For such a small part, the valve guide is pretty important. In fact, I’d say it’s the foundation for the rest of the cylinder head.
Valve guides support the valves and help keep them running cool. If the guides are worn, you’ll have increased oil consumption and decreased cooling of the valves. Remember 25% of a valve’s cooling depends on heat transfer from the stem to the valve guide.
Worn guides will also allow engine oil to be drawn past the valve stem, resulting in excessive oil consumption. Oil drawn past the valve guides can coke on the port side of the valves causing reduced air flow, exhaust or intake, compromising engine performance. Oil build up (coking) can also accumulate in the combustion chamber and on the piston top. Results here could be severe including a definite drop in performance and possibly mechanical damage as a result of pre-ignition. Pre-ignition being the glowing edges of carbon deposits igniting the incoming fuel mixture before the properly timed ignition spark.
Worn guides can also lead to another engine failure. Since the guides support and center the valves, a worn guide will allow the valve to flex as it closes against a now loosely aligned valve seat. After just so long, this can cause the head to break off the valve stem, damaging piston, cylinder head and cylinder block.
A general rule of thumb is that intake guides need guide-to-stem clearance of .001″ to .003″ and exhaust guides need from .002″ to .004″ clearance. The type of engine will make a difference. Air-cooled, water-cooled or diesel, as well as the material of the valve, the valve guide and application. Diesels don’t require quite as tight a clearance and if you have sodium filled valves you’ll need an additional .001″ to take care of the heat that’s conducted out through the stems. Air cooled engines also use a larger clearance. These figures are only general – always check the specification manual for the make and model you are rebuilding for exact figures.
It’s not enough to say the guides are worn. To do a quality job you need to know how much wear you have. Are they serviceable, within specs, or do they need replacing? Service limits are exact, so break out your precision tools, micrometer and split ball gauge (Goodson #SHG-5) at a minimum. You can use the gauge to measure each guide, or speed things up by setting the gauge to the valve guide maximum diameter and use it as a go, no-go gauge. Simply try the split ball gauge in each guide after setting it to size. If it enters the guide it is out of tolerance. NOTE: upon disassembly, keep valves in order. In my book, this is essential. While measuring guides, we might run into a larger I.D. This could reflect previous work, where a replacement valve with a larger stem might have been installed. Right, wrong, it is a method of repair; as long as the stem to guide clearance is correct. If not, replace the guide or repair by means of a false, Bronze Wall or Bronze-Liner guide repair system. All of these are available through Goodson Tools and Supplies.
Now can you see the importance of an organized tear down? Which valve came from which valve guide? Other valve train components should be organized too. By doing this we can test every thing systematically. Valve springs, stems, keeper grooves, and cam followers to name a few parts. Organizers are available from Goodson under part numbers, SHO-8, DOHC-1, and VTO-80.
It’s just like going to the doctor to see why you are ill. You probably would not like to see the results of your tests thrown around his office, mixed with every one else. Is that really mine or what? Same thing goes at a quality machine shop. Every thing is tested and organized so the customer can be shown exactly why his or her head is in a hurt and what it will take to repair it.
Now that you’ve got the head torn down and the customer’s go-ahead to do the repair, you have some repair options. That’s what we’ll be discussing in the next issue of Talkin’ Tech. And remember, if you have questions about any of what’s covered here (or just about anything else) contact one of the Goodson Techxperts™ at 1-800-533-8010 or drop us an email.
toll-free phone: 1-800-533-8010 • phone: 507-452-1830 • 156 Galewski Drive • Winona, MN 55987 • fax: 507-452-2907 • toll-free fax: 1-888-466-3909