By Chuck Lynch
I have the fortune of visiting a lot of shops from small to large. I visit shops that are Performance leaning and shops that are industrial or agricultural leaning as well as traditional automotive leaning. In my observation, I see that the valve guide is not given due respect or appreciation of its function and purpose. I feel this has to do with the fact there has always been a lack of emphasis on the importance of the valve guide from machine tool sales to technical training manuals.
Definitions of the word Guide:
- Noun – a structure or marking which directs the motion or positioning of something.
- Verb – direct or have an influence on the course of action of (someone or something).
These definitions may seem overly simple because we use the word guide for many reasons. You have a guide on a vacation tour, a guidebook, a reference guide, etc.
What we do not do well is think of the daisy chain effect that valve guides have on valve seat machining and valve control. Folks will put great emphasis on the correct seat width, pretty transitions, blends, radii and so on, but the guide sizes will vary widely. Whether they have taper, out of round, are not typically qualified because in my observations, many shops do not have the gauges to measure valve guides. Many are still trusting that a reamer will make a hole based on the size that is listed on the reamer or in the catalog, or they are using a feel method by putting a valve in a guide bore and rattle checking it.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ1/#p=36
If the guides are not uniform in size and cylindricity, no different than a cylinder bore, then they are not going to properly “guide” the pilot to cut or grind the seat. They are not going to properly control the valve trajectory off of the seat during the opening sequence or locate the valve to the seat in the closing sequence. Again, the guide is the guide, not the seat.
When guide geometry is not maintained, it is easy to start producing valve seats that have high eccentricity, or no longer concentric to the valve guide. Most shops have a process to vacuum test valve to seat contact, but you can have remarkably high eccentricity numbers and the assembly of valve to seat can pass vacuum test. If there are no abrupt changes like chatter, scratches, dirt between the valve and seat, it is quite common to see valve seat with excessive runout. Ideally, you never want valve seat runout (eccentricity) that exceeds the specification for stem to guide clearance. That means, regardless of the head diameter, you do not want most Intake valve seats to have more the .002″ (0.05mm) runout/eccentricity.
Why is this? A simple relatable example is that of a morse taper tool (cone in cone) used in drill presses, lathes, etc. The high energy created by the taper not only retains the tool in the spindle, but also aligns the tooling given the tool is not worn out.
Now, think about 45° valve seats and the ability of the taper to influence the valve stem. There is supposed to be clearance between the valve and guide, but the taper will win, and the clearance will now become friction points. If you are lucky, you wear out the valve guide. What sometimes happens is the stress becomes too high, and valve materials fail. Valve heads pop off and that can cause catastrophic failure. This is the perfect storm, if the valve is not properly seated, then the valve to seat contact is marginalized. The temperatures soar beyond the limits of the material of the valve, the guide, the seat. All of these parts will incur some level of damage, and you will struggle to find the root cause.
I have heard many folks speak to their disgust when trying to measure valve seat runout. This goes back to why the seats cannot be properly machined from valve guides that have poor geometry. This deficiency will also impact your ability to measure runout because the same pilots are used to locate your gauge. Your destination can only be determined by having a point or origin or more simply, I need to know where I am when determining where I am going. Once again, the guide is the guide.
What is different today that makes alignment more critical than ever before?
- The materials. The valves, seats, guides are all higher-grade materials due to the power demands. Soft iron valve guides that used to wear/break-in no longer exist in modern engines. Valve steels are high strength alloys for both intake and exhaust. The valve seats are high alloy materials to endure extremely high heats.
- The dimensions/sizes of parts. Gone, even in the heavy-duty engines, are the large stem diameters, fat valve margins, wide seating areas. We have condensed size and space for weight savings while using forced induction and fuel delivery to increase power.
- Power output. I was recently asked, due to the current bearing availability issues, if you could pour babbitt in the shells for some of these modern bearings. I shared that it would not be a good idea. Consider the compression ratio of an engine that came equipped with babbitt bearing…probably nothing more than 6.5:1. We are more than double that today in many cases. The higher cylinder pressures mean higher load, which means higher HEAT load on the smaller parts that we discussed!
Engines are a system. You will be hard pressed to find any two components that do not rely on the interrelationships of another component. That means that you need to always think systematically when performing any singular process from machining to assembly or when analyzing a failure. We often use maxims without understanding the intended meaning. Like a “flash in the pan”…many do not realize that this phrase refers to flintlock rifle construction and function. The old maxim of “the devil is in the details” is absolutely true when separating what will make an engine live reliably or die prematurely.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ1/#p=36