1974 mercruiser 3.0 cylinder head interchange

[Chevyls6]

Hello everyone I've got a 74 Merc 140 in a StarCraft aluminum V Hull. the engine had a blown head gasket and a cracked head.

my question is can I use the late style small square port head or does it need to be the early style rectangle port head ? I believe I need a new manifold and elbow anyway so fitment there shouldn't be an issue as I'd just order the corresponding style.

 

[Scott06]

Hello everyone I've got a 74 Merc 140 in a StarCraft aluminum V Hull. the engine had a blown head gasket and a cracked head.

my question is can I use the late style small square port head or does it need to be the early style rectangle port head ? I believe I need a new manifold and elbow anyway so fitment there shouldn't be an issue as I'd just order the corresponding style.

Yes you can if you can still use the existing manifold there is an adapter https://www.michiganmotorz.com/mercruiser-exhaust-manifold-adapter-plate-kit-high-output-3-0l-engine

 

[Chevyls6]

well I'm not sure about the manifold and elbow. they seem pretty heavily corroded along with the exhaust ports and valve seats on the old head. how can you tell if they're bad ?

 

[Scott06]

Pressure test by blocking off the water inlet and outlet. Need to fab a blcok off plate ate outlet to elbow…

if they are that rusty and you are spending for a new head might be better off to get new manifold or clean fresh water take off.

if you have that much rust inside the manifold and head should you just get a new base engine or reman longblock? I know easier for me to say than you to pay but may be more reliable long term

 

[Chevyls6]

I believe this was a freshwater only engine as im hundreds of miles from the nearest ocean.

I did look into a new longblock but at $2500 its a hard pill to swallow. It looks like this engine was seized at one time then broken loose by the previous owner. I was hoping to throw a new top end on there to squeeze this season out of it then reconsider boring/sleeving it as a winter project.

The leak down test revealed 20-30% leakage in 1 and 4 with the gasket being blown between 2 and 3. I'm not sure how much of that figure is ring/bore related vs head gasket or valve related.

I like this hull a lot and it also came with a very good condition alpha one outdrive to swap in place of the pre alpha unit on there now.