I may be replacing the 5.7L Mercruiser (Chevy) so am trying to get ahead and
ask some questions early. I understand that a 4 bolt block is stronger, but if
you build a 300hp (or less) motor, will it really make any difference? At what
point do you see a 2 bolt block breaking (what HP)? Will a 2 bolt go up to
350hp and hold up? If I indeed need to replace the motor, I plan to look for
an automotive short block and install a marine cam, use all of the mercruiser
other parts (manifolds, distributor, starter, alternator, etc.) so I should
pretty much have a "marine" engine but not at the prices Mercruiser wants.

Thanks,
bobby
bbusselman at hotmail dot com

4 bolt main caps truss the webbing in the block and reduce flexing under
extreme torque and rapid RPM changes, as in bracket racing. Little
positive effects in a marine type operating environment.
JR

bob wrote:
I may be replacing the 5.7L Mercruiser (Chevy) so am trying to get ahead and
ask some questions early. I understand that a 4 bolt block is stronger, but if
you build a 300hp (or less) motor, will it really make any difference? At what
point do you see a 2 bolt block breaking (what HP)? Will a 2 bolt go up to
350hp and hold up? If I indeed need to replace the motor, I plan to look for
an automotive short block and install a marine cam, use all of the mercruiser
other parts (manifolds, distributor, starter, alternator, etc.) so I should
pretty much have a "marine" engine but not at the prices Mercruiser wants.

Thanks,
bobby
bbusselman at hotmail dot com

JR NORTH wrote in message ...
4 bolt main caps truss the webbing in the block and reduce flexing under
extreme torque and rapid RPM changes, as in bracket racing. Little
positive effects in a marine type operating environment.
JR

bob wrote:
I may be replacing the 5.7L Mercruiser (Chevy) so am trying to get ahead and
ask some questions early. I understand that a 4 bolt block is stronger, but if
you build a 300hp (or less) motor, will it really make any difference? At what
point do you see a 2 bolt block breaking (what HP)? Will a 2 bolt go up to
350hp and hold up? If I indeed need to replace the motor, I plan to look for
an automotive short block and install a marine cam, use all of the mercruiser
other parts (manifolds, distributor, starter, alternator, etc.) so I should
pretty much have a "marine" engine but not at the prices Mercruiser wants.

Thanks,
bobby
bbusselman at hotmail dot com


Bob, while I agree with JR, given the choice, and not much differance
in price (at the block level), I would still go with the 4 bolt block.

I would just rather know I have the better block. I had two 350's
built up (rebuilt) for me on my old boat. I think it ran something
like another $150 to $200 total for the two 4 bolt blocks (used). When
your spending like $2500 an engine to have it built up (350's are
cheap to build), what's another $75 to $100. Your only talking about
4% more.

John

"JR NORTH" wrote in message
...
4 bolt main caps truss the webbing in the block and reduce flexing under
extreme torque and rapid RPM changes, as in bracket racing. Little
positive effects in a marine type operating environment.
JR

bob wrote:
I may be replacing the 5.7L Mercruiser (Chevy) so am trying to get ahead
and
ask some questions early. I understand that a 4 bolt block is stronger,
but if
you build a 300hp (or less) motor, will it really make any difference?
At what
point do you see a 2 bolt block breaking (what HP)? Will a 2 bolt go up
to
350hp and hold up? If I indeed need to replace the motor, I plan to
look for
an automotive short block and install a marine cam, use all of the
mercruiser
other parts (manifolds, distributor, starter, alternator, etc.) so I
should
pretty much have a "marine" engine but not at the prices Mercruiser
wants.

Thanks,
bobby
bbusselman at hotmail dot com



Actually they will help in any marine setup. The webs between the mains are
thicker and stop flexing of the block. You still run marine motors at a lot
average RPM than a car motor. They blocks are not much different in price
from a wrecker. Most of the motors of the last few years are 4 bolt mains.
Bill

bob wrote:

I may be replacing the 5.7L Mercruiser (Chevy) so am trying to get ahead and
ask some questions early. I understand that a 4 bolt block is stronger, but if
you build a 300hp (or less) motor, will it really make any difference? At what
point do you see a 2 bolt block breaking (what HP)? Will a 2 bolt go up to
350hp and hold up? If I indeed need to replace the motor, I plan to look for
an automotive short block and install a marine cam, use all of the mercruiser
other parts (manifolds, distributor, starter, alternator, etc.) so I should
pretty much have a "marine" engine but not at the prices Mercruiser wants.

Thanks,
bobby
bbusselman at hotmail dot com

Thanks for all comments and advice. Looks like I got lucky as bolted on new heads
and the water in the oil appears to be gone (my threads took on different paths as
I went through the motor but I started with hydrolocked motor, water in pistons 5,
7 and 8. Took heads off 1 at a time and went to 2 shops, both said "looks like if
froze". Cracks in 5 and 7 combustion chamber and cracks on outside of both).

Story was:

* overheated last year and found water in oil. Changed head gaskets and heads
checked, resurfaced and rebuilt. No cracks found. Ran boat maybe 20 more hours
last year.

* Ran boat many more hours from May to late July and hydrolocked and had water in
oil. Water in cylinders 5, 7, and 8. Hoped for cracked exhaust manifold but both
heads cracked and appear to have froze. I was (and still am) afraid I had
forgotten to drain block at end of season. Still not sure.

I installed new heads and ran motor for 2-3 minutes on trailer without water on
exhaust manifold. Let sit overnight, pulled plugs and turned motor over. No
obvious water in cylinders. Connected water to exhaust, ran 10-15 minutes for temp
gauge to come up, let sit overnight, pulled plugs and saw dribble of water on #7
(back cylinder and lowest on that side due to tilt). Assume I have a cracked
exhaust manifold on that side. Also, only saw water on #8 once so that water could
have come in the exhaust valve also and only show up if the valve was open when I
shopped motor??)

Anyway, has anyone seen a frozen motor that cracked both heads and maybe exhaust
manifolds but not the block? When a block freezes and cracks, where does the Chevy
350 usually crack? Into a cylinder, into the lifter valley or in the lower half?

Still not sure what happened so not confident it is permanently fixed. Can freeze
cracks take a while to open up enough to cause problems? I ran it from May to July
if I froze it last winter (Dallas winter was rather mild last year also, but boat
is stored in driveway with a cover).

Thanks again,
bb