Is my 5.7 4-bolt main?

[L31MaxExpress]

I once had a 350 in my old G20 van that broke the crank. Engine was making a knock at idle. Sounded alot like a broken flexplate. When I disassembled the engine the crankshaft came out in two pieces. I drove the van 20 miles home at highway speeds after discovering the broken crank.

 

[Crazydavez28]

Got the thing backed off the trailer and drove it around the yard. Vehicle functions as it should. Knocked like crazy but quiets down at idle. I'm thinking the flex plate is messed up somehow. I forgot to mention the previous owner wrecked it on the front end smashed up the bumper and grill. He said the knock started after that happened. Listen here

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[likestrucks]

I'm not sure on the TBI's but when it came to the Vortec's in the GMT400 platform, the 2 bolt block was used under 8600gvw, and that 4 bolt was anything 8600gvw and over. Yes, there may have been times when a 4 bolt could have been substituted in production for a 2 bolt, but that would be the exception. Under 8600 = 2 bolt and at or above 8600 is a 4 bolt was the standard.

When I replaced my engine in my 98 I put the 4 bolt in because it was only $50 more.

 

[Crazydavez28]

I had some time to fiddle with this and after unbolting the torque converter i saw no cracks in the flexplate and the noise persists. We're gonna find out real soon if this is 4-bolt main...

 

[evilunclegrimace]

The way that I have always been able to tell if a block was 4 bolt or 2 bolt on the TBI engines is if it has a fuel pump block off plate installed on it, MOST 1500/2500 under 8600 GVW did not have the block bored for a fuel pump push rod and MOST 3500's had the block bored for the fuel pump push rod even though the trucks had electric fuel pumps.

And for all of you that are buying 4 bolt TBI blocks you are also dropping your compression ratio from 9.3:1 down to 8.3:1. You also can use your RPO codes to find out what block/compression ratio that your truck has. RPO NA4 emissions = 8.6:1 ratio/4 bolt block and RPO NA1 = 9.3:1 ratio 2 bolt block.

 

[Crazydavez28]

The way that I have always been able to tell if a block was 4 bolt or 2 bolt on the TBI engines is if it has a fuel pump block off plate installed on it, MOST 1500/2500 under 8600 GVW did not have the block bored for a fuel pump push rod and MOST 3500's had the block bored for the fuel pump push rod even though the trucks had electric fuel pumps.

And for all of you that are buying 4 bolt TBI blocks you are also dropping your compression ratio from 9.3:1 down to 8.3:1. You also can use your RPO codes to find out what block/compression ratio that your truck has. RPO NA4 emissions = 8.6:1 ratio/4 bolt block and RPO NA1 = 9.3:1 ratio 2 bolt block.

RPO code is NA1 so not looking good for the 4-bolt

 

[evilunclegrimace]

4 bolt main bearing block are not necessary for 99% of the engines out there. Very few engines generate enough cylinder pressure to even come close to pushing the crank out of the block.

 

[L31MaxExpress]

4 bolt main bearing block are not necessary for 99% of the engines out there. Very few engines generate enough cylinder pressure to even come close to pushing the crank out of the block.

The problem is not so much the crank coming out the bottem as the fact the 4 bolts prevent the caps from walking in the block at higher rpm. 4 bolt caps are more stable because of the increased length and the extra clamping force on them from the additional outer bolts. The aftermarket splayed caps and the deep skirted block cross bolted caps are an additional improvement to this as well as girdle designs.

 

[evilunclegrimace]

The problem is not so much the crank coming out the bottem as the fact the 4 bolts prevent the caps from walking in the block at higher rpm. 4 bolt caps are more stable because of the increased length and the extra clamping force on them from the additional outer bolts. The aftermarket splayed caps and the deep skirted block cross bolted caps are an additional improvement to this as well as girdle designs.

And again 99% of the stock engines do not benefit from 4 bolt mains, they don't make enough horse power, don't turn enough RPM's or generate enough cylinder pressure. The newer LS style block is abit different from a SBC.

Four bolt blocks were developed by Chevrolet mainly for trucks. The two bolt main bearing retainers were failing due to the excessive "lugging" that was typical with a truck that was loaded. This was strictly a low RPM problem.

Racers of course liked the idea of the extra bolts retaining the main caps, so they became commonly used in racing and were installed in many factory Muscle Car engines.

As the others have said...a fine bottom end can be built from either. The 2 bolt is plenty strong for most street driven and mildly modified racing engines. If you are getting into serious big power modifications, then the aftermarket caps may be the way to go...which will require a 2 bolt core block.

john56h, Apr 2, 2007

 

[Schurkey]

The way that I have always been able to tell if a block was 4 bolt or 2 bolt on the TBI engines is if it has a fuel pump block off plate installed on it, MOST 1500/2500 under 8600 GVW did not have the block bored for a fuel pump push rod and MOST 3500's had the block bored for the fuel pump push rod even though the trucks had electric fuel pumps.

I've got a Vortec engine with fuel pump blockoff plate, drilled for engine-mounted fuel pump. 4-bolt mains.

I've got a service-replacement "crate engine" for a '91--'93 TBI Caprice with a blockoff plate, NOT drilled for an engine-mounted fuel pump. 2-bolt mains

I've got a Vortec engine, no blockoff plate, of course not drilled. 2-bolt mains.

I've got a TBI block that someone threw Vortec heads on, and sold it to me as a "Vortec" engine, has a blockoff plate, but not drilled. 2-bolt mains. (Just like the TBI Caprice engine)

Four engines is not much of a sample. It seems that the blockoff plate by itself means very little. It's the drillings for the fuel pump pushrod and the oil drainback BEHIND the blockoff plate that might indicate 4-bolt mains.

But again, with a sample size of four blocks, and sketchy history on a couple of them, I wouldn't bet too much money on anything. Pull the pan, know for sure.

And for all of you that are buying 4 bolt TBI blocks you are also dropping your compression ratio from 9.3:1 down to 8.3:1. You also can use your RPO codes to find out what block/compression ratio that your truck has. RPO NA4 emissions = 8.6:1 ratio/4 bolt block and RPO NA1 = 9.3:1 ratio 2 bolt block.

The light-duty pickup TBI 5.7s had dished pistons. The heavy-duty TBI 5.7s had deeper dishes, or bigger chambers, or SOMETHING to reduce the compression even more. The Vortec 5.7s have dished pistons, and as far as I know they're all the same compression ratio.

However, the TBI 5.7 in the '91--'93 Caprice, Roadmaster, etc. "B-Body" have flat-top pistons, for about a 1/4 point increase in compression ratio. The B-Body engine also has the roller cam. I had a B-body engine in my K1500 for about 80,000 miles, and it ran FLAWLESSLY on the stock K1500 computer and chip. (but no big increase in either power or mileage.)

The problem is not so much the crank coming out the bottem as the fact the 4 bolts prevent the caps from walking in the block at higher rpm. 4 bolt caps are more stable because of the increased length and the extra clamping force on them from the additional outer bolts. The aftermarket splayed caps and the deep skirted block cross bolted caps are an additional improvement to this as well as girdle designs.

In most (not all) cases, 4-bolt blocks/caps aren't really needed. If the application isn't fretting the caps, more bolts aren't going to help.

Who spins a pickup truck engine high enough to need four-bolt mains? I think it'd take some seriously heavy load to move the caps around at 4K rpm. Of course, detonation will do that. It's never a good idea to detonate an engine at heavy load. Light-load detonation was called "The Sound Of Economy" by GM in the '70s.

Another thought...The 1963 Daytona 500 "Chevrolet Mystery Engines" set speed records in qualifying, and a car powered by the "Mystery Engine" (prototype for the 1965 "Mark IV" Big Block 396) won the race. They all had two-bolt mains.