Engine replacement confusion '79 GM350

[Nivekt]

So I've been shopping around for a replacement for my '79 OMC 260(GM 350) and while I can get a remanufactured marine long block for $1500 from Carolina Machine, I would rather try to save the coin and get a good used block for much less.

Of course this brings up the variable of having the correct cam and head gasket installed right?

My understanding is that my motor has a very specific cam that is good for marine use and that any used motor I get will need to have my cam swapped into it, correct? If I find a good used motor, is it as simple as pulling my cam out and putting it in the new motor, or is there a bearing replacement situation to consider?

So, lets say I get a motor with the correct cam, wont I have to replace the head gaskets to a type suited for marine use? If so, once I pull the heads of the auto engine, wont they then have to be resurfaced before re-installing with the correct gasket?

Also, what age range of block am I looking for as a direct replacement? My understanding is that GM went from a 2-piece main seal(what I have now) to a 1 piece as of '85 or '86. So what year range should I be shopping for when digging through the classifieds for used motors? Also, where can I find corresponding block casting ID's to identify what I am shopping for?

This is all new stuff for me guys and any help you can give me would be greatly appreciated. If shopping around for a used block is just too much of a gamble to be worth my time(with my lack of experience) then Ill just save the coin and get a reman. engine.

 

[matt167]

Re: Engine replacement confusion '79 GM350

If you can find a good pre '86 GM truck engine, you can knock the core plugs out, and put in brass core plugs, add in your marine accessories and drop it in. The truck cam is very close to a marine grind, so it's not really worth it.. Their is some conflicting information on the rear seals. I'v always known it as '87-later ( which is a year that a lot of changes came ), but some say, '86's have 1 piece rear mains. I have never done work to an '86 SBC so I can't confirm one way or another

 

[Boomyal]

Re: Engine replacement confusion '79 GM350

Beware of going to the later GM crank and rear main seal model. Word has it that OMC never made an engine coupler that fit the later GM crank bolt pattern.

Also, IMHO, unless you are using a closed cooling system, change the head gaskets and casting plugs. You won't need to resurface anything to run the Stainless Steel Marine headgaskets.. I am running a stock block 2001 5.0 Explorer roller cam in my 20 ftr. I am also running a closed cooling system so I left the original head gaskets and casting plugs. Every time I take the boat out I revel in the thought that my motor is running in coolant.

A truck cam would certainly serve you so you would not have to go far to just replace the head gaskets. If you do get a block without a truck cam, you would not necessarily have to replace the cam bearings to change the cam.

 

[Nivekt]

Re: Engine replacement confusion '79 GM350

If you can find a good pre '86 GM truck engine, you can knock the core plugs out, and put in brass core plugs, add in your marine accessories and drop it in. The truck cam is very close to a marine grind, so it's not really worth it.. Their is some conflicting information on the rear seals. I'v always known it as '87-later ( which is a year that a lot of changes came ), but some say, '86's have 1 piece rear mains. I have never done work to an '86 SBC so I can't confirm one way or another

Sounds simple enough. I do not know what kind of vehicle the engine came out of. Ill send him another email to find out. The engine is sitting at the guys shop so perhaps I can contact them for more details. It is purported to be a perfectly good engine pulled for replacing with a 454. The engine supposedly has less than 79K miles on it.

Beware of going to the later GM crank and rear main seal model. Word has it that OMC never made an engine coupler that fit the later GM crank bolt pattern.

Also, IMHO, unless you are using a closed cooling system, change the head gaskets and casting plugs. You won't need to resurface anything to run the Stainless Steel Marine headgaskets.. I am running a stock block 2001 5.0 Explorer roller cam in my 20 ftr. I am also running a closed cooling system so I left the original head gaskets and casting plugs. Every time I take the boat out I revel in the thought that my motor is running in coolant.

A truck cam would certainly serve you so you would not have to go far to just replace the head gaskets. If you do get a block without a truck cam, you would not necessarily have to replace the cam bearings to change the cam.

Thanks Boom. If I pick it up, Ill do just that. Im using a raw water cooling system. The brass core plug kits are cheap. Is it as simple as knocking out the old and pressing in the new or does some kind of sealant have to be used when installing the new ones?

Also, by "later" are you referring to the post '86? I have been told on multiple occasions that my housing adapter will only accommodate a block that is a 2 piece main seal motor so that is what I'm aiming for.

Can anyone say with certainty that the '71 motors were 2 piece main seal?

What would I need to look at/be certain of in determining whether or not the cam bearings would have to be replaced should I need to swap the cam?

Thank you all for your help with this. $500 sounds a whole lot better than $1500 if I can pull this off.

 

[matt167]

Re: Engine replacement confusion '79 GM350

'71 Motor is definitely a 2 piece RMS. Knock the core plugs out with a screw driver and pull them out. Install with some hi-tack spray and use a socket that is just smaller than the diamater of the plug. As far as Cam bearings, you kind of have to guess if they are bad or not. You can look and see the first couple bearings with a flashlight, and you would be looking for a copper color to tell you if they are worn or not ( copper= worn ). If the old cam shows signs of wear on the journals, then you might want to put bearings in. Worn cam bearings are pretty uncommon, generally done during a rebuild as preventative maint only

 

[Nivekt]

Re: Engine replacement confusion '79 GM350

'71 Motor is definitely a 2 piece RMS. Knock the core plugs out with a screw driver and pull them out. Install with some hi-tack spray and use a socket that is just smaller than the diamater of the plug. As far as Cam bearings, you kind of have to guess if they are bad or not. You can look and see the first couple bearings with a flashlight, and you would be looking for a copper color to tell you if they are worn or not ( copper= worn ). If the old cam shows signs of wear on the journals, then you might want to put bearings in. Worn cam bearings are pretty uncommon, generally done during a rebuild as preventative maint only

Thank you for the details. I just found out that the vehicle the engine came out of was an El Camino. I cant imagine the cam type in that type of vehicle would be what I need for my boat. Im a little hesitant taking the risk of having to swap out the cam once I buy this motor only to find out I have to tear down the engine if the bearings are bad. I guess Ill keep looking.

 

[matt167]

Re: Engine replacement confusion '79 GM350

Cam bearings can be replaced without rebuilding. Just need the bearing driver, which can be had for $40. If your concerned about the crank/ rod bearings. For a $500 used motor you should be able to have the motor put on a stand to where you can check them. Your only expense if you didn't buy the motor would be an oil pan gasket to put it back together . If owner is not willing to do that, $150-$250 is a fair price. 350's are too common to take chances on like that

 

[Bamaman1]

Re: Engine replacement confusion '79 GM350

Just pay the $1500 and get a marine specific engine.

Boat engines run very much differently form a El Camino engine, and they are highly stressed when run at high rpms all day. The auto engine just won't last as long without a marine camshaft and valve train.

I have a friend that worked at GM's engine casting factory in Michigan. He said Mercury would come in a couple of times per year and cast engines. Then, they'd take the rough castings back to their factories to grind and dress on the blocks. He said that the metal used by Mercury was a custom mix--different from the metals used in auto engines. In other words, Mercury I/O engines may look like Chevy engines, but there are differences.

You can pay now, or pay later. The marine engine is actually a good value.

 

[smitty28]

Re: Engine replacement confusion '79 GM350

Just pay the $1500 and get a marine specific engine.

Boat engines run very much differently form a El Camino engine, and they are highly stressed when run at high rpms all day. The auto engine just won't last as long without a marine camshaft and valve train.

I have a friend that worked at GM's engine casting factory in Michigan. He said Mercury would come in a couple of times per year and cast engines. Then, they'd take the rough castings back to their factories to grind and dress on the blocks. He said that the metal used by Mercury was a custom mix--different from the metals used in auto engines. In other words, Mercury I/O engines may look like Chevy engines, but there are differences.

You can pay now, or pay later. The marine engine is actually a good value.

Yeah Mercury uses a top secret blend of meteorites and moon rock to make the strongest blocks for marine use

 

[Howard Sterndrive]

Re: Engine replacement confusion '79 GM350

I have a friend that worked at GM's engine casting factory in Michigan. He said Mercury would come in a couple of times per year and cast engines. Then, they'd take the rough castings back to their factories to grind and dress on the blocks. He said that the metal used by Mercury was a custom mix--different from the metals used in auto engines. In other words, Mercury I/O engines may look like Chevy engines, but there are differences.

Mercury maybe (?).
OMC just used regular blocks and regular GM internal parts. That's why the OMC parts books all show GM numbers for the engine parts, and direct you to a GM dealer for parts, including the block part number.
same part number = same part

 

[matt167]

Re: Engine replacement confusion '79 GM350

I have heard that Mercury specified a specific metalurgy for their blocks in the '70's, and they did clean up the castings to some degree, but beyond that, I'v never seen proof of it, or a Mercruiser SBC of that vintage apart to see the differences..

I do know that Mercury marine has in the past ( if not currently ) assembled GM's crate engines for them. I think Mercruiser did their own casting on the 502/572 CI big blocks, and then in turn also sold them to GM as crate engines.



In this case I think a used truck engine is more than adaquate for a repower, converted to marine with marine gaskets, core plugs, carburation, ignition and accessories.

 

[Bamaman1]

Re: Engine replacement confusion '79 GM350

Matt: You're right about Mercury doing some custom assembly work for auto manufacturers. Yamaha used to build Taurus SHO engines too.

Their employees are absolutely fanastic at building engines--simply put. Some of the Mercury Racing engines going into offshore racing boats are running incredible horsepower, and they actually stay together running at redline for hours on end. I would say Mercury's building engines almost to the quality of a NASCAR Sprint Cup engine--only 250 cubic inches larger.