Do I need special adapters for different spark plug or glow plug sizes?

Adapters are included in the kit, but confirm your engine's plug thread size matches the universal adapter set before testing. If your plug size isn't covered, you may need a separate adapter.

What pressure readings tell me the engine is healthy?

Compression specs vary by engine design and displacement. Check your vehicle's service manual for the spec range. This meter gives you the number — you compare it to factory specs for your specific engine.

Can I use this on both gas and diesel engines?

Yes, the universal design works across both fuel types. Diesel engines run higher compression naturally, so reference the correct spec sheet when you test.