pistons for a 350 4bbl mercruiser engine

[lime4x4]

Looking at building a 350 marine engine for my boat. I have the lower end. block,crankshaft,rods and pistons.Pistons are pitted. Which type of pistons should i get? Its for an 86 mercruiser with a alpha 1 drive setup

 

[Bondo]

Re: pistons for a 350 4bbl mercruiser engine

Ayuh,... Cast will work,...
Hyperutectics are Better,... Forged is even Better, but probably overkill....

As to Which 1s,.... Ask your Machinest...

 

[lime4x4]

Re: pistons for a 350 4bbl mercruiser engine

thanks. My current motor when warmed up and tested shows only 125 to 130 psi of compression.That motor is only 2 years old.So i don't know if the wrong pistons or heads were installed and that's why my compression is low. Adding oil to the cylinders doesn't help so i know it's not a ring issue.I posted before on having low power from this 350 engine.I even checked compression on my tow vechile that has 350 in it with a 161000 miles and that engine has 175 to 180 psi compression. So i'm going to assume i have a low compression engine currently in my boat. Which chevy did make by the way..

 

[Bondo]

Re: pistons for a 350 4bbl mercruiser engine

My current motor when warmed up and tested shows only 125 to 130 psi of compression.That motor is only 2 years old.So i don't know if the wrong pistons or heads were installed and that's why my compression is low.

Ayuh,.... That's the Beauty, or the Curse of the SBC....

There's literally 1000s of possible combinations,+ None of them are necessarily Wrong,....
Just used in the Wrong Applications.....

About the Best option going around right now is a GM Goodwrench long block with Vortec heads....
The 1s spec'ed for pickups even have brass coreplugs,... Plug & Play with your old motor's Dressings...
With a 4bbl. Carb, it'll do a pretty Honest 275+hp.... Plenty for the Alpha drive, but not Too Much....

 

[TilliamWe]

Re: pistons for a 350 4bbl mercruiser engine

Just used in the Wrong Applications.....

GM Goodwrench long block with Vortec heads....
Plug & Play with your old motor's Dressings...

Wouldn't he need a newer style coupler and a different starter?

But regardless, maybe you have a cylinder head issue? Maybe it got large combustion chamber heads that have lower the compression ratio? But 125-130 is not terrible for a Marine engine. They tended to keep the comp ratios low, cause of the need to run on 87 octane fuel. Maybe your compression isn't "low"? I don't know, i am just asking.

 

[lime4x4]

Re: pistons for a 350 4bbl mercruiser engine

Current engine is a new style block one peice rear main seal

 

[wca_tim]

Re: pistons for a 350 4bbl mercruiser engine

by new engine, what do you refer to? Was it a reman form somewhere? or a buddy or you build it, etc... so many things can be a factor... combusion chamber volume, was the cam degreed off a bit (have seen that done before... can result in much lower static compression and a dog of an engine even though it runs), etc, etc, etc...

what kind of pistons do you have in it? 9as in dished, domed, flat, flat with valve reliefs. etc...

Oh, what are the casting numbers off the heads.


a factory long block has a lot of potential advantages as well...

 

[lime4x4]

Re: pistons for a 350 4bbl mercruiser engine

i removed the valve cover. the casting numbers are 333882 which show up as 76 cc chamber heads.The current engine uses regular stock dished pistons so that explains why my compression is low.i'm going to install 64 cc heads and see what happens

 

[dan t.]

Re: pistons for a 350 4bbl mercruiser engine

put in flat top pistons, a small chamber head woth dished pistons is asking for detonation as a dished piston has no quench. a flat top with these heads and a shim type gasket will give you around 9.1 comp ratio

 

[JustJason]

Re: pistons for a 350 4bbl mercruiser engine

i wouldn't say your compression is low at all. It's on the lower side but not low.
Anything below 100psi is low.
Before you go through the expense of swapping heads, grab some Yamaha combustion chamber cleaner, or some sea foam or something to the same extent and run it through the motor.
You may just have rings that are sticking a bit.
Spending 10 bucks on a can first makes alot more sense then 1000 bucks on heads.

 

[lime4x4]

Re: pistons for a 350 4bbl mercruiser engine

i have a set of 64 cc heads so i only have the expense of the gaskets.Upon farther research i found that my current heads are generally used in 150 to 180 hp engines.But i will try the seafoam first thou.Like i said my main concern is this engine at full throttle will not go over 4000 rpms and i even tried a 13 pitch prop that should've brought my rpm's up and didn't. I can swap out the heads in a half a day.Ive had this power issue since i put this engine in.

 

[JustJason]

Re: pistons for a 350 4bbl mercruiser engine

even with 130 psi a 4bbl 350 with the correct cam should get you 260ish HP.
If i'm reading your signature correctly.... you have a 26ft searay?
A single 350 isn't going to move a 26'er all that fast, but you should be able to get 4500 rpms out of the motor.
Have you been through your fuel and ignition systems yet?
Have you confirmed your gear ratio yet?
I work on an awefull lot of boats that have compression in the range you stated and I never have any problems getting full rpms.
Boosting your compression from 130 to 180 may require a rejet of the carb.

 

[mtnrat]

Re: pistons for a 350 4bbl mercruiser engine

I bought a reman 350 a few years ago. It worked ok but didn't have the power I thought it should. Turns out it only put out 200 hp. Switched it out for a 383 I built myself.

 

[lime4x4]

Re: pistons for a 350 4bbl mercruiser engine

I know it's a heavy boat and it's not going to be a speed demon.With the current engine it takes so long to get the boat up on plane plus i have to keep it at max throttle to keep it on plane.If i floor it from a stand still it takes a good 2 min to get it up on plane. My goal is to try and get her to run a little better with the least amount of money. Me and the wife have been talking about getting a bigger boat so sinking a couple of thousand into this one doesn't make sense. If i can replace the heads and the compression comes up hopefully the power curve will to. I've talked to other searay owners with this size boat and engine combo and they all keep it on plane at 2800 to 3000 rpms

 

[JustJason]

Re: pistons for a 350 4bbl mercruiser engine

may want to think about having your boat weighed... then launch the boat and weigh the trailer and see if you can get some specs from bayliner about what the boat should weigh.
It's just part of the troubleshooting process... if your weight is what it's supposed to be then you can at least rule it out.
Its a 23 year old hull.... and more importanly it's a 23 year old bayliner.
Not trying to pzzzz on your parade or anything... but i've seen alot of them that have been waterlogged.

 

[wca_tim]

Re: pistons for a 350 4bbl mercruiser engine

putting a different set of heads on a motor that won't turn a 13 pitch prop anywhere near redline... might make it a little better, might make it worse. I think others aqre right on track and that you have other problems.

Have you checked the left rear side of the outdrive to see what gear is stamped on it? It's also easy to take the top cover off (12 point socket) and count the teeth on the gears in the upper half.

I would stat there and then do tune up, weight boat, etc... lots of good ideas. if you're systematic about it it'll wind up being much faster and cheaper to get running at its best...

Just my two cents, not an expert, just a guy that has a lot of experience with what not to do...

 

[lime4x4]

Re: pistons for a 350 4bbl mercruiser engine

the drive is new and verified 1.50 ratio. The boat was weighed just over 7000 lbs with a full tank of fuel and water.It's a searay by the way. Some glass was removed from the bilge and transom no signs of water.Carb was rebuilt engine just had a complete tune-up filters,spark plugs,coil,cap,rotor and wires were replaced.Mercruiser manual shows compression should be no less then 150 psi.
When we boat the boat almost 3 years ago it had a blown motor.I replaced the engine with a short block and reused the cylinders head from the blown engine which by the way was a jasper rebuilt engine. Since the block is using factory pistons it's throwing the compression off. I looked a couple of weeks ago at the block i took out and sure enough that block had flat top pistons.That's why i'm looking at replacing the cylinder heads do to the fact i currently have a mismatch of parts.

 

[lime4x4]

Re: pistons for a 350 4bbl mercruiser engine

well pulled the engine out of the boat today. Hooked up a battery to the starter and retested the compression.All cylinders were between 96 and 105. Pulled the heads found 7 rocker studs started to pull out of the heads.Installed the correct heads and retested compression.Now all cylinders are 165 to 170 which is WAY better then what i had. Tomorrow morning going to reinstall the engine then take it to the lake and c if the power is better