Shot peening works

September 1, 2016 | By Staff


I am building a 1950s-style hot rod, hopefully with a Ford 59A flathead in it. But I am having some problems finding a good re-buildable engine that is crack-free. Also, once I find my engine I am wondering what I can do to make it tougher and less crack prone. No doubt I will be pushing it beyond its original limits now and then, and I want it to hold up. I have heard some of my buddies talk about shot peening. What is it, and does it work?


Here is a condensed version of what I wrote regarding shot peening in my book “How To Build a Small-Block Chevy For The Street” published by Motorbooks International ISBN 0-7603-1096-3. It is available from Amazon for $17.95.

Shot peening does make an engine as well as chassis components more durable, though most old flatheads of the vintage you are talking about have cracks due to heating and cooling over time, and shot peening won’t help that. However, it will make a sound engine more durable.

In fact, with this one simple, inexpensive process you can increase the resistance of your engine’s heads, connecting rods, bearing saddles, and crankshaft to cracking by between 10 and 45 percent. It works like media blasting except for the point of it is to relieve stresses and increase crack resistance rather than to remove material.

Shot peening is a form of coldworking metal, similar to the way medieval sword makers used to hammer steel blades to make them more resilient. The difference is, instead of hammering, shot peening involves tiny spherical bee bees that are blasted at a component using a special machine. This relieves the item of tensile stress, which is bad and adds residual stress, which is good.

Now you may ask: If shot peening is such a great process why didn’t Ford do it to their rods, heads, and crankshafts in the first place? The answer is it wasn’t necessary for their application. And, in fact, Ford and other manufacturers do use shot peening to increase the resilience of their springs, gears, and suspension parts these days.

But Fords were and is designed and built to a price, just like other production cars. The bottom ends of

shot peening
To prepare for shot peening, grind off all casting lines and sharp edges.

sharp edges and corners
Make sure all sharp edges are rounded. Cracks form where there are sharp edges and corners.

shot peening
Here is a rod prepared for shot peening versus a stock one. The rods will have to be balanced after all the grinding and peening.

classic Fords were more than rugged enough to handle the punishment of normal service, and that was all that was required. Ford’s engineers knew that the only time shot peening would be advantageous is if the engine were going to be pushed beyond its design envelope.

Peening takes time, and time is money. A lot of money, when you build as many engines as Ford does. But for street rodders, it’s a different story. They will be pushing their engines beyond their engineering parameters occasionally, and this relatively simple and inexpensive process can make the difference between a motor that can handle higher rpm and one that might send a rod through the side of the block.

The steel shot is perfectly round. It’s manufactured by dropping tiny blobs of molten steel from as high as 150 feet into water which quenches it and shapes it into perfect little balls. This shape makes them ideal for hammering metal rather than abrading it.

In the peening process, the surface of the part doesn’t get as hard as the immediate subsurface which becomes compressed, creating residual strength. Shot peening also closes the pores and tightens the grain of the metal, and that helps keep cracks from forming. Peening relaxes cast-in stresses that can cause rods and crankshafts to warp. It is especially beneficial when it comes to crankshafts because they are generally cast flat, then wrenched into shape while still hot and malleable, thus creating a great deal of cast-in stress.

When peening is done professionally, it is not blasted from nozzles, but flung by spinning flapper wheels. Special test strips called Almen gauges are even used in some cases to determine exactly how much peening is necessary to achieve specific results. These strips are sent to a lab to be analyzed before any work is done on the component. If we were building engines for NASCAR we might want to go this far, but for street use such exacting work is not necessary.

Have your heads, block, rods, and crankshaft and bearing saddles Magnaflux tested at a machine shop for cracks before you do anything with them. Shot peening won’t help cracked parts. Also have your rods straightened and trued. Your machinist will have a special jig for this. Only when you know you have a good set of straight rods—and before you have the rotating assembly balanced—should you go to the next steps which you can do yourself at home and save money.

Contrary to what most people think, connecting rods don’t usually break at their slimmest point, nor necessarily in the area that takes the most stress. Instead, they often break at points where surfaces such as the rod bolt bosses intersect with the main casting. These areas form sharp angles that invite cracks. One reason such areas are so prone to fracture is that at extreme rpm the rod journals take such a pounding that they stretch downward, pulling the big end cap away from the rod.

Sharp corners should be carefully radiused, parting lines should be ground off, and casting roughness should be ground away. (Don’t remove those little numbers on rods that tell you which cylinder they fit into though.) After that, polish your rods to a satin finish using fine sandpaper or glass beads in a blast cabinet. Your connecting rods don’t need to be shiny, but they should be smooth. Your rods are now ready to go to the machine shop for peening and balancing. Crankshafts should be cleaned of slag and roughness and then shot-peened too, before they are machined. This can help cut down running friction and will strengthen the crankshaft considerably. Once again you will want to radius sharp edges, remove slag and grind away roughness.

Shot peening your crankshaft is probably not necessary on crankshafts that won’t be seeing tough duty, but it certainly won’t hurt. Once your crankshaft is cleaned up you are ready to have it shot-peened. Some shops will do the job in a blast cabinet, but the bigger shops will have a special machine that uses a paddle wheel to throw the shot.

Don’t forget to tell the machinist to a radius around the oil holes in the bearing journals too. This will help avoid damage to the bearing shells and make the crankshaft a little stronger at the same time.

Heads are especially prone to cracking due to the stresses caused by heating and cooling and being clamped to the block. Have your machine shop bake your heads and Magnflux test them for existing cracks before going to the trouble of peening them. You will want to shot peen your heads before you have any machining done on them.

The sharp downward force exerted by combustion creates a great deal of pressure on main bearing saddles, and that can cause them to elongate and fail. Sharp edges and irregularities invite fractures that can widen and lengthen, allowing the bearing journal to elongate and deform until the engine spins a bearing.

Cleaning up sharp edges and shot peening bearing saddles can help avoid this problem. Only remove what is required to smooth the bearing caps and radius the edges. A smooth, shot-peened surface will be much stronger than a rough surface that still harbors tensile stresses.

You may never push your street rod hard enough to stress it to a critical point, but the extra insurance may save you from having to find another 59A block. And when you red-line your engine you can rest a little easier knowing that its rods, heads, and crankshaft are that extra bit stronger.