Are new main, rod and head bolts necessary?

When it comes to performance engines? Absolutely! Just remember this; the ONLY thing holding your engine together, (the rod caps in place, the heads onto the block, and the crank in the main caps, and so on), are the bolts. If you use old, worn, rusted or cheap bolts, then they are sure to stretch or let-go. When bolts stretch, things get loose and start to move, or “walk”. When things get loose or walk, failure is eminent, and that eminent failure can be REALLY expensive!

Look at the image and see what happens when a perfectly good looking bolt loses its temper or hardness. It stretches. Yes, those are the same two bolts from the same connecting rod, but are years apart from each other.

The one on the right was ran hard for a number of seasons. It "looked" just fine to the naked eye before a new freshen-up. I mean, look at it. It looks all nice and clean. It's shiny and the threads look perfect, just like the bolt on the left, except it was fatigued. That bolts would have for sure led to the rod cap not staying in place, which would have led to a catastrophic failure, all over a bolt that costs less than a cafe latte' at your favorite coffee joint. So is it worth it to run old bolts that might "look" okay to the eye? No! 

When it came time to torque it back to spec in the rod, it just wouldn't get to its final torque value because it began to stretch, and stretch, and stretch... until it elongated and got super thin in the middle. Again, your eyes can't see metal fatigue, loss of temper, or small amounts of stretch that weaken it. Luckily this one gave indication by not letting me get to the final torque value before I knew something was way off. 

I can’t begin to tell you how many home rebuilt or “bargain priced” engines I've seen in my day that had old, re-used bolts in them or "bargain priced" inferior quality aftermarket bolts.

A bolt might “look” good, but what do you have... X-ray vision or something? You can't see fatigue or minor stretching by simply looking at a bolt. The only way to see if a bolt is strong or not is to test it in a Rockwell tester, and the only way to test one in a Rockwell tester is to stress it until it breaks, and then say; “Gee, that bolt was strong”, or “Gee, that bolt sure was weak”. So testing a bolt isn’t exactly an option, therefore - new, strong, high quality bolts are an absolute must in any performance engine if you want it to survive and last. And as you can see, even the best quality bolts fatigue or over-stretch sooner or later under use and abuse, so it's always a good idea to change-out all of the old bolts and put some new, stronger ones in. This goes for rod bolts, main bolts, and head bolts. It's what I call "cheap life insurance". 

Almost all high-end aftermarket connecting rod manufacturers use ARP bolts their rods. Most of them have their brand name stamped into them along with ARP's name as well. The reason I always chose to use ARP bolts in all of the engines I built, and why almost all high-end connecting rod manufacturers choose ARP too, is because of their quality and strength. Again, the only thing holding your motor together are the bolts, so they had better be strong and the best quality possible. They definitely have my stamp of approval! 

Never forget that the only thing keeping your main bearings in place and your main caps from walking is how tight your main caps are swedged in the ways of the block, AND the bolts holding it all in place. If the ways machined in your block are loose and your main caps can easily slip into place with a little bit of looseness, you have a good chance of having main cap walk which will cause major failure to the bearings. Good / strong bolts help hold everything in place.

On connecting rods; (aside from making sure the rods are machined to size correctly...) the only thing keeping the caps-on and from having failure, are the bolts. If the bolts stretch, they will allow the cap to loosen a little bit and the bearing will not maintain its crush to keep it in place. What you'll end up with is a spun rod bearing. Contrary to what a lot of people think though; rod and bearing failures are not usually caused by the down force of the piston, because that puts the load on the rod beam, not the cap and bolts. Rod beams rarely fail on their own. What kills rods is when they hit TDC and go from an upward direction, then stop, and then get yanked on by he crank to suddenly go in a downward direction between the exhaust and intake stroke. The weight of the piston and rod itself are now being yanked down and all of the load is on that rod cap. If those bolts aren't strong, the kinetic energy from the rod being stopped on its way up the exhaust stroke, then a sudden stop at TDC and then back down again is all loaded on the cap and bolts.

When a rod get's "pitched", it's rarely because the rod beam itself failed. 99% of the time it was the cap that got loose, the bearing spun, and then the cap comes-off. The piston and long-end of the rod stops, but 7 other cylinders are still spinning that crank, so the rod journal comes back around and hits the lower end of that now hanging main part of the rod and tosses it right out the side of the block. The main culprit was the cap coming loose and then coming off, not the long-end or beam side of the rod. It, along with the piston, were just along for the ride once that crank came around again and smacked it, and out he side of the block they go! The reason rod caps usually come loose, or come off, is because the bolts stretch and can no longer hold the cap in place under extreme stresses. This is why when you're building a performance engine, you always run good, strong rod bolts. 

Deceleration also puts stress and load on rod caps and bolts, so going from an engine under a load at high RPM, to sudden deceleration, such as slowing down using the engine itself as a brake, also puts tremendous loads on the caps and bolts. 

Head bolts are pretty obvious in that the only thing keeping the heads clamped down and the head gaskets sandwiched in place are the bolts. Yes, deck surface texture, flatness, etc, as well as the surface textures and flatness of the heads are also important to maintain a good seal, but all in all, at the end of the day, the only thing keeping the head gaskets sandwiched good and solid between the block deck surfaces and the heads - are the bolts. If they aren't torqued to spec right, or are weak and are stretching and aren't maintaining their clamping force any longer because they are too old or have been heat cycled or re-torqued too many times, (even if they look good to your eyes), you will have problems blowing head gaskets. Again, it's cheap "life insurance" to just install new ones and call it a day. 

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