I bought my '54 GMC just over a year ago and the 261 Chevy engine came with the truck and I do not know what had been done to the engine in the past. I do know that the main bolts were not not all from the same manufacturer. All of them had grade-5 markings though. As it turns out the two that stripped and the one that stretched all had the capital letters TR in the center. The four longer intermediate front and rear cap bolts, which I have not had any problems with yet, all have a smaller font capital F in the center. I also have two other bolts on the front main cap, which also have not been a problem and they are marked with a capital M in the center. So those people who thought I wasn't playing with a complete/matched set, were correct. I guess I bought a Frankenstein engine; 261 block, 235 head, and they got the bolts to put it together from a guy named Abe Normal. Does anyone know what markings are correct for main bolts in a '58-61 261 Chevy engine? My head bolts all have the same markings, a capital A inside of large circle and no Grade markings, are these original to the 261?
