What We Learned After our Small-Block Exploded
By Jeff Smith – Photography by the Author
In the real world, things sometimes just go terribly wrong. Like when the engine that you so carefully assembled rewards you by pushing shrapnel pieces out through the oil pan. If there is a silver lining to the dark cloud of oil that just shot out the exhaust, it would be that we learn more from our mistakes and miscues than from success. This episode was not our first exploded engine learning moment and most assuredly won’t be our last.
We had built what was nothing more than a very basic, cast piston, 355ci small-block Chevy that was intended for my ’93 GMC pickup truck to replace its ailing 290,000-mile 305. Originally, we intended to buy a standard GM crate engine. But after discovering that GM no longer offers that two-piece rear main seal crate engine, we decided to build our own.
Our friend Bill Irwin offered a used, one-piece rear main seal 350ci engine and we immediately had a machine shop clean it and perform the standard rebuild procedure that included 0.030-over cast pistons and rebuilding the stock rods with new ARP bolts.
The standard connecting rod rebuild procedure is to first replace the bolts, torque them in place, and then resize the big end for the proper bearing clearance. All that went well, and the machine shop then rebalanced the entire rotating assembly and soon it was ready for reassembly. We’ll just touch on the relevant parts of this story rather than recount the entire procedure.
Once the machine shop resized the rods and pressed the wristpin in place through the pistons, it was ready for reassembly. We preassembled the bearings and torqued the bolts in order to check bearing clearances. Altogether, all eight sets of ARP connecting rod bolts were torqued into place at least three times. The first was when they were resized, the second when we checked bearing clearance, and the third during the final assembly.
Many years ago we learned never to trust torquing rod bolts because we had an engine fail after a rod nut loosened up, let go, and the engine failed catastrophically. In that situation, we discovered our torque wrench was not calibrated and was under-torquing the fasteners by almost 20 percent. This allowed the rod bolt nut to loosen up and the rod cap broke.
From then on, we always used an ARP rod bolt stretch gauge to ensure the rod bolt was properly torqued by measuring its stretch. Each ARP rod bolt is intended to be stretched to a certain figure based on the bolt material, its cross section, and its length. These factors combine to establish the bolt’s proper stretch figure.
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The spec for our particular ARP small-block 3/8-inch-diameter rod bolt was a stretch figure of between 0.0055 and 0.0060 inch. Our procedure was to slide the piston and rod over the crank, install the cap, and then zero the free length of the bolt with the stretch gauge. With that zero baseline, we then tightened the bolt to a certain torque number, measured the stretch of the bolt, and then further tightened the assembly until we had the proper stretch spec. After each pair of rod bolts were stretched, we used a Sharpie to mark an X on the end of the rod. We accomplished this for all eight connecting rods and then assembled the rest of the engine, paying attention to all the details.
Summit Racing had supplied most of the internal parts to complete the build and we wrote a story on the buildup for Summit Racing’s website called “On All Cylinders.” As a final evaluation, we wanted to see how much power this engine would make, so Summit invited us to test the engine on their dyno located at Trick Flow Specialties (TFS) in Tallmadge, Ohio.
Before we left for the test, we placed our small-block on our test stand to make sure it was happy with no problems. We did make one change from the original build on the engine by swapping the flat-tappet camshaft for a mild, Summit Racing hydraulic roller cam. We then delivered the engine to TFS and bolted it on the dyno.
We began the test using Summit’s Break-In SAE30 oil, and from the startup we had an oil leak. It turned out that when we changed the cam, the area between the front timing cover and the pan didn’t seal properly, which required dropping the pan down and re-sealing the connection with a bead of Permatex Ultra Gray.
With the leak resolved, we measured power with three average pulls to come up with 420 lb-ft of torque at 4,100 and 378 hp at 5,400 rpm. These weren’t bad numbers but from previous testing we knew that if we drained the Break-In oil and put in good 5W-30 hot rod oil, we could pick up perhaps an additional 9 to 10 hp.
That’s when we experienced what SpaceX recently described (when their rocket exploded) as a rapid unplanned disassembly. And that’s what our engine did at around 5,000 rpm. It went “boom” and spit a chunk of connecting rod right out of the oil pan onto the dyno floor along with about a 1/2-quart of oil.
It’s funny how everyone gets real quiet after something like this happens. After some embarrassment on this author’s part as the engine builder, we decided to do a quick forensic investigation to see if we could figure out what caused our engine to part ways so spectacularly. This engine should have lasted 75,000 to 100,000 miles, instead only surviving for about 75 minutes!
We removed the engine from the dyno and pulled the pan to discover a fully intact ARP rod bolt and nut lying in the bottom of the pan surrounded by a pile of broken piston and connecting rod shrapnel. The bottom of the oil pan looked like a gravel pit and the largest piece of the piston was the wristpin. Even the connecting rod was in multiple pieces.
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It was clear that the rod bolt nut had backed off and allowed the rod cap to fly apart that created the ensuing chaos. It sure looked like the engine builder had screwed up and not properly torqued all the rod bolts. Short of swearing on a stack of Bibles, I can only say that I know without a doubt that all 16 rod bolts had been properly stretched.
Nowhere during this process did we consider that it was the fault of the ARP fastener. It was clear that insufficient bolt torque was the cause. We then checked the remaining rods for torque. We did this by zeroing the rod bolt stretch gauge and then loosening the rod bolt nut. Only one other bolt did not come back with a spec between 0.0055 and 0.0060 inch. That one checked at barely 0.0035 inch or 0.0025 inch below spec.
After some further soul searching, I remembered a situation many years before when I was torquing the rod bolts on an engine and during the process the bolt seemed to become “soft” where it felt like it was pulling apart. But then just as quickly, it tightened back up again. When I questioned my machine shop, they said they’d experienced this before and that the rod bolt had not fully seated until I put more torque to it and then the bolt head fully seated against the connecting rod.
We contacted ARP and spoke with Chris Raschke and explained what happened. He asked us to remove a couple of the surviving bolts from the rods. This required us to hit the bolts very hard with a hammer to remove them from the connecting rods. Raschke told us that he has seen situations where the upper portion of the ARP bolt will be slightly oversized to fit inside the stock rod. He said that this could cause the bolt to bind inside the rod and not allow the bolt to completely seat to the connecting rod.
This would allow 0.0010 to perhaps 0.0020 inch of a gap between the underside of the bolt head and the flat portion of the connecting rod where the bolt should meet the rod. In that earlier situation where the bolt did not fully seat, applying sufficient torque had seated the bolt, which is what we felt when the torque wrench seemed to “let go” for a moment and then again get tight. This movement was the bolt head seating against the rod.
We then called a couple of machine shops to find out how they mount new bolts in the rods. Both shops told us they use a large hydraulic press to install the new bolts using a fixture to ensure they are installed straight and apply at least 2,000 pounds of force to ensure the bolts are fully seated.
We next spoke with the machine shop that rebuilt the rods in the failed engine and the technician said that they had merely used a hammer to pound the bolts into place. Some basic force calculations will easily foretell that a 20-ounce hammer will not come close to the force of a hydraulic press at 2,000 pounds or more.
These loads are necessary to ensure the bolt is fully seated. This is important because the force inside the engine that places the greatest load on the rod bolts occurs when the piston reaches top dead center (TDC) on the exhaust stroke and then pulls the piston downward. This movement attempts to pull the rod cap off the connecting rod, placing enormous stretch on the bolts that engineers call tensile load that attempts to pull the bolt apart.
In our case, our forensics pointed us to the conclusion that after several 6,000 rpm runs on the dyno, the higher forces across TDC placed more than enough load on the rod cap to then seat the rod bolt that had not properly seated. If this bolt head was, for example, 0.002-inch away from fully seated in the rod, then when this occurred the bolt stretch dropped from 0.0055 inch to something closer to 0.0035 inch. This is what we saw with one of the remaining rod bolt stretch values.
Again, this was not the fault of the ARP rod bolts but rather due to the rod bolt not fully seating in the rod. The lesson here is that the bolts must be installed using a hydraulic press to ensure the bolts fully seat. ARP recommends a minimum of 1,600 pounds of force on the press to ensure this occurs. In our case, this didn’t happen.
Lessons like this can be painful and expensive, which is why we decided to write this story to offer an opportunity to learn from others’ mistakes. If you’re not positive the rod bolts are fully seated, take them to a shop with a hydraulic press and make sure. It’s inexpensive insurance so that your engine will live a long and powerful life. We overlooked that and the penalty was expensive.
Automotive Racing Products (ARP)
Trick Flow Specialties