What is the motor mount bolt size on a 1965 289 engine block?

There isn't a single "motor mount bolt size" for a 1965 289 Ford engine. The size varies depending on the specific motor mount type used (there were different designs over the years, even within the 1965 model year depending on the vehicle it was installed in) and whether you're referring to the bolts that attach the engine block to the motor mounts themselves or the bolts that attach the motor mounts to the frame.

To determine the correct bolt size, you need to:

1. Identify your specific motor mounts: Look at the mounts themselves. Pictures can be helpful for comparison with online resources or parts catalogs.

2. Consult a parts catalog: Use a parts catalog specific to your 1965 vehicle (e.g., a Ford parts catalog for the specific car or truck the 289 was in). This is the most reliable method. The catalog will list the correct hardware for your exact model.

3. Measure the existing bolts (if still installed): If you can safely access the bolts, measure their diameter and length. Note the thread pitch as well. This is a less reliable method but gives you a starting point.

Without knowing the specific vehicle and motor mount type, providing the correct bolt size is impossible.