bolt? The original size and strength is what is appropriate. I, on the other hand, have always changed them out to a 316 SS bolt and 304 nut (currently 3/8) to eliminate corrosion and rust. SS is not nearly as strong but then I don't slam my transmissions around and in 30 years have never broken a bolt. I use the standard size SS lock washer and have never used a lock tight product. Again, 30 years and never had a nut and bolt set up come loose. I elbow touque the nuts and call it good. I never match a nut and bolt as it leads to galling when tightened. 316 is more noble the 304 so the mixed combination is less likely to to give you trouble when you need to unbolt. Hay, it's worked for me for so long that I see no reason to change. It's just MHO.