You can also soak them in a bath of evaporust. www.evaporust.com
- I cleaned up my 17 year old fresh water manifolds and risers and even ran it thru the block...it's non corrosive... see http://www.mastercraft.com/teamtalk/...=work&page=193
post 1924. Other thread has test on penetrating oil - 50% ATF + 50% acetone showed best results.
+1 on the duty cycle on the manifold/risers - but might be worth talking a close look at the channels in manifold/risers if cleaned up.
Regarding the evaporust - I used a gallon to get my results. You could use it to at least remove the rust from the bolts on the manifold before you applying penetrating solution.
My plan is another gallon of evaporust thru manifold in risers during after next season.
Your manifolds/risers might be a cheaper replacement then buying enough evaporust to fill a vat. But I would at least get a few gallons and run thru block to clean out water jacket.