7.4 engine replacement

R

Rncab218

New Member
Aug 19, 2017
18
Boat Info
310 Sundancer 2001. Mercruiser 7.4 twin I/O Bravo 3
Engines
Mercruiser 7.4 bravo III drives
2001 310 Sundancer I/O Bravo 3’s. One engine block blown. Salt water cooled. Mechanic and myself cannot find a 7.4 long block anywhere. All back ordered and heard mercury either has may stop making them? Mechanic suggesting replace both engines with 6.2’s. Love the 7.4 engines. Any thoughts on replacements or any information on when 7.4 long blocks might be available? Thanks
 
Rncab218 said:
2001 310 Sundancer I/O Bravo 3’s. One engine block blown. Salt water cooled. Mechanic and myself cannot find a 7.4 long block anywhere. All back ordered and heard mercury either has may stop making them? Mechanic suggesting replace both engines with 6.2’s. Love the 7.4 engines. Any thoughts on replacements or any information on when 7.4 long blocks might be available? Thanks
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Is the block itself damaged? If the cooling jacket is still solid it can be rebuilt. A machine shop can resleeve the cylinders to bring it back to spec or they can bore it out and you can install oversized pistons. Lots of options on how to rebuild as long as the block isn’t cracked.
 
Be careful going from 7.4 big blocks to 6.2 small blocks. Mechanic needs to verify the outdrive gear ratios, among other things. I replaced 7.4s on my 290AJ with 6.2 383 MAG strokers and blew through 5 of them over a 2yr period. Gear ratios on 7.4 are typically 2.0 while the 6.2's should have at least a 2.2.
 
Go find a rebuilt one. There are plenty of reputable engine rebuilders that offer the same or better warranties than the new engines.
 
AJSB said:
Be careful going from 7.4 big blocks to 6.2 small blocks. Mechanic needs to verify the outdrive gear ratios, among other things. I replaced 7.4s on my 290AJ with 6.2 383 MAG strokers and blew through 5 of them over a 2yr period. Gear ratios on 7.4 are typically 2.0 while the 6.2's should have at least a 2.2.
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@Coz interesting
 
The chance finding a “new” 7.4 block is slim and it will be pricey. A Properly remanufactured 7.4 is equal to to a new engine and some argue better. Either way make sure you get a 4 bolt main block which can be verified by the “HI PERF” cast on the outside of the block between the #1 and #3 cylinders.
 
Wanderer 290 said:
The chance finding a “new” 7.4 block is slim and it will be pricey. A Properly remanufactured 7.4 is equal to to a new engine and some argue better. Either way make sure you get a 4 bolt main block which can be verified by the “HI PERF” cast on the outside of the block between the #1 and #3 cylinders.
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4 bolt mains are not as common as you think. The 4 bolt block is not needed unless you are building a high performance race engine at 500hp or more. It's also a myth that all Crusaders use 4 bolt mains. I can tell both of 1986 Crusader 454/350s are 2 bolt mains. I had them bolt rebuild from the ground up 8 years ago at a very good performance engine rebuilder. He is known as the top race car engine shop in the area and confirmed that 2 bolt blocks with cast crankshaft are fine up to 500hp. He installed a "mild" performance camshaft in my engines and they dyno tested at 380hp witch has made a difference in my 390EC. They have run flawless for 8 years and don't burn or leak oil. I am a fan of having you existing engines rebuild as long as they are close cooled and you find a good engine shop.
 
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These are new
 
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Hoplite808 said:
Is the block itself damaged? If the cooling jacket is still solid it can be rebuilt. A machine shop can resleeve the cylinders to bring it back to spec or they can bore it out and you can install oversized pistons. Lots of options on how to rebuild as long as the block isn’t cracked.
Click to expand...
I’m with @Hoplite808, there is A LOT of money to be saved looking into those options. Block cracked? Yeah, it’s gotta go. Not cracked, can be repaired and still a match without replacing the perfectly good twin, again, a huge savings!
 
Rncab218 said:
2001 310 Sundancer I/O Bravo 3’s. One engine block blown. Salt water cooled. Mechanic and myself cannot find a 7.4 long block anywhere. All back ordered and heard mercury either has may stop making them? Mechanic suggesting replace both engines with 6.2’s. Love the 7.4 engines. Any thoughts on replacements or any information on when 7.4 long blocks might be available? Thanks
Click to expand...

I went through this two years ago on my 340. Bought the boat very cheap with very suspect 7.4 horizons. Ended up junking both of them. Choices are as follows: Mercury 8.2 Big Block, 7.4 long blocks, 8.1s, or small blocks.

Because this boat had been sitting for YEARS, a lot of the accessory parts were in really bad shape, thus a 7.4 long block was really not an option. There are no new 8.1s available, and the used ones are really a crap shoot. That left new 8.2s or small block V8s (6.2s, 5.7s.) After conducting some research, the 383 bobcat engines are total crap. Additionally, the small block v-8s make up for the lack of torque with higher RPMs. The 340 is a heavy boat relatively speaking. We figured I'd be running the small blocks at 4,000 plus to achieve similar torque that the big blocks provided. Might be different with outdrives, but I still think the small block v8s are problematic for this size boat over the long term.

That left the 8.2s, which is what I ultimately chose. In California, I had to get the ECT versions with the gigantic cat on each manifold. They fit, but it's tight. Hooked them to the existing transmissions.

It's an expensive job. They don't make 8.2s without digital throttle and shift, meaning the transmissions had to have electronic actuators added, and you needed all the smart craft harnesses, in addition to the analog to digital conversion equipment to work with the existing gauges. It was a JOB.

Additionally, with that extra torque provided by the 8.2s, over the next two years I broke both existing propeller shafts after long runs. I attribute the breaks of at least one to the age of the shafts, the inferior alloy used in this era of Sea Ray for the shafts, AND the increased torque of the 8.2.

All in all, however, I would have made the same decision. The 8.2s provide plenty of power for this boat and the digital throttle and shift and smart craft are awesome. Have had minimal issues with the engines, which now have about 200 hours on them.

HOWEVER, unless you're up for a relatively large investment (at least 60k I would imagine to do both), my advice would be to go with a 454 long block reman. There is no such thing as a "new" 454 anything. They stopped making them years ago. Same with the 8.1 (496). Indeed, the only new 'big block' V-8 you can get is the Mercury 8.2 and that's only because Mercury licensed the molds from GM and builds their own engines. I suspect, however that the delay in getting a 454 reman will be just as bad as a delay in getting two new 8.2s. I believe it took 6 months to get the engines from Mercury when I did it, and that was pre-COVID.
 
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