Marine vs Auto blocks.

Can anyone tell me the differences between a marine block and an auto block. I have a boat that has a 305 gm marine engine. The block has a small crack. I guess I can pull the engine and weld to crack up or I can find another marine block or if there are no real differences just use an auto block. Any info will be appreciated. David


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hi

it is my understanding, that the top 10% of all blocks produced by gm are sold direct to mercruiser. ( please coorect me if someone else has more precise info)

if yours is a merc than i would replace it with one.

the extra load that is forced on a marine engine ,,,,,it needs to be strong....do not trust an auto block if you take your boat offshore

you may end up with even more probs


aussie bob

I dont think there is difference in the BLOCKS. I have used auto blocks in pleasure boats before and they held up fine. People throw some crazy horse power into there street/strip engines and make them spin at 6k-7k rpms. Unless there is a major mechanical problem the blocks hold up. A pleasure boat generally only spins up to 5500 rpm due to thats about what the outdrive can handle ...so Im sure it will be just fine.
I spun a bearing in my boat with a merc 350 two weeks ago. Im throwing a 400 in it that lived its life in a car.
I think the marine industry attempts to leave quite a few things a mystery in an attempt to make you think you need to buy marine products. Electrical components such as dist., starter, alt. do need to be coast Guard approved.. I dont think much else differs from auto though.
Its just my 2 cents...take it or leave it.

all i know is that is easier to walk to the repair shop when the engine blow up in a car

Don't know about the GM products but I would guess the blocks were standard automotive. I know the Ford 460 and 302 used the standard automotive casting and machining, all the marine uniqueness was in final assembly components.

Some of the marine manufacturers built their own engines with custom blocks, cranks, and heads, buying the other parts from standard available OEM stuff. I sold a lot of Ford connecting rods to marine manufacturers back in the 80s.

i have changed engines in three different boats ranging from 4cyl. to chevy 350's.the difference that i found was a windage tray that you will need to take the main cap bolts for the studs.the head gasket is a special one that is required because you are running river or lake water through.we thought we could get away with a felpro gasket,so i talked to a buddy that races boats and he said that if you want it to last to use the marine gaskets he said he don't but he runs copper gaskets and also running nitro.you will need exhaust gaskets intake water pump any thing running water through it. the exhaust is a one piece gasket that the manifolds only use 4 bolts.

Thanks guys.

Sounds to me like someone needs a cell phone, a friend or two
and if we're going far off shore, a gpm and a ship to shore radio by .02

I have them all but the gpm. I do have a gps though that ought to work.

one of the other differences is that i BELEIVE that marine blocks have coated coolant passages everywhere, that way salt water doesnt create a horrible rusty mess inside the block

automotive blocks realy shouldnt be used in salt water apps' but they will work fine in fresh water

Or you could just use a rad. Using water that cold to cool your motor will cause overcooling and a consiquent loss of fuel economy.

^^^boat engines still have themostats, even if they are "open" systems. the only way they would run to cold is if the thermostat was stuck open

I haven't seen coated coolant passages in production marine engines..... and when you flush them you get a rusty mess.....

Maybe somebody does it but I haven't seen it yet.

If it isn't a mercruiser 350, then its just a 350. Thats what I was told anyway. My dad had a Mercruiser 350 in a lobsterboat once and they told him when he bought it that the blocks were better built, they had steel cranks and rods, etc. Could be wrong though.

^^^ ah ok, luckily i phrased it in a way that alowed me to be wrong

i know i saw that "fact" online someplace while i was researching using an automotive engine for a boat i was drawing up plans for, but i think i only saw it written on one site