Seized Mercruiser 3.7 (470) Swap to 165 Inline 6

[Justafarmer]

I have lurked on here for some time but this is my first post. I recently purchased an old Starcraft Holiday 18 with a seized 3.7 engine and an alpha one drive. It was supposedly running well when "parked" 5 or 6 years ago and properly winterized. The boat is going to need some work and I was hoping to get lucky freeing up the stock engine. No water in the oil and coolant is topped up. I've been adding small amounts of oil/penetrant to spark plug holes over the past several weeks. At first the oil would seep through on cylinders 1, 2 and 4 (counting from bow to stern). Cylinder 3 held the oil for the first couple days and now it drains out as well. I am using a large breaker bar to put gentle pressure on the crank bolt to try to rotate the engine but it is still stuck solid.
I realize that ideally the engine should be torn down and investigated but is there any other suggestions of what I could try to free it up. I am thinking about pulling the front pulley to see if anything external could be causing it to be stuck. I have read on here about all of the flaws of the 3.7 which is why I am looking to find an inexpensive way to assess this engine. If it's not going to free up then I may spend my money on something that had a better track record.

I also have a chance to buy an inline 6 mercruiser 165 for a reasonable price. Could anyone help me with figuring out what engine would be a good swap for what I already have (3.7 and alpha one)? Is there an engine that is compatible with the existing transom plate and drive or do they each have different set ups?
Thanks!

 

[nola mike]

What year is the 3.7? You'll have to fab new mounts regardless of the engine. Not sure about the I6 transom, but IIRC the transom plates were changed when the MR drive came out around 1983 on the 470. I6 is at best a lateral move. Same power, same obsolescence. Better going with a 4.3 or 5.7, both still supported. Remove the outdrive to make sure that isn't causing your issues. And don't turn the engine over using the crank bolt...

 

[Scott Danforth]

welcome aboard

first and foremost, quit trying to turn the motor from the front crank pulley. when you break the bolt off, you will need to pull the hole motor apart

next, pull the drive to make sure its not the drive that is locked up.

do a search, the swap has been done before. however a better swap is find a 4.3/A1G2 complete and swap the whole thing

 

[Justafarmer]

Thank you for the replies so far. I realise that using the crank bolt is not a wise way to put force on the engine. I have only used gentle pressure.

Hadn't considered the drive being stuck. If it was would there be a small amount of rotation/play in the engine still? This is locked solid. Not a mm of play.

I found some numbers on my engine and drive. Can these be used to find out if other engines would mount to my transom plate?

These numbers are both on a sticker on my engine. The transom assembly number matches the number on a sticker on my external drive.

engine drive unit serial #0C344617
transom assembly serial #0C431886

Thanks!