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BASIC ENGINE BLUEPRINTING
The following are the original recommendations released by Chevrolet engineering in 1969 for the preparation and blueprinting the 302 cu. in. Z/28 engine for “heavy-duty” use (“heavy-duty” meaning “competition use” in GM-speak).

These suggestions reflect the current engine building wisdom of that time. Included are recommendations using premium Chevrolet engine components and upgrades with specifications and assembly tips derived from Engineering’s extensive research, tests and actual race experience. These Chevrolet specifications are especially pertinent for us since they are specific to the 302 and list parts and combinations that were developed specifically for that engine.

While over the years engine building techniques have advanced from a technological point of view, these specific recommendations are still relevant and will produce a high-powered engine with maximum durability. Try a 458hp 302 small block engine that will run 7 grand all day...!

CLEARANCES AND SPECIFICATIONS
To build a 302 cu. in. engine for heavy-duty service, it is easiest to start with a Z-28 Engine Assembly No. 3932413. This engine is of high-performance design featuring: four-bolt main bearing caps, forged high-compression pistons for 4-inch bore with full floating wrist pins, 3-inch stroke specially heat-treated crankshaft with 8-inch harmonic balancer. (In 1968, main and connecting rod journal sizes were increased on all small-block Chevrolet engines.) Selected high-quality connecting rods, large port cylinder heads with 2.02” diameter inlet valves and 1.6” diameter exhaust valves, aluminum tuned-runner design inlet manifold, 800 CFM Holley 4-barrel carburetor, special oil pan baffling, deep groove belt pulleys, and a .455” lift mechanical lifter camshaft with special pushrods and rocker arms. Partial short block engine number for the 302 is No. 3961490 for 1969 (later changed to No. 3970657). Clearances for the 302 should be as follows:
Piston to Bore:
.0055”-.0065” measured at centerline of wrist pin hole, perpendicular to pin. Finish bores with No. 500 grit stones or equivalent (smooth).

Wrist Pin:
.0004”-.0008” in piston, .0003-.0005 in rod (for floating pin)

Rod Bearing:
.002”-.0025”, side clearance .010-.020

Main Bearing:
.002”-.003”, minimum preferred,
.005”-.007” end play.

Piston to Top of Block: (Deck Height)
.010”-.015” average below deck. No part of piston except dome to be higher than deck of block. Deck height specified is for a .025” steel head gasket. If a thicker head gasket is used, a piston to cylinder head clearance of .035” should be considered minimum.

Valve Lash:
.030” intake, .030” exhaust for production cam No. 3849346; .022” intake, .024” exhaust for optional service camshaft No. 3927140 and No. 3965745.

Valve to Piston:
.010” intake, .020” exhaust checked at zero lash during valve overlap cycle. These are absolute minimum clearances to allow for heat expansion only and will not accommodate valve float from overreving the engine.

This close-up shows the “SO” alpha-numerical stamped on the oil pan rail, denoting this as a Service Order block -- the perfect starting point.

This hefty boy is the large journal 1969 302 crank. The 1178 casting number gives its identity away. Rod and main journal sizes were increased for 1969 to enhance durability.

PREPARATION OF COMPONENTS
Cylinder Block:
Inspect, clean, de-burr the block. Main bearing caps should fit tightly into case notches to prevent cap misalignment. Additional main bearing bulkhead durability may be achieved by using studs instead of bolts in the 3 intermediate main bearing caps. Studs should fit snugly the full length of the block threads and should be installed with Loc-Tite for better retention. 1969 high-performance blocks are equipped with 4-bolt main bearing caps. Torque inner bolts to 70 ft.-lb., outer bolts to 65 ft.-lb.

Crankshaft:
Remove any burrs from oil holes and passages and polish journals with No. 400 sandpaper. The production 302 crankshaft is “Tuftride” heat-treated to improve journal hardness and give greater fatigue strength for high-performance durability. This feature is an improvement to any high-performance forged crankshaft and should be included in any engine build.