I cracked one of the 990 casting heads in the 454 in my boat. Wasn't able to find a reasonably price replacement quickly and bought some 088 casting heads (Gen V) that were sold to me with the quarantee they would work. The machine shop I bought them from swears up and down they will work. I am now getting water in my cylinders and oil and cant figure out where from. I used Mark IV head gaskets like the guy that I bought the heads from told me too and in inspecting the head gaskets I do not see anyplace water could be getting into the cylinders. In reading on this website it seems that they say GEN V heads wont work on a Mark IV block but doesnt say if the same is true the other way around. I was asking about some 088 heads on ebay and he said that, " GM makes adapter plates that press into the GEN V heads to make a perfect hole match." Does anyone have any info about this or if these adapter plates exist. Thanks, Jim

Does a leak down test and or compression test verify the leaking head gasket? More than likely your intake is leaking.

thanks for the reply... Did a leak down, no leaking and did compression check hit 150-160 throught all the cylinders. do you think it is in the intake because of the casting of the heads or possibly badly torqued intake. I read a post about water in the oil and how the standard 2 or 3 rotation torque down is not sufficient with marine.. It suggested torquing down 10 rounds or so. thanks again for the help.. Jim

Jim,
Try to find someone local who has experience with this as I have been told- yes they will work if you use right gasket, and no they won't work. Here's a little bit of information on The Mark site- http://www.chevyhiperformance.com/te...67/index5.html . Maybe you have to use Mark V gaskets not sure... good luck, Rick

Jim,
You probably saw some of the other posts about leaking intake gaskets and water ingestion. US1 Fountain suggested going around several times. I am using his former B&M 250 and I torqued it several times around. It leaked. Then I used Copper RTV on all intake gaskets surfaces and torqued around 10 times and then a few more. Had a beer and did it again. That cured it and it has been several years. Use a Fel Pro gasket. A marine motor seems especially prone to intake leaking due to the higher block pressures. I am not the most knowledgeable about engine building but if your compression shows good I would sooner suspect the intake gasket. You need to clean those surfaces and really goop them up good with RTV, the Copper or the Red seems to seal up pretty good. The Gray is for imports and cures much harder due to the number of bolts the Jap engines use on sealing surfaces. Don't use the import RTV, it is very stiff when cured and is like permanently gluing the surfaces together.

JIM, The cork's that mount above the lifter galley, throw them out. What happens sometimes is that cork will raise the intake manifold and you will not have a good seal towards the bottom of each runner. I've seen where they cause an internal vacuum leak as well with water leaks.
Place the intake manifold with gaskets in place to give you an idea on the silicon bead thickness. Make sure you use the silicon on both sides of gaskets around all 4 water ports.

Did you pressure test you exhaust? If you're exhuast is leaking you'll be chasing your tail forever. Reversion will milk out your oil in a hurry.

In addition to what Cig1988 said about the cork intake gaskets, it's been a LONG time since I've gone near them. I've seen where with them the bolt holes in the intake are really off relative to the threaded holes in the heads (can we all spell pulled threads?). Obviously, properly machining the intake will make any combo work but why bother? Using Silicone instead of the end gaskets lets you cocentrate on getting the intake and head surfaces parallel.One indication that you might have an intake leak (vacuum not water) is that you'll have full power at WOT but poor midrange power/response. As I said in a related post, when you find an intake gasket set that works for,stock up so you're covered for future teardowns. I could say more but I'll stop.