You can look up the hardness of bolts. If I remember correctly Grade 8 bolts can be close to Rockwell 30 hardness. You could probably buy the proper length bolt online and have it delivered to you sometime next week for around $5.
Could you stack some flat washers under the nut or under the bolt head so you could use the longer bolt without modifying it? That would retain the full strength of the bolt.
As others have said, bolt threads are roll formed before heat treating. Cutting longer threads on a bolt does work, but it introduces stress risers that weaken the bolt. In an application where there can be a lot of shock I would try to avoid using a modified bolt, or have a plan to replace it before it breaks.
Same-Day Shipping! Most of our stocked parts ship the same day you order (M-F). Expedited shipping available, just call! Most prices for parts and manuals are below our competitors. Compare our super low shipping rates! We have the parts you need to repair your tractor. We are a Company you can trust and have generous return policies! Shop Online Today or call our friendly sales staff toll free (800) 853-2651. [ More Info ]
TRADEMARK DISCLAIMER: Tradenames and Trademarks referred to within Yesterday's Tractor Co. products and within the Yesterday's Tractor Co. websites are the property of their respective trademark holders. None of these trademark holders are affiliated with Yesterday's Tractor Co., our products, or our website nor are we sponsored by them. John Deere and its logos are the registered trademarks of the John Deere Corporation. Agco, Agco Allis, White, Massey Ferguson and their logos are the registered trademarks of AGCO Corporation. Case, Case-IH, Farmall, International Harvester, New Holland and their logos are registered trademarks of CNH Global N.V.