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Chevy 409 Mark I Big Block Engine Build - Mighty Fine 409

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With its distinctive scalloped valve-covers, Chevy's Mark I 348/409ci big-block blazed a legendary trail across America's race tracks during the early '60s. Today, it retains a faithful following among resto guys and nostalgia racers. Although finding parts for an engine that ended production in 1965 might seem like a chore, in reality the 348/409 is a transitional engine that shares design characteristics with small-block and newer big-block engines, many of whose parts bolt into the 348/409 with few or no mods.

Major (and unique) components like heads and blocks can still be found through enthusiast publications, swap meets, and truck salvage yards. There's also the one-stop 348/409 niche specialist CH Enterprises, which offers all internal components plus rebuildable cores. CH also maintains complete 409-based engine cylinder-head porting, engine machining, and engine assembly services. Owner Curt Harvey has updated the venerable engine using today's technology, developing modern cam profiles and big-inch stroker kits. With high-perf 409 heads, a relatively mild mechanical cam, and 10.0:1 forged pistons, a 474ci street stroker makes about 500 hp and 500 lb-ft of torque. Sound intriguing? Let's take a closer look at this classic motor and what it takes to build it into a 474ci stroker ready for the 21st century.

Blocks You Can't Knock-Off
W-Series or Mark I big-blocks were built in three different displacements:

Ci Year Bore Stroke Con Rod Length
348 '58-'61 4.125 3.25 6.125
409 '61-'65 4.3125 3.50 6.010
427 (Z-11, rare!) '63 4.3125 3.65 6.135

These days, Harvey sees more 409s than 348s. Either core costs about $400 cleaned and magged, so it pays to find and build the larger 409. As Harvey puts it, "Why build a 283 when you can move up to a 350?"

Although some 348 blocks have thick cylinder walls, most cannot be bored out to the larger 409 diameter. ID a 409 block by the last three digits of the block casting number that's located on the rear driver-side bellhousing flange:

Year 409 Block Casting Nos.
'61 723
'62 068
'63 814 (also 427 Z-11)
'64 422
'65 656

Cylinder-wall thickness varies widely because casting techniques back then weren't up to current standards. Usually 0.030-over is no problem, but 0.060 requires sonic-checking the block (minimum acceptable wall thickness is 0.110-inch). Occasionally, Harvey gets a block that can safely go 0.132-over-but don't hold your breath!

Retain the two-bolt main-caps using GM Mark IV big-block main cap bolts plus hardened washers or equivalent ARP bolts and studs. The oiling system is typically Chevy-reliable. All you need is a Melling M55HV small-block high-volume oil pump. Big-block pumps also fit, but they'll suck the stock pan dry. A late big-block heavy-duty oil pump driveshaft (PN 3865886) works.

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