[Miles in MN]

The spacers that Charlie Yapp sells (Scalded Dog Speed Parts) are 3/16 inch thick machined rings that have an ID that matches the OD of the standard two-piece Model A valve guides. I did not achieve the 60 lbs. of seat pressure recommended by Jim Brierly with stock (new) valve springs and Charlie's spacer. I found that I needed another 3/16 inch spacer to get the installed spring pressure to 60 lbs. I made spacers out of 3/4-inch black iron pipe to get the additional 3/16 inch that I needed. One good thing about the Yapp spacers is that they can be installed between the block and the valve guide; that is above the step in the original or Snyder's original style guide. This drops the guide relative to the ports and (IMO) provides the opportunity for additional flow through the ports.