Correct, the throw-out is actually bolted to the clutch adapter. I used a spacer that a local machinist made for me (actually, he made 2), they are 0.4" worth of spacing. They slip over the slave mount just as the slave would, and they are stackable. Then the slave is stacked on the spacer and bolted in place. If small amounts of shim is still needed after installing the spacer, flat stock can be placed under the slave and spacer. I had my spacers made to fit snugly into the slave base and each other just for this possibility.
