Basic Engine Block Prep 101

Prepping a Used Cylinder Block at Speed-O-Motive

By guest contributor Tom Dufur

used big blocks

Here's what a typical junkyard refugee big-block looks like - years of use, followed by more years of neglect. As long as the block is structurally sound, we can breathe new life into it with the right equipment and an operator who knows how to use it.

It's a time-honored tradition in the hot rod community- the day you take your block to the machinist for boring, decking, and line boring the mains. Sounds simple enough, but there is an amazing amount of precision measuring and machine work involved to do the job correctly. The good people at Speed-O-Motive in West Covina, California do this on a daily basis, and after 40+ years in the trade, have got the procedure down pretty well. Follow along as we show you just what it takes to transform that rusty and crusty big-block into a precision piece of high performance hardware, ready to bolt together.

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Speed-O-Motive uses a state-of-the-art block cleaning system that starts with a high temperature (650° F) oven to literally burn off all residual oil and sludge, followed by a specially designed bead blaster that bombards the block with steel shot to remove everything that isn't solid metal. After removing the main caps, cam bearings, head dowels, freeze plugs and pipe plugs, the block is bolted to this special cradle which rotates in the oven first, followed by the bead blast cabinet, exposing all surfaces the high pressure cleaning media. Quite a transformation: already the block looks almost as good as the day it was first cast! You can see that this block, casting #445, is a mid-80's four-bolt main 454, identifiable by the 7.4 designation cast on the sides. Note that not all blocks with this casting number will have four-bolt mains - you gotta look!


The block is checked for cracks with a magnaflux machine, which is a strong electromagnet that temporarily magnetizes the iron block between the two poles. The operator then sprinkles a very fine iron powder over the area to be checked, and any cracks in the block create a disturbance in the magnetic field which attracts the powder.

Next step is to carefully tap the main cap bolt holes with a 1/2"-13 bottoming tap, and the head bolt holes with a 7/16"-14 bottoming tap. The air-powered driver is not only quick, but the torque setting is low enough to stop the tap before it breaks if you encounter an obstruction. Chips are blown out with an air gun frequently.

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On the passenger's (right) side of the block, the second head bolt hole back of the outer four holes aligns with the inside casting boss for the motor mount holes. You can see the silhouette of the boss as a dark shape when we shined a light into the water jacket. Numerous blocks have been ruined when someone used a long plug tap, instead of a bottoming tap, and kept cranking on the tap handle as the nose of the tap hit this boss. That causes the tap to tilt at an angle, ruining the threads. Then the block would have to be heli-coiled®.

Lifter bores get a quick hone with a 7/8" ball hone, lubed by WD-40. After honing, all lifter bores are checked for fit with a spare lifter. This is an important step. Worn lifter bores will cause the lifters to wobble, creating additional wear and reducing oil pressure due to excess leakage from the lifter bores. That means more oil dripping on the crank too! Good engine builders actually check the clearance of each lifter in its bore.

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All oil passages are thoroughly cleaned with a long stiff bristled brush, including the lifter galleries, main oil gallery, and all intersecting oil passages.

Oil holes, main cap bolt holes, and head bolt holes get chamfered to improve alignment with the oil hole in the bearings and to prevent raising the top thread out of tapped holes when tightening bolts.

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In preparation for line boring, it's important that any small metal chips or debris which might get between the block and the clamping surfaces are removed. The block is cleaned in the hot tank, followed by a quick high pressure water rinse, then blown dry with compressed air.

The edges of the main bearing bulkheads are carefully beveled with a file, then the main cap mating surfaces are scrupulously cleaned with a wire wheel.

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The main caps and their bolts are glass beaded, leaving them spotlessly clean. This also compacts the surface of the iron caps, adding a bit of strength as well.

The main caps are ground flat on this precision cap grinder. The grinding wheel is CBN (cubic boron nitride) which is almost as hard as diamond and produces a perfectly flat surface (plus or minus .0001") where the caps bolt to the block.

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Note the shiny spots where the grinder removed uneven material by the two outer bolt holes and the rear main seal. A  rear main bearing inserted in the cap ensures that the cuts are perfectly square with the thrust surface of the bearing. Rear main cap material removal is usually held between .006" and .010" so that the rear seal is not over compressed when the engine is assembled. The other four caps are usually ground between .010" and .012".

The freshly surfaced main caps get beveled with a file to break the very sharp edges, remove burrs and eliminate stress risers that could ultimately lead to a main cap failure.

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Speed-O-Motive's Eddy Cheung centers the main caps over the main bulkheads carefully, feeling the alignment between the two pieces with his left hand while tapping the cap into place with a rubberized T-handle wrench. Try to duplicate this precision when you bolt your big-block together.

After lubing the bolts and torquing the mains to 110 ft./lbs., Eddy attaches this cut-down oil pump housing to the rear main cap to duplicate the stresses of bolting the real pump on the cap. This attention to detail results in main bores that are perfectly round when the engine is assembled.  

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Here Eddy is zeroing the block on the Rottler computer-controlled machine for line boring. The combination of a skillful and experienced operator, plus a dead-nuts accurate machine, produces the kind of first rate work that makes the difference between a top quality big-block and a so-so engine.

You can just see the bright ring of freshly cut metal left by the cutter as it travels through the rear main. Eddy sets up the line boring machine to remove only .002" to .0025" from the block, with most of the metal removal coming from the caps. Because of this small change in the center-to-center dimension between the crank and the camshaft, a standard timing chain or other cam drive assembly can be used, even after two line borings. This is the main reason that line boring is preferred over line honing, which removes equal amounts of metal from the block and the caps, changing the cam-to-crank distance by more.

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After line boring, the mains are cleaned up with a few quick strokes of the ball hone to break up sharp edges and improve the surface finish. No measurable material is removed; this is just a surface treatment.

With the block still clamped to the same fixture, the main caps are removed and the tool head is changed to a vertical milling setup to clearance the block for a stroker crank. Eddy drilled a hole dead center in this bolt, which is then inserted into the inner bolt hole of the front main. This serves as a reference point for the machine's probe and establishes a true "zero" point from which the computer program will run.  

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Here, a roughing cutter is used for quick metal removal from the bottom of the cylinders. Since this is only a clearancing step, surface finish is not a big issue. It's still way quicker and looks better than doing this tedious step by hand with a die grinder. In the second photo, you can see the finished clearance slots, which provide enough clearance for up to a 4-1/2" stroke (you need to check rod/counterweight clearance with your combination, of course).

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Next, the block is transferred to a second Rottler machine which is set up for boring, honing and decking. Massive steel rods are inserted in the mains and cam bores as their alignment establishes the true vertical access of the block, from which the deck and bores are located by the heavy steel fixture visible here. After carefully checking the set up, Eddy determined that the original bores were .010 in. off center. Even though this block was a standard 4.250 in. bore to begin with, the amount of offset did not allow for a .030 in. overbore, which would only remove .015 in. from each cylinder wall; not enough to get into "clean metal." It cleaned up nicely at .060 in. over and the cylinder walls are still thick enough to prevent distortion. The block is getting bored to within .004 in. of the final bore size and the rest will come off during the honing process.

Here the block deck is getting cut with a fly cutting head. In the next photo, you can see just how much the factory deck was off as the first cut took off the high spots. Note that most of the metal removed was toward the outside of the block illustrating that the deck was not at a true 45° angle from the block/cam vertical line. This block required a .015 in. cut to finally clean up the entire deck surface. The final cut is only .001 in. deep at a much slower feed rate and higher RPM to produce the smooth surface finish required for optimum head gasket seal.

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After boring and decking, the bores are chamfered quickly with this king-size abrasive cone. This is absolutely necessary to help guide the piston rings into the bore during final engine assembly.

The final step in the block prep is to hone the bores after installing a torque plate and head gasket to simulate the localized stresses that the head bolts impart. Eddy periodically checks the bore size with a dial bore gauge to make certain everything is progressing smoothly.

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Honing the bores is a two step process. First the bores are honed with 400 grit CBN stones, followed with a plateau brush finish. In this photo, the bore on the left is the unfinished bore and the bore on the right is the final finish left by the plateau brush. Note that the plateau finish is darker and "not as pretty" as the unfinished bore. That's because the 400 grit hone leaves microscopic peaks in the metal which catch the light and have a very bright appearance. The plateau brush hone simply removes these peaks, transforming them into plateaus. Even though the finished bore doesn't look as bright, you can feel the difference between the two with your fingers. It's like the difference between a freshly washed fender and a freshly waxed fender; the plateau brush finish is that smooth. It's the perfect finish for most modern moly piston rings.

Here it is: the finished fully machined block. All that's left is another trip to the hot tank to remove any honing grit, chips and machining oil, and installation of the cam bearings. Of course if you were to take delivery of the block in this condition, you'd want to remove excess casting flash with your die grinder, but that's not the sort of job you'd want to pay a first class machinist's wages to do. Do it yourself. It'll give you a chance to admire the quality work of a first class machine shop.