Editing Vortec L31 cylinder head (section)
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==Valve guide boss== [[File:900 guide boss1.jpg|thumb|450px|left|Stock Vortec boss is 0.845"OD]] For performance use, often the guide boss will be shortened to allow more lift without retainer to seal clearance problems. Along with that, the guide boss OD can be machined down from 0.852" to a smaller common diameter, like 0.785" of the original Gen1 SBC heads, so aftermarket springs (double or with dampers) can be used. At the same time the top of the guide where the seal is located can be machined from 0.555" (stock) to a smaller diameter like 0.530" to use commonly available seals. <br style="clear:both"/> [[File:VORTEC GUIDE BOSS MODIFIED.jpg|thumb|250px|Before (right) and after]] In order to fit one of the commonly available smaller diameter positive type valve stem oil seals, the tops of the guide boss needs to be turned down. To fit springs with a smaller ID like double springs or springs that use a damper, the guide boss needs to be modified by shortening and decreasing the OD of the boss. <br style="clear:both"/> ===Tooling=== [[File:Tool.jpg|thumb|450px|Comp tooling]] Crane, Comp Cams and others sell cutters to machine the guide boss diameter using a hand drill. The arbor and cutters should be used with valve guides that are in good condition because the arbor uses the guide to locate the cutters. ;Comp Cams<nowiki>:</nowiki> *p/n 4718- spring seat cutter, 1.440" OD *p/n 4726- valve guide boss seal cutter, 0.530" OD *p/n 4732- 11/32" arbor for cutting tools ;Competition Products<nowiki>:</nowiki> *p/n 7020 (includes 11/32" arbor)- valve guide boss seal cutter 0.531" OD <br style="clear:both"/> ;From an article by editor Jeff Smith: <blockquote>The Comp Cams cutter increases the spring-seat diameter while simultaneously reducing the outside diameter (OD) of the guide boss. The Crane tool also cuts the valve-guide-seal diameter at the same time. Comp sells a separate tool that cuts a new guide seal and reduces the guide height to create additional retainer-to-seal clearance. Both cutters machine the top of the guide down to a 0.530-inch diameter, which is the standard size for positive-type aftermarket seals. This requires purchasing new valve-guide seals, since the factory uses a 0.555-inch guide to locate the seals. Another way to go is to have a machine shop cut the guide boss diameter to around 0.760 inch and reduce the seal height and use either the Crane H-11 tool steel spring (PN 99846) or the Comp spring (PN 981). </blockquote>
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