7.4L Replace/Rebuild

[FreeBeeTony]

I am considering purchasing a boat w/ a 7.4L Mercruiser I/O.
The boat is being advertised as having the engine replaced in 2010 but I'm pretty sure it was the longblock that was replaced as it still has a carb.
I'm going through a broker and it is being reported to me that a head gasket needs to be replaced, and asked if I would be willing to buy the boat as is. This raises a BIG RED flag to me and am now assuming I would be replacing/rebuilding if the price is right and the boat is worthy of such an effort.

My question is: if I was to make the purchase, and I wanted to replace the long block....what would be the recommendation?
I'm pretty well versed in the SBC's, this is my first BBC project.
I'm assuming the long block was replaced w/ a GEN V6. If my assumption is correct I may elect to rebuild: would like to Bore&Stroke to a 8.1L (496) or replace the long block.

Any/all opinions welcome!

 

[Lou C]

Tony what's going on? First yer lookin' at outboards now this! Lol!
How's the rebuild running after all the work you did last year?
I am not that well versed in big blocks but if that engine needs a head gasket, if they let it sit with either water (if raw water cooled) or antifreeze (if closed cooled) it may need to be completely gone through due to corrosion in the cyl walls and bearings etc may be all in need of replacement. I'd check what Michigan Motorz has as far as long blocks.

Is the price of the boat right enough that even with a rebuild it makes sense financially?

 

[alldodge]

The BBC is the same as the SBC its just bigger, for the most part. It gets rebuilt the same way and has similar issues.

Don't see you making it a 496, but can bore and stroke it and get 468 like Scott

Just cause it was rebuilt/installed in 2010 doesn't mean its a Gen 6, get the casting numbers from the motor to determine

 

[FreeBeeTony]

Thanks.......That's the kind of info I'm looking for!
But........I think a 468 is a 454 just bored out to a 4.310 bore (+0.060) and not stroked?
Is it OK to bore a 454 0.060 over in a marine application? I know 0.030 is the max recommended for a SBC.
I think the stroker crank of a 454 is 4.250 compared to 4.000 on a std 454. Do these blocks require clearancing as do the SBC?

 

[alldodge]

Yes can make 496 with a stroker kit 4.25 cranks and 6.385 rods

Do these blocks require clearancing as do the SBC?

A motor is a motor is a motor, if your building anything, clearances are checked, and building major power then even more so, because piston and deck height is critical

 

[Scott Danforth]

two ways to a 496.... the 4.25" bore, 4.375" stroke of the Gen VII (8.1) motors or a 4.312 bore (.060 over) and a 4.25" stroke crank shaft

a .030 over stroker is a 489

yes, if you are stroking a BBC, you need to spend a few minutes with a die grinder clearancing the pan rail for the rod bolts just like a SBC

stock stroke .030 over 454 is 460, and .060 over is a 468

the amount you can over-bore is based on the block you start with . a stock GM production block is about .060" without problems

a GMPP block is usually able to go bigger on the over-bore

a DART block can go really big

if you start with a 502 block, a 4.5" bore is a 509. go stroker and your 540

if you go with a DART tall deck block, you can go up to 632 cubic inches - then you can add forced induction and have 4 digits worth of HP if wanted

however prior to dreaming of spending money, as building a BBC is more expensive than an SBC, determine what you need, determine what you have, and determine what you want.

you have determined that if you buy this boat you need a rebuild. you have an existing boat, broken motor and a drive. What do you want? 400hp, 500hp, 1000 hp? above 500hp, and you need a different drive

Head gaskets generally dont go in a boat unless something catestrophic went first. most likely its a salt water boat, the manifolds were not maintained and they leaked salty water into the engine

assume the block and heads are a good core, and you simply need a rebuild.

stock rotating assembly is only a few $ less than a SCAT stroker (most machine shops prefer SCAT over Eagle)
new crank is about the same as getting a crank ground
new rods are generally cheaper than reconditioning existing rods
work with your local machine shop on pricing for line-hone, bore & hone, decking, balancing and headwork

or assume you have to start from scratch
a core motor for a production L29 is about $350-$400

a 502 is a bit more

new 502 crate long blocks are about $8200
used in the $3000 range

factor in the cost of exhaust manifolds, etc. and your labor.

 

[FreeBeeTony]

A motor is a motor is a motor, if your building anything, clearances are checked, and building major power then even more so, because piston and deck height is critical

I understand, just thought there may be differences in the BBC accepting a bigger crank than the SBC.

 

[tpenfield]

You could be spending $5K and up to get things sorted out. Do you really want to do this? What is so great about this boat that is justifying all of this potential effort and cost?

 

[FreeBeeTony]

I'm leaning towards this boat because I think I can get it VERY cheap......it's in a marina, owner wants to sell it, now is told bad head gasket....may just want to dump it and cut his losses.

If I do get it and decide to do a rebuild.....here's what I'm thinking assuming the block is usable:
1) Bore/hone/deck/clearance as required for a stroker
2) Install a stroker rotating assembly
3) Use "L29" Vortec heads (can/should I use LCQ pistons as I did on my SBC?) **Maybe these are not the optimal heads for a BBC......I'm in new territory.
4) Use a CompCams Marine cam (XM284-HR)

Those are my thoughts at the moment.....just thoughts at this time.

 

[Scott Danforth]

BBC's have different combustion technology than the SBC's the L29 only works with nearly flat tops as they are 99cc heads. so you are looking at a mild dome on the pistons at best

what heads are on the motor in the boat now? the L29's were in only a few configurations. rectangular port "088" heads in others. the head will determine the piston. if you have "088" heads, you will need more dome.

before you bore/hone/deck/etc. you have the block checked to make sure its good, then you get a cheep set of bearings and set the crank in there, identify where you need to clearance.

then you put the crank away, take teh bearings out, and clearance the block
then put the bearings in, put the crank in, and check

if all is good, then you send the block back to the machine shop. they should align-hone the crank bore, then dress the bearing caps, then deck true, then bore and hone.

use coated bearings for your engine assembly