Modern Engine Blueprinting Techniques. Mike Mavrigian
rel="nofollow" href="#fb3_img_img_52477834-d9ae-5420-84b3-5a1a2c2c49f5.jpg" alt="Aftermarket performance blocks commonly provide a bit of additional rod clearance (more than an OEM block), since the block manufacturer is aware of the possibility that a longer stroke crank may be used. This Dart 351W block already has a convenient notch for big-end rod clearance. Depending on your specific crank and rods, some additional clearancing may still be needed, but it is minimal."/>
Aftermarket performance blocks commonly provide a bit of additional rod clearance (more than an OEM block), since the block manufacturer is aware of the possibility that a longer stroke crank may be used. This Dart 351W block already has a convenient notch for big-end rod clearance. Depending on your specific crank and rods, some additional clearancing may still be needed, but it is minimal.
If using a stroker crank and/or a high-lift camshaft, don’t forget to check for possible rod-to-cam lobe interference. This photo (taken without cam bearings or cam) helps to illustrate the potential for clearance issues. With cam bearings, camshaft, crank and rods, and timing gear test installed, carefully rotate the crank while observing cam lobe clearance. It may be difficult to see, but possible with a skinny inspection light. If clearance is a problem, you may be able to remove material from the rod big ends or move to a cam with lower lobe height. This is why aftermarket blocks are often available featuring a raised cam tunnel.
Once you’re sure that the crank clears the block properly, install one rod/piston set, starting with the number-1 cylinder. There is no need to install rings at this time.
Slowly rotate the crank, check for the rod big-end clearance to the block during a full-stroke travel. Rotate the crank a full 360 degrees. Again, minimum clearance of the rod to the block at any given point should be about .060 inch. Typically, if you do find a rod clearance issue, it is likely in the rod bolt/rod cap shoulder locations. Don’t be tempted to grind material from the rods; remove material (if needed) from the block.
Also, check the clearance between the crank counterweights and the bottom of the piston skirts. If clearancing is needed, the crank counterweights can be lathe-machined (prior to crank balancing). Mock-install the camshaft and timing assembly and check the rod big end clearance to the camshaft, especially if a high-lift cam is used (this is the reason that aftermarket performance/race block manufacturers often offer a “raised cam” block, which places the cam bore higher, farther away from the crank centerline). The cam bearings must be in place in order to properly center the cam in its bore.
After carefully removing block material with a handheld electric grinder (and after washing the block and reinstalling the crank and rod), clearance was again checked. It is a more-than-healthy clearance of approximately .080 inch. When checking rod clearance, you must be careful to rotate the crank slowly as you observe the rod approaching the block. In some cases (depending on the block), only a couple of spots may require clearancing. In some cases you can get lucky, with no additional clearancing needed, or you may need to provide clearance for the majority of rod-to-block areas. Just take your time and check each and every location.
Never assume anything. Just because the block’s main bearing saddles have oil feed holes, it doesn’t necessarily mean that these holes properly align with the oil holes in the upper main bearings. Test fit the new upper main bearings to the block and check for oil hole alignment. If necessary, the oil holes in the upper bearings may be enlarged to achieve correct alignment. It’s easier and quicker to do this than enlarging the saddle holes. Hole enlargement must be performed carefully to avoid damage to the bearing, and make sure that the new hole is carefully deburred to remove any high spots on either side of the bearing shell.
Whether you’re modifying the upper bearings or not, once you obtain correct oil hole alignment with each upper bearing, mark each bearing for block location (number-1, -2, etc.) so that each bearing is installed in the same location during assembly. Mark each bearing backside with a felt-tip marker, or place each bearing in a clearly labeled plastic bag. In addition to checking for oil hole alignment, run a small-diameter rifle brush through each main saddle oil passage to make sure that there are no obstructions.
Check each threaded hole in the block for thread cleanliness and integrity, especially on a used block. A hole with contaminated or burred threads prevents you from achieving proper torque values and clamping loads during assembly. Prior to, during, and after block cleaning/washing, run the appropriate-size rifle brush through each threaded hole, along with a hot soapy water solution, followed by a rinse and a blow-out with clean compressed air.
When checking or correcting slightly burred threads, do not use traditional cutting taps; they are designed for removing metal. Instead, and especially on the most critical threaded holes, such as cylinder head holes in the block decks and main cap bolt holes, use a dedicated “chaser” tap. Chaser taps are designed to re-form threads instead of cutting threads. These taps are available through quality automotive machine shop supply companies such as Goodson Tools. Using a standard cutting tap, depending on the condition of the existing threads, can remove enough material to weaken the threads.
Threaded Inserts
If any less-critical threaded holes are stripped, cross-threaded, etc., use quality thread inserts to repair the hole. In basic terms, there are two common types of thread inserts: helically wound stainless steel (Heli Coil and other brands of the same style) and solid steel or stainless steel. In either case, first drill the damaged hole oversize following the spec provided with the intended repair insert. Then tap the hole, using the specific cutting tap supplied with the thread repair kit. Screw the thread repair insert into the newly tapped hole.
Install a helically wound insert using a specialty driver tool supplied in the kit. When fully installed with the top threads located immediately below the top surface, remove the driver tang at the bottom of the insert. The insert has a driver tang that’s notched, which provides a stress point. A quick tap with a skinny, flat punch knocks the tang loose, which must then be removed from the hole. Install a solid-wall insert that has internal and external threads.
Solid inserts, depending on style, may or may not require application of a thread-locking compound to prevent it from backing out. Other styles may require staking at the top using a small, sharp punch.
There’s nothing wrong with saving damaged threaded holes with inserts. Sensible applications include mountings for water pumps, oil pan, bellhousing, timing covers, etc. Thread-repair inserts also offer good methods of saving otherwise damaged holes for areas such as valve cover bolt holes, exhaust manifold/header flange bolt holes, intake manifold bolt holes, and spark plug holes.
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