4.3 to 5.7 swap

[77searay197]

I wasn?t sure where to post this but My 4.3 Yamaha sterndrive has low compression. I have a complete sbc with a mild can just sitting in the garage waiting to be used. I currently have a 180 bowrider. Now I know I have to at minimum change props, get manifolds, etc and know that the engine has to come forward some. My question is if there isn?t enough room on the mounts to mount the sbc to the engine stringers what material do you use to make the engine stringer mounts? Can you add wood to the current mount and fiberglass it or is it better to cut the engine stringer completely out and completely make a new one? And if I did have to do that how would I go about doing it? I am mechanically inclined and have lots of fiberglass experience. I know it?s a smaller boat and a 6.7 will make it scoot pretty good but I wanna make sure it can handle it. Would glassing the whole engine area beef it up anymore? I?ve swapped a 5.7 from a 4.3 in my s10 before but I know this is another world. Thanks

 

[Scott Danforth]

Your drive will need to be regeared. It would actually be easier to sell all the yamaha stuff to some poor sole trying to keep his no longer supported yamaha drive running and get a complete mercruiser settup from controls to prop

As far as fiberglassing, plenty of threads in th ehull restoration forum

 

[77searay197]

Is the cutout the same for a mercruiser as it is for a Yamaha?

 

[Scott Danforth]

yes

 

[Scott06]

The 5.7 will be about 4" longer than the 4.3, motor mounts will also be 4" farther forward. If your boat was offered with a V8 it is likely that the mounts are already there. If mounts aren't long enough you need to build them out of wood and glass in place, just add to what's there no need to remove old ones.. Bigger issue to check is if you have enough room in front of the pulleys.

While you should be able to swap over all the accessories from the 4.3 to the 5.7 get the V8 exhaust and drop it into the yamaha drive, if you're doing that much work I think it would be worth it to swap in a mercruiser set up so you can find parts easily going forward.

 

[77searay197]

Yeah I?ve already done the measuring I?m gonna have to move the back seat a few inches forward but that?s ok. Do I just buy some plywood and layer them? And if so do I use outdoor glue to layer them and then epoxy over all of it after layering it and then glass it to the boat or is there another way? I just wanna make sure they last

 

[Scott Danforth]

I would take some time and look thru some of the restoration threads.

Woodonglass has a great link in his signature covering stringers, etc. the information will be valuable in building your motor mounts http://forums.iboats.com/forum/boat...1961-lonestar-flamingo-splashed-w-pics/page42


as for the 4.3, you realize you can simply drop in another 4.3, and be boating by Saturday. Then sell your boat and buy a bigger boat with a bigger motor for what you will sink in repowering your boat.

 

[Scott Danforth]

Here is some reading on the 4.3 if you decide to simply replace it http://forums.iboats.com/forum/engi...history-of-the-4-3-liter-with-casting-numbers

 

[77searay197]

Yeah I realize I could do that and I have thought about it. What sucks is I spent all summer rebuilding the floor in this boat and putting all new interior in it and setting the interior up the way I want it. I?ll look into those links

 

[QBhoy]

Would you not just be tempted to keep it simple and throw in a good direct replacement Merc or VP 4 barrel GM V6 carb engine ? Plenty about for much cheapness.
Plenty power there for a 18 footer ?

 

[77searay197]

Yeah but what it cost for one It would prob be cheaper for me to stick a 5.7 in it considering I already have one built.

 

[Scott Danforth]

a 1996 or newer 4.3 from a pick-a-part salvage yard and a 4-barrel marine intake with a 4-barrel carb will set you back $1000 sticking with a pre 1996 motor and you can swap over all the stuff from your yamaha upfitted motor and you are in it for $400 (300 for long block and 100 for gaskets/core plugs)

swapping your yamaha drive and all the other stuff will cost you much more.

and by having one built. did you build the 350 as a marine motor or was this a project for a car? depending on what its built with it may or may not work anyway.

 

[77searay197]

If I bought a junkyard motor I would have to tear it down and replace who knows what because I don?t know what I am buying. As far as engine internals go there?s no diff between a marine engine and a boat engine. Honestly it doesn?t even have to have marine gaskets because it?s a freshwater boat and not a salt water boat They are both gm motors with a diff cam. My 350 I have is a mild built 350. I built it and I know it?s a good engine. The cam that is in it would work fine for my application and already has the intake that I need also.

 

[Scott Danforth]

Actually it would need marine gaskets for any marine applications unless you run a heat exchanger. Even in fresh water. Same for the circulating pump. If the cam has more than 270 degrees, you will have reversion issues.

Does your 350 have brass or stainless core plugs?

Low mileage running take out motor with a 90 day warranty is only $300.

 

[77searay197]

Only gasket you would maybe need for marine would be the head gaskets but even then you really only need it for salt water and that’s the only marine gaskets I’ll probably buy. My cam is 268 and has brass freeze plugs

 

[Scott Danforth]

Yes, head gasket needs to be marine (stainless core)

 

[Scott06]

What's causing the low compression? Understood you have a good engine sitting in the garage, but if you have to move the seats doesn't seem to be a money saver. May be cheaper and easier to pull the 4.3 run it through the machine shop to freshen it internally and go back in than moving the seat?

 

[77searay197]

I’m not sure I noticed it when I was winterizing the boat. I’m guessing bad rings or something maybe cause it had a little smoke coming out of the vent tube. And the compression was 100psi and when I squirted a little oil in the cylinders It would jump up. I?ll prob do a leak down test to determine the issue. Yeah but I would only need to move the back seat a few inches I believe. I’m gonna pull the motor out this weekend and set the 5.7 in there and see what kind of room I’m exactly dealing with and then that will ultimately decide what to do

 

[Scott Danforth]

have you verified your gauge? I have had compression gauges read 50psi low, and some read 20 psi high

if the gauge is correct, I would also pull the heads on the 4.3. would hate for you to jump thru hoops only to find carbon buildup on the valves

 

[77searay197]

I?ve tried 2 diff gauges. Plus my boat seemed sluggish and wouldn?t get up to the Rpms like it was supposed to get up to. I could barely hit 4K with just 2 people in the boat. And I had the carb rebuilt so I know it?s not that and I checked the fuel psi and it?s good. I?ll pull the heads when I pull the motor tomorrow