are 305/350 mercruiser heads interchangeable?

My friend has a low hour 305 engine with a cracked block. He can get a high hour running takout 350 cheap. Was wondering if he could put his lower hour 305 heads on the 350.

Assuming that this is for a cruiser type boat.

You'll first want to determine which pistons are used in the 5.7L engine.
If these are F/T's, then in order to maintain a suitable static C/R, you'll need the 76cc chamber cylinder heads.
If these are the dreaded full dished pistons, then you'll need the smaller 64cc chamber cylinder heads.

Take the casting numbers from the 5.0L heads and research this.
http://www.google.com/search?hl=en&...pw.r_qf.&fp=62ec7320b59ce67f&biw=1406&bih=765
See if they are suitable for use with a 5.7L.


If the pistons are F/T's, then you have the ability to create a Quench Effect or Squish Zone.
Take the piston deck height, and then purchase your head gaskets (for the correct compressed head gasket thickness) that will give you a quench dimension of approximately .038".

If full dished pistons, you will not be able to create a quench area.


Bottom line..... make sure that the cylinder head chamber volume is correct for the 5.7L piston selection, and you'll be OK.

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Thanks. Also sorry for posting in crusader forum, but this is were i always go. Creature of habit, i guess.

Just be carefual about the 'flavors' of the blocks/heads...I'm getting at if the old block is pre-vortec and the young block block is vortec you may want to swap more than just the heads...

Thanks. We are talking mid 80's engines so that would be well before vortec right?

Mid 80's (GM date) is pre-vortec...I'd have more interested in getting the vintage of the "low hour" 305...

stock 305 Mercruiser heads and stock 350 Mercruiser heads from the 80's are not interchangeable.

sammi said:

Thanks. We are talking mid 80's engines so that would be well before vortec right?

Click to expand...

Yes, this is well before the Vortec engine.
In the mid 80's ('86 and '87... and you'll find conflicting information on this), both the 5.7L and 5.0L underwent a change regarding the 1pc rear main seal, and the center valve cover bolt pattern.
Somewhere in the mix here, the intake manifold bolt angle changed, and from what you can read, some Camero 5.7L engines got the roller camshafts.

Like said, first determine which pistons are in the 5.7L, then make your cylinder head choice.

If the 5.7L pistons ressemble this, then you're pretty much limitted to the 64cc chambers.





If the 5.7L pistons ressemble this, then I'd use the 76cc chambers, the correct head gaskets, and give this a quench or squish.