PN 24502495 is a tall-deck aluminum Bowtie case with a deck height of 9.525 inches and it can be decked to 9.0 inches. Deck thickness is 1.125 inches. Four-bolt splayed 8620 alloy steel caps are used on the center three caps, while the front and rear caps are straight four-bolt caps. Use 2.65-inch main journal cranks and a 400-style two-piece rear main seal. It has sleeves that are semi-finished at 4.117 inches, and it can be bored to 4.135 inches. The pan rails are spread 0.800 inch and take strokes of 4.00 inches without modification, strokes of 4.125 inches with minor grinding, and a 4.25-inch stroke with major grinding and modifications.
Additional tin or nickel is added to the cast iron in some blocks. These high-tin and -nickel blocks are very desirable. In this photo, the extra tin is noted on the rear face of this 350 block by the “010” signifying that 1 percent more tin was used to help the molten metal flow into the casting mold. The “020” indicates that 2 percent more nickel was used in this block, which helps make the cylinder barrels harder and more durable. These casting identifiers are also found on the front face of these blocks under the timing cover.
Buying a used small-block can be a gamble. Do you know the history of the engine? Has it been stored inside or in the open? Can you tell if a block or head is cracked? Also, you’ll need to research what the engine was used in to help make sure your new replacement parts work!
Bowtie Casting Numbers
Casting numbers generally can be used to identify most Chevrolet parts. However, I must caution you about two casting numbers that have been used on one Bowtie block and another casting number that has been used with some aluminum Bowtie 18-degree and 15-degree heads. I bring this up because you may be looking at a used casting and should know the differences.
On cast-iron Bowtie blocks, casting number 10051184 was used to indicate a block with standard lifter bore angles, and the same block and casting numbers were used to identify the SB2 block with SB2 aluminum heads. There are also other variations between PN 24502503 and PN 12480175 (which are both based on casting number 10051184). One block uses nodular caps (PN 24502503) while the other uses steel caps. Also, the “503” uses a two-piece seal, while the other uses a one-piece seal. One casting number but two completely different pieces. ■
This block uses big-block Chevy-type cam bearings (Federal-Mogul PN 1811M), and the camshaft tunnel itself is raised 0.391 inch from the standard location. This dry-sump block has priority main oiling. There is no provision for an internal oil pump. The oil filter boss has been deleted and a starter can be fitted to either side of the case. There is no provision for a mechanical fuel pump, either, because the fuel pump boss has been deleted. You need an aftermarket oil pan in order to fit the wider pan rails. Also, the camshaft should have big-block journal sizes and a longer aftermarket timing chain or belt to fit the raised cam tunnel.
Cylinder Sleeves
So you look at your cast-iron block and find that a cylinder barrel is cracked or has a window. What now? Some folks believe that when this happens the only thing left to do is cart the bare block off to the dump or local recycling center. An alternative is to try to save the block by using a repair sleeve.
A machinist overbores the cylinder barrel and then press fits a barrel sleeve into the block. The sleeve is then bored to size. It’s done all the time by competent machinists, and iron sleeves are used routinely in aluminum blocks. The question is: Is it worth the expense and bother?
If the motor is going to be used for stock or moderate street performance, then a sleeved cylinder works just fine. If the block needs more than one sleeve, you probably shouldn’t bother. It’s cheaper and easier to get another undamaged cast-iron block. However, if you’re working with a rare, numbers-matching block for a restoration project, it may be worth replacing more than one sleeve to retain the original numbers-matching engine.
Aluminum blocks, on the other hand, have iron cylinder liners in all of the barrels. Replacement sleeves are available from Chevy Performance or the aftermarket. PN 12480004 is a standard bore steel sleeve for late-design aluminum blocks, including PN 10134400. This sleeve has a 3.980-inch bore and can go up to 4.135 inches.
Freeze Plugs
Small-blocks use 1⅝-inch-diameter freeze plugs, which are available in brass as PN 94673017. Aluminum race blocks, however, use a threaded freeze plug with an O-ring seal. Other freeze plugs are available through the aftermarket.
Some 400 small-blocks used three freeze plugs per side. Other 400s had a boss in the middle of the side of the block for the third freeze plug, but the boss was not drilled. You can use this unique freeze plug design to help identify 400 small-blocks. Other small-block displacements only had two freeze plugs per side.
Cam Bearings
With the exception of the 1955 and 1956 265 blocks, which used a unique rear cam bearing and rear cam journal, Gen I production cases can use the following cam bearings: PN 474005 fits position number-1; PN 474006 fits in positions number-2 and number-5; and PN 474007 fits in positions number-3 and number-4. Remember, each cam bearing goes in a particular position front-to-back, since three different sizes are used. If you cannot find these older Chevy part numbers, turn to the aftermarket bearing companies.
Crate Engines—UPDATE!
Chevrolet has always offered small-block “crate motors” over the counter. That expanded tradition continues today and enables the buyer to purchase a complete or partial motor. These crate motors are all new from the oil pan to the valve covers and come with warranties. Chevrolet also realizes that a number of the currently offered late-model production engines appeal to enthusiasts who want to put these engines into earlier-model vehicles, whether they are classic hot rod coupes and trucks or to punch up the power in vehicles that never had factory-installed V-8s.
Likewise, the aftermarket performance industry has done what it can to provide engine parts that improve performance of late-model vehicles and still meet the required national, state, and local vehicle emissions standards. The following is just a sampling of what is currently available from Chevrolet Performance.
Before getting into the performance side of Chevrolet’s crate engine program, there are three economical choices if you’re simply looking for a quality replacement engine at a great price. PN 12681429 is a classic 350-ci engine designed to replace worn out small-blocks from 1973 to 1985. It is based on a new four-bolt block, flat tappet valvetrain, iron heads, and perimeter-style valve covers. For 1996–2002 replacements, there’s the L31 Vortec 5.7L PN 12530283 (4-bolt) and PN 12530282 (2-bolt) engine with cast-iron Vortec heads. Finally, there’s the 350 ci for 1985–1987 light-duty trucks as PN 12681430.
Now, let’s get into the performance side of the offerings, starting with the 350/290 Base (PN 193556580). Bang for the buck starts here with a four-bolt block with aluminum pistons working with 76-cc iron heads to deliver a safe 8.0:1 compression ratio. The engine is rated at 308 hp at 5,100 rpm and 347 ft-lbs of torque at 3,900 rpm. The engine is basically a long-block but Chevy offers a Deluxe model (PN 19355659) that includes a 4-barrel intake complemented with chrome valve covers and air cleaner.
PN 19955662, the 350 HO Turn-Key engine, is topped with a set of iron Vortec heads fit with 1.94/1.50-inch valves. Compression is set at 9.0:1 and a flat tappet cam is supplied with peak power coming on at 5,100 rpm with 333 hp and 381 ft-lbs of torque. The Turn-Key crate engine comes with a 4-barrel intake, HEI distributor, a Holley carb, fuel pump, plug wires, and a complete front serpentine belt assembly including a new alternator, power steering pump, and air-conditioner compressor. A long-block package is also available as PN 19355660.
Next in the crate list is the SP350/357 that delivers 357 hp and 407 ft-lbs of torque. Based on the same block and Vortec heads as the 350 HO, the addition of a hydraulic roller