I've got twin 420's, just went thru them last year, I noticed they have a small amount of rusty water marks that seem to start at the Heads and drip down the block, I made sure at rebuild that marine head gaskets were used. Can't figure out whats up, I do wash down alot to keep them looking good, also note I had ton's of surface rust on the block under the heads before rebuild when I bought the boat. Any thought's???

This has been posted before about 6 months ago and I don't rember if they ever got to the bottom of it.I to have the same thing going on and I had the same problem on my last motor.Both motors had the heads off.The new motor has GM gaskets and the last had Felpro.Hope this thread has a good answer to it and it's not any big thing James

Thanks,good or possably bad to here some other folks have seen this, I am thinking/hopeing it's coming from condensation or wash down water getting between the open gap between the head and Block where the engine paint didn't get too, Then sits there and rust's, then finaly drips down the side of the block.

What kind of exhaust do you have? Stock Merc. manifolds have plastic plugs tapped into the back side of them ( the side facing the head). I have seen them leak from there.

Mine Stainless Marine.

On BBC depending on the year you have to seal the threads on the head bolts because most go through the water jackets.

My Gen V 454, yes had sealer. The Gen VI 502 does not need it.

I have 454 as well with CMI headers, and sealer was used on the head bolts, wouldn't the head gasket keep the head bolts from leaking?

Were the heads and block resurfaced as part of your rebuild? If not, this could be the problem, especially if there was evidence of a prior leak.

My advice would be to run the engine for a while and visually inspect for leaks. Try it while the engine is cold, as well as after it warms up. I went through the same thing after I rebuilt my 454 - water was "weeping" from the interface between the head/block. Builder said that some of the silicon-faced gaskets rely on engine heat to establish a seal. I went as far as installing a 180* T-stat to try to get my temps up, with no success. I finally started seeing water in the oil, at which point I took it back to him and had him replace the head gaskets. Turns out that his flunkee that he had install the heads neglected to use thread sealant! He replaced the Ferea gaskets with FelPro's, and used thread sealant, and the problem was solved.

Thanks Budmad, yes the head's were surfaced and the Block was decked, the guy that built them is a very reliable builder and said he used sealant , I do remember asking him if Brass valve guides would be a good idea during the rebuild, and he said he dosn't like to put them in just because the valve guide cast is thin and he dosn't like to take the chance of water getting / eating the cast between the Cast and Brass guides, with this said I figuerd he did all possable to keep the bolts sealed. I guess I'll go have another talk with him..Thanks.