You can lay two screws together along the threads. If the mesh along the length, the thread pitch is the same. If you lay your 1/4-28 screw, crank end, etc against another the theads will fit . If you lay it along another close, but not exactly the same pitch, the two will not mesh but set tip to tip along the threads. Once you have the thread pitch, the possible nominal sizes are going to be very limited and you can easily eye ball the corect one by compairing your threads to the bin ones. Just be careful that someone hasn't slipped the wrong screw in the bin,

The local hardware store becomes a very effective place to determine pitch. In your case, check the Ace Hardware bins of metric screws and match one to your engine. They will probalby have a nut that fits also if you need one.

Don