Magazine for the auto service professional
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56 | ASP June 2016 Technical forgiving, capable of sealing Ra fnishes even in the 30 to 70 Ra fnish. Tis allows repair shops to install heads with less than ideal surface fnishes. However, regardless of today's gasket technology, even the best Ra fnish won't allow proper sealing if the surface is deeply scratched or gouged. So, while cleanliness and attention to surface fnish remains critical, the average repair shop will be able to restore cylinder head sealing in real-world situations, without the need to meet extremely fne surface fnishes. According to Rotunno, a typical OE fnish today calls for an Ra of 30, but the coatings today can easily handle a range up to 50 Ra and sometimes higher. When installing an MLS head gasket, make sure that all surfaces are clean and dry. Do not apply any type of additional sealant. A high-tack or copper sealant can dry too quickly and result in uneven thickness, and can become trapped between MLS layers. In short, always install an MLS gasket dry. Head bolts Cylinder head bolts (or studs) are absolutely vital in achieving proper cylinder head installation. Even with the highest quality head gaskets and ideal deck surfaces, head gasket failures can occur if clamping loads are not at the correct value. Female threaded holes, bolt threads, thread lubrication and proper clamping loads are all critical factors, and are far too often overlooked during head installations. Clean the threaded holes in the block deck to remove any contaminants and thread deformation. Use a chaser tap to clean female threads. Unlike a common cutting tap, a chaser tap is designed to clean and restore existing threads without removing material. Use a solvent and wire rife brush to clean threads, followed by blowing with compressed air. Chaser taps are readily available through any decent tool supplier. Also, if the block deck has been resurfaced, make sure that the top of all female bolt holes are slightly chamfered to remove a sharp, and possibly raised, edge which could potentially be drawn upwards above the deck mating surface. Te tensile strength and elasticity of head bolts is extremely important, since the bolts, when properly tightened, are designed to stretch and provide clamping force. Te bolts don't just sit there. Tey are exposed to dynamic forces as the head tries to pull away from the block during engine operation. Tis requires the bolts to be somewhat elastic as they are tightened, but if tightened too far, they will stretch beyond the elastic range into a yield, at which point they lose their ability to clamp. Many production engines today feature TTY (torque-to-yield) bolts that are designed for one-time use. TTY bolts should never be re-used. Even if you are unsure if the bolts are TTY, it's always best to replace all cylinder head bolts whenever installing a head. Re- using an unknown-quality bolt is simply not worth the risk. Entire articles can easily be devoted to the subject of cylinder head bolts. Once all bolts are clean, lubricate the bolt threads, the bolt head underside, and each side of the bolt washers with an appropriate lubricant. Be aware that any tightening specs are based on a certain type of lubricant. Com- monly used bolt lubricants include engine oil, a moly-based lube or other high-pressure lubricant such as CMD. Before installing cylinder head bolts, apply the appropriate lube not only to the threads, but to the underside of the bolt head and onto any washers as well. Whether you're using torque or torque-plus-angle tightening, reducing unnec- essary friction at all contact areas will improve tightening accuracy and consistency.