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Procedures for Checking Straightness on 1992-2003 Ford Aluminum Cylinder Heads

Aluminum cylinder heads and blocks require flatness checks prior to gasket installation to ensure the component part is not out of factory specification.

The surface quality (finish) of the cylinder head gasket surface is machined to close tolerances that enable an effective sealing joint for the MLS (multi-layered steel) gasket. To ensure all head gaskets seal properly, be sure surfaces are clear of any gasket debris, RTV, oil, and coolant using Motorcraft Silicone Gasket Remover (ZC-30) and Motorcraft Metal Surface Prep (ZC-31). Use a lint free rag to mop up the deck surfaces. Shop vacuums are unacceptable for mop up as the cleaner is volatile. The block or head surface must be clean and dry before running a flatness check.

Use a straightedge that is calibrated by the manufacturer to. be flat with 0.0002˝ (0.005 mm) per running foot length. For example, the straightedge may be 24˝ (61 cm) long. That means the machined edge must be fiat within 0.0004˝ (0.010 mm) from end to end.

Special care is requlred to ensure the tool Is not damaged in handling or storage. Be sure that the machined surface is free of nicks and dents. If these properties are found, the cost of tool re-calibration most likely will exceed the cost of a new straightedge. Machine shops may not be able to hold the specified tolerance within the original cost of the straightedge.

When checking for flatness, avoid running the feeler gauge butt end against the straightedge. This action will curl or warpthe leaf and prevent entry under the straightedge. Erroneous readings will result.

Instead, lay the leaf on the suspect area, placing the straightedge on top of the leaf. A very easy pull orlbooseness of the leaf under the edge will indicate a depression in the surface. Turn the edge 90 degrees to the area just checked. Confirm any suspicion by checking the area more than one time.

Figure 1 and 2 demonstrates a 12 point check-system using a grid format on a 4 cylinder bank. Important to note is that a localized area of less than approximately 1.5 to 2.0 sq.in. will influence the effectiveness of the MLS gasket sealing capability. Depressions or marks measured to be over 0.001˝ deep will not seal properly. The overall flatness check (head end to other end) is not as critical.

As shown with Planes 1 through 3 (Figure 1 and 2)check these areas locally with the gauge looking for suspect depression areas of 0.001 inch or more. Then, check these same areas again, perpendicular, using Planes 4-12. The second pass will confirm a suspect area because you are using a shorter span of the straightedge. The first pass (Planes 1-3) is necessary because the overall end to end flatness must be accounted for before doing the secondary passes (Planes 4-12).

Ford Motor Company approved straightedges are commercially available through the Rotunda Tool Catalog at special pricing, or can be purchased from Ihe local tool distributor or truck at regular pricing.This edge meets or exceeds the requirements of testing Ford engine components.

Some or all of this information was provided by the Automotive Parts Remanufacturers Association (APRA). For more information on technical bulletins available through APRA call 703-968-2772 or visit www.AutoBulletins.com.