Engines are manufactured on Earth by humans, meaning that there are variances between them-new or used. Also, milled or surfaced heads or blocks even brand new, various intake manifolds, align boring and many other factors can cause your distributor from not sitting down flush to the intake manifold. These nylon shims take the place of double-gaskets which may compress with usage. If your distributor doesn't rest flush with the manifold without a gasket, these shims may be needed to prevent the distributor when clamped down from placing undo pressure on the oil pump.