Jim Smart
September 15, 2007

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Should You Do A Mock-Up?
We learned the mock-up drill from McAfee. A mock-up is designed to make sure everything fits together properly while there's still time to make corrections. Long after machine work and during final assembly isn't the time to discover things don't fit. Do it early in the going before all machine work is completed. A mock-up:

  • Ensures piston skirts clear crank counterweights
  • Verifies piston/connecting rod to camshaft clearance
  • Ascertains rods clear cylinder skirts
  • Makes sure crankshaft counterweights won't hit the oil pump
  • Confirms piston-deck and compression height
  • Determines crankshaft endplay and bearing clearances
  • Confirms piston-to-valve clearances
  • Determines valve-to-valve clearances
  • Confirms valvetrain clearances (pushrod to cylinder head, rocker arm to valvespring retainer, spring coil bind, and more)

Dry Wit
When McAfee starts his day, he checks his inventory for everything, including a good sense of humor. He brings the latter to work with him because things don't always go well in the engine-building business. There are parts that don't fit, parts that don't show up, and times when there are miscommunications with machinists and suppliers. That's when McAfee looks to a can of Dow Corning 321 dry lubricant to reduce daily friction.

All kidding aside, McAfee coats all moving parts (except piston-ring package) with Dow Corning 321 dry lubricant. Dry lubricant is nothing more than a spray-on graphite lubricant designed to protect moving parts during initial startup. Bearings, lifters, rocker arms, camshafts, and other moving parts get a coating of this stuff for one reason-to eliminate any chance of metal-to-metal contact during startup before oil has a chance to flow under pressure. You can find 321 at almost any professional auto parts store or at Dow Corning's Web site (www.dowcorning.com).

Smooth Fit
Whenever you build an engine, it's important to understand proper fit.The first rule of fitment: If you have to force it, something's wrong. Never beat something to death with a hammer to make it fit. If a cam gear won't slide on the crank or cam, the fit is incorrect. If a distributor gear wobbles on the shaft, it's too loose and unacceptable for use. If any part exhibits sloppy fit, it's not suitable.

McAfee University teaches us that fit is pretty fundamental. Cam sprockets should slide on and off cranks and cams without wobble. If you have to beat them, the fit is too tight. Dress the crank and clean up the snout; then check fit. Ditto for a stubborn camshaft drive gear. There's one exception to this rule: The crank damper must be an interference fit (pinch fit: 0.0005-0.0008 inch). Other items, such as cylinder-head dowel pins, rod caps, main caps, timing cover, and the like should also exhibit smooth fit. When-ever you force something, you're creating unnecessary stress and the potential for failure.

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