I would hate to guess since half of them are metric now. I would eyeball some of the other bolts on the engine (like a alt bracket bolt or something), and see if it will fit thread wise. It should have a good fit in the threads, with no slop. If you do find one that fits, measure how thick the starter flange is, and get a bolt twice that thickness(approximately). Take the bolt that fits down to the hardware store, and try their selection of nuts till one fits. Then you will know the thread pitch and if it's metric, and then you will know what bolt to buy.