looking for curable/pourable rubber

I have an application where I need to seal a cylinder which passes through an aluminum block. It is a tight fit but not leak proof. There is constant
contact with anti-freeze on one side and oil on the other side. The temperature can reach about 220 F and the pressure probably never exceeds 30 psi. I would like to machine a counterbore in the block and apply a pourable rubber type material in the counterbore to provide the seal. The people at Dow tell me that the RTV silicones will soften over time if they are in constant contact with the water/coolant. This is not my field of expertise. So if some one has any ideas, I would be grateful to hear them
Alan Krem Krem Speed Equipment, Inc.
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