What is the cylinder numbering on 1990 F150 351 Windsor engine?

The 1990 Ford F-150 351 Windsor engine uses a standard firing order of 1-5-4-2-6-3-7-8. However, the cylinder numbering itself is sequential, starting at the front of the engine on the passenger side (right side in North America) and working towards the rear, then repeating on the driver's side (left).

So, the cylinder numbering would look like this:

* Passenger Side (Right): 1, 3, 5, 7

* Driver's Side (Left): 2, 4, 6, 8

Keep in mind that this is a general description. Always double-check your specific engine by consulting a Ford repair manual or a reliable online resource with diagrams for your exact year and model. There's always a slight chance of variations, though highly unlikely for a standard production engine.