| From time to time, this issue arises: how can I tell what pistons are installed in the engine I just bought/found/inherited? Here’s how to tell the difference between early stock 9:1 pistons (found in US Alfa Romeo Spiders from approximately 1972-1989), 10:1 “Motronic” pistons (in US cars from 1990-1994) and high-compression, 10.4:1 pistons. - First, the Motronic pistons (center photo) are easiest to identify, since they have a raised, flat crown, with the valve cut-outs straight across the crown (and parallel to one another). The valve cut-outs are approximately 1-7/16” apart.
- Stock 9:1 pistons have a subtle dome shape, with a wider distance between the valve cut-outs (they’re about 1-11/16” apart at their closest point). They may have the code “6675” on the crown.
- 10.4:1 pistons have a more aggressive dome, with the valve cut-outs only about 1-3/16” apart at their closest point. They may have the code “7222” on the crown.
Please note that the markings on AE/Borgo/Federal Mogul pistons may vary.
- Mark Lee |
