350 mpi rebuild questions

[vroom ZOOM]

Hello, I was posting regarding my 5.0 saltwater motors, and it seems like rebuild time arrived too fast. One of my motors on my new to me boat is stuck solid. Time to build new ones!

Since the old motors are saltwater 5.0 mpi's, I think I will use them for parts and junk the blocks. I will sell the 5.0 ECMs and buy 350 mpi ECMs, so that shouldnt cost me anything. I just picked up a low mileage 1998 5.7 vortec 5700 truck motor to use as a core for peanuts. I want to rebuild the short block, and then rebuild the heads in the winter or something if they are really bad. I am actively looking for parts right now and could use some advice on what to buy.

Parts that I currently have:
Mercruiser cams and lifters from the 5.0 engines (5.0 and 5.7 same merc part number)
Mercruiser 5.0 cranks (I think they are the same cast cranks as in the 5700 engines)
Mercruiser MPI harness, brand new complete intakes, accesories etc
All the parts from a complete 5700 engine

I want to buy new pistons, rings, bearings, gaskets, etc. I can either buy them individually or in a rebuild kit. The rebuild kits include brass core plugs which is nice. I am not really concerned about the gaskets or bearings in the kits, and definitely not the core plugs, but more so on the pistons and rings. Here are my piston options so far (all are hypereutectic, I don't think I need forged for a back to stock 350 mpi):

Summit
speed pro
sealed power
federal mogul

Any ones to avoid here? or are they all good? What piston head volume do I need? Compression height? Needs to all be stuff to work with the stock 350 mpi computer.

Next question is the top end. Does merc use 062 or 906 heads? are the vortec 5700 and merc 350 mpi rockers, lifters, and pushrods the same? Also my machine shop suggested to go with 1.6 ratio rockers, apparently it gives more valve lift and power without messing up the stock 350 mpi tune?

Any other considerations?

Thanks

 

[Scott06]

Hello, I was posting regarding my 5.0 saltwater motors, and it seems like rebuild time arrived too fast. One of my motors on my new to me boat is stuck solid. Time to build new ones!

Since the old motors are saltwater 5.0 mpi's, I think I will use them for parts and junk the blocks. I will sell the 5.0 ECMs and buy 350 mpi ECMs, so that shouldnt cost me anything. I just picked up a low mileage 1998 5.7 vortec 5700 truck motor to use as a core for peanuts. I want to rebuild the short block, and then rebuild the heads in the winter or something if they are really bad. I am actively looking for parts right now and could use some advice on what to buy.

Parts that I currently have:
Mercruiser cams and lifters from the 5.0 engines (5.0 and 5.7 same merc part number)
Mercruiser 5.0 cranks (I think they are the same cast cranks as in the 5700 engines)
Mercruiser MPI harness, brand new complete intakes, accesories etc
All the parts from a complete 5700 engine

I want to buy new pistons, rings, bearings, gaskets, etc. I can either buy them individually or in a rebuild kit. The rebuild kits include brass core plugs which is nice. I am not really concerned about the gaskets or bearings in the kits, and definitely not the core plugs, but more so on the pistons and rings. Here are my piston options so far (all are hypereutectic, I don't think I need forged for a back to stock 350 mpi):

Summit
speed pro
sealed power
federal mogul

Any ones to avoid here? or are they all good? What piston head volume do I need? Compression height? Needs to all be stuff to work with the stock 350 mpi computer.

Next question is the top end. Does merc use 062 or 906 heads? are the vortec 5700 and merc 350 mpi rockers, lifters, and pushrods the same? Also my machine shop suggested to go with 1.6 ratio rockers, apparently it gives more valve lift and power without messing up the stock 350 mpi tune?

Any other considerations?

Thanks

Thee is very little diff between a truck engine and marine. No you dont need forged pistons as stock is not forged.
if you have a low milage core if compression is good swap head gaskets, core plugs, cam, and circ pump and drop it in

 

[Scott Danforth]

merc used what ever vortec head GM bolted to the motor on the assembly line. some motors got one of each casting as GM considered them interchangeable. 062 = 906 Just like Mercruiser Phantom Black = Rustoleum gloss black. they are interchangeable.

as stated above, you dont need forged unless you want to bolt on a blower. cast is fine.

the cranks from your 5.0 may have rust on them (nothing special about them). I wouldnt use them. use the cranks that come in your new to you 5.7 blocks. or buy new cranks (talk to your rebuilder about the cost of refurbishing your cranks vs the cost of new.) your 5.0 cranks come out of the same foundry as any gm truck crank. there is absolutely nothing special about the GM motors used in boats other than a marine cam (truck cams are almost identical) brass core plugs, marine head gaskets and marine circulating pump. exactly what Scott said above on the shortblock

if you are bound and determined to rebuild the shortblocks, the cost to build a 5.7 and the cost to build a 6.2 is within $5 of each other

things I would do to the vortec heads:
cut the valve stems for better seals
use screw in rocker studs

If you have good ECM's just send in to Bob at OBD2 and have him reprogram them. then you can build a 5.7 or 6.2

things I would do with the cooling system. the hypercomplexified drain system which kills all mercruisers would be the first thing I would get rid of. I would also get two heat exchanges and at least do a half system

 

[vroom ZOOM]

merc used what ever vortec head GM bolted to the motor on the assembly line. some motors got one of each casting as GM considered them interchangeable. 062 = 906 Just like Mercruiser Phantom Black = Rustoleum gloss black. they are interchangeable.

as stated above, you dont need forged unless you want to bolt on a blower. cast is fine.

the cranks from your 5.0 may have rust on them (nothing special about them). I wouldnt use them. use the cranks that come in your new to you 5.7 blocks. or buy new cranks (talk to your rebuilder about the cost of refurbishing your cranks vs the cost of new.) your 5.0 cranks come out of the same foundry as any gm truck crank. there is absolutely nothing special about the GM motors used in boats other than a marine cam (truck cams are almost identical) brass core plugs, marine head gaskets and marine circulating pump. exactly what Scott said above on the shortblock

if you are bound and determined to rebuild the shortblocks, the cost to build a 5.7 and the cost to build a 6.2 is within $5 of each other

things I would do to the vortec heads:
cut the valve stems for better seals
use screw in rocker studs

If you have good ECM's just send in to Bob at OBD2 and have him reprogram them. then you can build a 5.7 or 6.2

things I would do with the cooling system. the hypercomplexified drain system which kills all mercruisers would be the first thing I would get rid of. I would also get two heat exchanges and at least do a half system

Thanks for the info. Regarding building the 6.2, it seems like the cams are different? I found three different part numbers for sbc cams:

811658 (I think I have these, used on 5.0 and 350)
866022T (Used only on 350)
862562T1 (Used only on 6.2)

Does anyone know the difference in these cams? Some places list the last one as compatible with all models, others say only for the 6.2? Again, my main concern is for the parts to work well with the mercruiser ECM without hiccups. Would a 350 with an ecm programmed for the 866022T ecm work well with the 811658 cam?

 

[Scott Danforth]

The Mercruiser cams for the 5.0 and 5.7 were the melling cams supplied by GM. The 6.2 used to be a crane cam, Crane 104224. Crane no longer exists as it's now Comp. They bumped the duration a bit

stock GM cam should be as follows

here is the stock marine cam for GM motors that was manufactured by Melling. https://www.summitracing.com/parts/mel-22119

from one of the service manuals.

202 duration intake
214 duration exhaust
395" lift intake
404" lift exhaust
112 lobe separation.

Merc Service manual 24 supplement:
Engine 377 CID (6.2L)
Lifter Type Roller Hydraulic
Rocker Arm Ratio 1.50 to 1
Valve Lift
Intake 0.3114-.3153
Exhaust 0.3160-.3200
No duration spec.

Black Scorpion motor
From Merc Racing Service Manual 7:
Engine 377 CID (6.2L)
Lifter Type Roller Hydraulic
Rocker Arm Ratio 1.50 to 1
Valve Lift
Intake 0.510
Exhaust 0.530

Lobe lift +- 0.002 (0.051) I= 0.340 (8.636) E=0.3534 (8.9763)
Dur @ 0.050 I=222 E=230

here is the cam spec for the Crane Cam

 

[vroom ZOOM]

Well the project is moving along. Not as fast as I want it to, but nonetheless moving along. After finally getting two good cores (was not easy) I am transitioning to the building stage. First issue I had with figuring out what connecting rods to use has been finally solved, I want the one on the left. View attachment DSC_0732[1].JPG
Although the one on the right also came from a vortec engine, it is not one of the powdered metal rods. Different sources gave conflicting information, and although I thought all the vortec rods were powdered metal, it is not so. I asked my machinist whether I should use aftermarket 4320 rods, and the answer was a definitive no, apparently the PM rods are good up to 500 hp, and are based off a GM aftermarket performance rod made a couple of decades earlier. I should be ok here.

Now with the rod situation figured out, I have still to figure out the piston situation. I was going to go with the speed pro H815DCP, but because of the lower compression height of 1.55, I would either need to deck the block or use a shim gasket. My machinist said to absolutely avoid decking the block, as it creates all kinds of alignment problems down the road. Apparently silvolite makes a 1.56 piston that will work, just waiting on what number that will be. Need to make sure my compression stays at 9.4.

While my blocks get line honed and bored, I am working on zyglo inspecting the heads. Again, there was a ton of conflicting information here, with many people online saying that the vortec heads are prone to crack. Machine shop said they rarely crack and that they would need to be overheated really bad to crack. Not sure on who to believe but they are all getting crack tested anyways. Not going to do the valve job at the moment as I can easily pull the heads and do it later if I want to. The haters can cry all they want to about me bolting the heads right up . Will post more info later for anyone going through the same kind of build later on.

 

[Scott06]

Well the project is moving along. Not as fast as I want it to, but nonetheless moving along. After finally getting two good cores (was not easy) I am transitioning to the building stage. First issue I had with figuring out what connecting rods to use has been finally solved, I want the one on the left. View attachment 401162
Although the one on the right also came from a vortec engine, it is not one of the powdered metal rods. Different sources gave conflicting information, and although I thought all the vortec rods were powdered metal, it is not so. I asked my machinist whether I should use aftermarket 4320 rods, and the answer was a definitive no, apparently the PM rods are good up to 500 hp, and are based off a GM aftermarket performance rod made a couple of decades earlier. I should be ok here.

Now with the rod situation figured out, I have still to figure out the piston situation. I was going to go with the speed pro H815DCP, but because of the lower compression height of 1.55, I would either need to deck the block or use a shim gasket. My machinist said to absolutely avoid decking the block, as it creates all kinds of alignment problems down the road. Apparently silvolite makes a 1.56 piston that will work, just waiting on what number that will be. Need to make sure my compression stays at 9.4.

While my blocks get line honed and bored, I am working on zyglo inspecting the heads. Again, there was a ton of conflicting information here, with many people online saying that the vortec heads are prone to crack. Machine shop said they rarely crack and that they would need to be overheated really bad to crack. Not sure on who to believe but they are all getting crack tested anyways. Not going to do the valve job at the moment as I can easily pull the heads and do it later if I want to. The haters can cry all they want to about me bolting the heads right up . Will post more info later for anyone going through the same kind of build later on.

I would also not deck the block unless needed , certainly not to make pistons fit, If you do stock replacement the hyperutectic are fine. Everyone thinks marine engines are 4 bolt main forged everything , but the same chit that went in a chevy truck is fine for most uses .
I have also seen on forums the cracking of certain vortec cores. I think you need to consider the use. I think most of the cracking comments come from guys who are racing vortec heads vs more sedate use.

if it is any consolation I am going through the same doing the engine mental masturba... with the engine for my 65 GTO. You can drive yourself nuts and everyone is saying go with roller not flat tappet cams and forged everything... except my engine builder ( have a cam from him in my current engine in there no issues). Roller on a Poncho is like a $2k investment and the lifter bores on a poncho are weak and will break if roller lets go.. So, I am basically except for rods and stroker crank (Eagle) doing a mild build just more cubes, want to keep it simple. Builder said cast eagle crank is fine for under 500 hp/6k rpm and I wil be way under

 

[Scott Danforth]

All chevy heads after the early 70's are light-weight castings and prone to cracking if overheated and a slug of water hits them. its not a vortec thing, its a chevy cylinder head after about 1973 thing

most GM mercruiser blocks are the exact same longblocks as the GM trucks (except the head gaskets, cam, water pump). nothing special about a marine motor. cast pistons, 2-bolt cranks, cheap cheap cheap cheap parts

 

[vroom ZOOM]

The piston situation is driving me nuts...

Basically I have three options, neither of which I like but I have to pick one. Both set the compression to 9.4-9.5

Option 1 is to run a dished 1.55 compression height speed pro H815DCP. I like this option because I get 50 thou quench with this setup. Also they are in stock at rockauto and I can get them right away for peanuts. What I don't like is that I will have to use a 15 thou shim gasket on an undecked block and heads to make this work. Not sure if that is a really bad thing, as the heads and block are really nice and flat.

Option 2 is to run a flat top silvolite 3437HC with a 50 thou head gasket. My machine shop recommended this method. The problem here is that I end up with about 75 thou on quench, not sure if this is a concern at 9.5 compression. Also the availability is kind of spotty.

There is also the silvolite 3457... kinda in between. Also a 1.55 CH but it will run with the stock head gasket thickness.

Let me know what you would run and why.

Thanks

 

[Faztbullet]

Way overthinking project, just build the 5.7's stock. All the extra work is not hardly going to be seen on that big of boat. Be different if it had a performance hull

 

[Scott06]

The piston situation is driving me nuts...

Basically I have three options, neither of which I like but I have to pick one. Both set the compression to 9.4-9.5

Option 1 is to run a dished 1.55 compression height speed pro H815DCP. I like this option because I get 50 thou quench with this setup. Also they are in stock at rockauto and I can get them right away for peanuts. What I don't like is that I will have to use a 15 thou shim gasket on an undecked block and heads to make this work. Not sure if that is a really bad thing, as the heads and block are really nice and flat.

Option 2 is to run a flat top silvolite 3437HC with a 50 thou head gasket. My machine shop recommended this method. The problem here is that I end up with about 75 thou on quench, not sure if this is a concern at 9.5 compression. Also the availability is kind of spotty.

There is also the silvolite 3457... kinda in between. Also a 1.55 CH but it will run with the stock head gasket thickness.

Let me know what you would run and why.

Thanks

What is quench with stock set up? I know .035 is ideal quench but don’t what it is from factory. option 2 if thinner head gasket for better quench but again don’t know what stock is

 

[vroom ZOOM]

Way overthinking project, just build the 5.7's stock. All the extra work is not hardly going to be seen on that big of boat. Be different if it had a performance hull

I wish I could... That is what I am trying to do. The thing is I am having a hard time finding pistons that replicate the stock ones. Either they are too short, the dish is too big or too small or they cost an arm and a leg.

 

[vroom ZOOM]

What is quench with stock set up? I know .035 is ideal quench but don’t what it is from factory. option 2 if thinner head gasket for better quench but again don’t know what stock is

0.051-0.053

 

[Scott Danforth]

The stock pistons are Mahle or Melling (interchangeable)

However any cast flat top with 2 valve reliefs from any of the piston manufacturers will be better than the cheap stock GM 4-valve reliefs

Don't over think a stock build

If you do not want to deck the block, get the slightly higher compression height, run a felpro 0.041 thick head gasket

 

[Scott06]

0.051-0.053

then option 2 your shop prefers with this gasket will get you to the factory quench https://www.summitracing.com/parts/mah-5776?rrec=true

I know every one wets their pants over the correct .035 quench but the OEM quench will work just fine for what you are doing

 

[vroom ZOOM]

Well at this point someone mentioned that a steel shim gasket will leak and I then said to hell with the quench. To make matters worse, most marine head gaskets are 0.039. I went ahead and got flattop pistons 1.548 high and that gives my 9.4 compression with a marine head gasket. If it ends up working without pinging this will be my go-to setup for the future. If it does ping I will swap in the dished pistons and put in the steel shim gaskets to get factory quench.

The H345DCP pistons just arrived yesterday and I dropped them off at the machine shop today. Hopefully I can start slapping this thing together next week.

 

[Scott Danforth]

Stock motors for past 30 years have need 9.4:1

 

[vroom ZOOM]

Well, I still havent got the block from the machine shop yet, but I am trying to figure out the closed cooling situation. Does anyone know the difference between the closed and open cooling version of the ECM? Any harm if I throw a RWC ECM on a FWC motor?

 

[Scott Danforth]

Different fuel mapping due to the different temperatures the block runs at

 

[vroom ZOOM]

Well, it seems like I am going down the rabbit hole of DIY. I have a partially burned ECM555 from an engine that caught fire. I cant really sell it because the plastic connector is a little charred, but it otherwise looked intact. I surely am not going to throw it out. You know what that means...
I drilled out the three rivets and pried the case open.