There’s also the problem of loose guides. Some cylinder heads such as those on Chrysler/Mitsubishi 3.0L V6 engines are notorious for loose guides. In such cases, it may be necessary to install oversized replacement guides to fix the head.Simply put, valve guides can have a big impact upon an engine’s performance.How you choose to recondition the guides will vary depending on what kind of engine rebuilding operation you have. Large production engine rebuilders typically have a set procedure for reconditioning all the guides based on which method costs the least and yields the best results for them. They may ream out the old guides and install new valves with oversized stems or rechromed stems, or they may ream out the old guides and install bronze guide liners to restore the original clearances. Performance engine builders often prefer bronze guide liners to improve lubricity and resistance to seizure. Custom engine builders may use either method depending on customer preferences and how much a customer wants to spend.On aluminum heads with replaceable guides as well as cast iron heads with non-integral guides, worn guides can be reconditioned by driving out the old guides and replacing them with new ones, reaming the original guides to oversize to accept valves with oversized or rechromed stems, or reaming out the worn guides and installing guide liners. Most shops that work on aluminum heads usually replace the guides if they are worn. Hard powder metal guides can be difficult to ream so that’s why they often replace rather than recondition the guides. But new solid carbide reamers are available that have a six-flute design and can cut powder metal guides as easily as cast iron guides.On cast iron heads with integral guides, the most popular methods of reconditioning guides is to ream out the guides to accept valves with oversized or rechromed stems, or to ream out the guides and install guide liners. In cases where the original integral guide is too badly worn or damaged to accept an oversize valve or a guide liner, some heads may be salvaged by installing a whole new guide.When used valves are salvaged and the stems are reground, grinding removes the chrome flashing. Chrome prevents the stem from galling when cast iron guides are used, and it helps prevent positive valve seal wear on intake valves. If the chrome is ground off and the valve stem is not replated, it must be used with either a bronze liner or guide.Knurling is another technique that can be used to restore guide clearances, but only if guide wear is limited (.006? or less). Most engine builders today see knurling as a “temporary” fix that only provides limited benefits and won’t last over time. When the knurling tool is run through a guide, it leaves behind a spiral groove that acts like a furrow to raise the metal on either side. This reduces the inside diameter of the guide so a reamer can then be used to resize the guide back to (or close to) its original dimensions. The grooves also help retain oil for improved lubricity, which means you can tighten up clearances a bit compared to stock. But the bearing surface area created by knurling is not as great as a smooth surface, so over time it will wear more quickly.So how do you decide which method is the best one for you? There’s no easy answer to that question because the answer depends on the type of head that’s being reconditioned, your labor costs, how competitive you have to be with your pricing, how much your parts cost, what percentage of valves you can salvage, whether you buy rechromed valves or rechrome used valves yourself, and most importantly what your customers want.The bottom line is that you want to use a guide reconditioning method that is reliable, affordable, profitable and doesn’t cause problems down the road.
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