On the later (more modern) smallblocks these spacers lodge in a recess cast into the engine block. From memory about 3/4 inch long, and the seem to act as a guide & centering device for the engine through bolt. I may be able to get up to the Shed today - if so, I shall check the bits I have around - a disassembled Nevada & a Breva motor are on the bench - and get the dimensions. From memory, a coated steel - I did find one on the floor after reinstalling an engine no too long ago. Did wonder for a little while where it came from until it twigged - they don't usually fall out easily.