Glad to hear someone else recommending this- I got raked over the coals for suggesting it a couple years ago. I put 'em in the block drains just as "an ounce of prevention". Cheap insurance, available at any marine dealer. With the prevalence of aluminum parts being added to the cooling circuit (manifolds, heads, radiators, etc.) it becomes even more critical. Dissimilar metals- iron blocks and aluminum heads, manifolds, t.c.s and rads-equals accelerated corrosion. Distilled water and a proper coolant mixture seals the deal.