again.
Bill

[azruss]

maybe the impeller shaft is sticking out too far.

[BillFz1]

maybe the impeller shaft is sticking out too far.

Interesting, the pump turned freely when bolted up. So the impeller wasn't hitting the block. If it was the other way I would think it would still seal but the the pulley would stick out too far.
Bill