Replacing a Volvo-penta GM 350 with 305

RJ in Utah

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Hi there. I'm in a bit of a situation here. I have a 78 Fiberform 19 foot open bow PROJECT boat. As I was replacing the floor and floatation foam which I originally thought was the reason it was a project, I found the real reason. The original Chevy 350 block has a huge bulge and a number of cracks in it.

So as I see it, I have 3 options
1) If I'm so lucky, the cracks in the original block are external only, I can have them welded and rebuild the motor. With my luck, this is not an option.
2) Find an automotive donor 350 block and rebuild it for marine use using the internals of the original 350
3) I have an 305 from a 1983 Chevy truck. I could rebuild it for marine use using the internals from the original 350. If I go this route using the cam from the original 255 HP marine 350, anyone have any reasonable ideas what type of power the 305 would produce.

My intended use for boat is tubing, maybe a skier and definitely a fishing/swimming platform, nothing extravagant.

Thanks in advance for the advice
RJ
78 19 ft FIberform inboard
 

alldodge

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I would find a 350 truck motor or from a donor boat. Don't think you'll like a 305 in the boat
 

kenny nunez

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If a Chevy is cracked on the outside I cannot believe that the thinner area under the manifold would survive.
With a little luck you should be able to locate a 350. Just be sure to reuse the original marine cam if it is still in good shape and install a set of brass core plugs. You probably will need a set of manifolds.
A marine 305 only has around 225.
 

Bondo

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I have an 305 from a 1983 Chevy truck. I could rebuild it for marine use using the internals from the original 350.

Nope,.... No you can't,..... While the motors are identical, Externally,.....

Internally they are different,......

I think yer confused 'bout what makes a marine motor, a marine motor,.....
To convert the 305 to marine, install brass core plugs, 'n composite head gaskets,....
 

Scott Danforth

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your choices are find a new 350 motor, from a truck (not a car) change core plugs to brass and the head gaskets to marine and bolt on your external marine accessories (circulating pump, all brackets and accessories, etc. and buy new manifolds and elbows

LKQ pull-a-part has motors for $300. add new manifolds, gaskets, etc. and your at $1000

or get a complete bobtail marine motor for about $1000

305's and 350's have completely different bores.

if you have a 350 now, I wouldnt use a 305.
 

RJ in Utah

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Thanks for the thoughts. I'm inclined to the 350 also. And I am VERY aware of the differences in the automotive and marine engines and the 350 versus the 305. I was just not certain about re-camming the 305 with 350 cam. Since I would be inside any motor I use, a rebuild with the appropriate marine parts would be mandatory....

Just asking, why the automatic assumption that the manifolds are shot?
 

kenny nunez

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Most times when a block cracks the freeze also takes out the manifolds.
There were several times down here that caught a lot of people off guard who did not take any precautions and paid the price. One year I replaced 15 complete engine/ manifold combinations.
 

RJ in Utah

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Jun 11, 2018
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Well, after having started stripping it down, I've discovered the starboard ( right side facing forward) side exhaust manifold has a crack in it. I've been looking and I'm having a bit of a time finding the correct one. The original manifolds are end discharge.

As I see it, I have 3 options.
1) Find the appropriate good manifold which may be easier said than done.
2) See if I can get the manifold professionally welded at a good shop. Yes, pricey but would possibly be the best option
3) Purchase a set of new center discharge manifolds and update the exhaust system.

Any opinions which option is best and
recommendations for boat wrecking yards? I'm in Utah so we don't have a lot of boat wrecking yards here....
 

RJ in Utah

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Jun 11, 2018
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This is the style of the original manifold. This happens to be the side I don't need (figures) but will give you the general idea. This is definitely an old school style manifold.
 

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alldodge

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Cast iron can be band-aided (welded) but it doesn't work very well. I would get new or a used low hours fresh water set
 

USA_boater

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Jul 31, 2018
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I can tell you I once put a stock replacement cam for a 350 TPI corvette into a 305 TBI camaro and it was a LOT hotter than it would have been in a 350. The motor lost all bottom-end torque but screamed like an OHC on the top-end...I had to re-gear to compensate...Since my understanding is boats need to run like a torque motor and not a racing motor, I'd probably not risk putting 350 cam in 305 unless you can get a 4 blade with the right pitch to make up for it somehow similar to changing gears in a car.
 
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