What Changes In Valve Shim Size Can Tell You About Your Engine
I hope you enjoyed my last post on ice tire studding! The season in my neck of the woods has been a bit short this year and I may be getting back to the dirt sooner rather than later. Nonetheless, Part II, which covers mounting ice tires is now up on my blog. You can view it here: Ice tire mounting.
In today's post I'm going to shift focus back to the engine and talk a little about valve technology. Valve technology and manufacturing techniques have changed substantially from the earlier days of engine development and I want to share with you some information about the current valve technology being implemented in your engines. I also want to discuss one way you can get a feel for how much life is left in your valves. Let’s get started.
The following excerpt is copied directly from my book, The Four Stroke Dirt Bike Engine Building Handbook. If you want to learn more helpful tips, which will bring your maintenance knowledge and engine building skills to the next level, I’d like to invite you to pick up a copy of my book by clicking here. Be sure to use the offer code tt2016 to get 15% off when ordering!
Alright, on to valves shim sizes.
The cylinder head assembly of most engines will wear out before it resorts to telling you it has had enough by catastrophically failing. Diagnosing these wear signs and knowing when it is time to replace components is the key to keeping the cylinder head assembly from failing. Due to the aggressive camshaft profiles, high compression ratios, and high RPMs required to make a lot of power, the valves and seats typically are the first parts to wear out within the cylinder head. Worn valves and seats will cause the engine to become difficult to start, have low compression, and have reduced power.
Modern valves found in dirt bike engines are made from either titanium or stainless steel alloys. Regardless of valve material, modern valve faces are either coated in a variety of anti-wear materials or hardened using various hardening processes. Common examples of trade names you might be familiar with include diamond like coatings (DLC) and black diamond coatings. These coatings are typically harder than the base material of the valve and help the valve resist wear, which occurs from ingesting dirty air and repeatedly contacting the valve seat. Coating and hardening processes are only present at the surface of the valve face. Depending on the type of valve and process used to harden it, the coating thickness can range from as little as 0.0001” (0.003mm) to around 0.003” (0.076mm). An easy way to visualize the thickness of the coating is to pluck a hair from your head and either measure it or feel it between your fingers. Most human hairs are around 0.002” (0.05mm) which should give you a good idea of how thick the coatings used on the valves are.
The important takeaway here is that if the coating is only a few thousandths of an inch thick, the valve can only be adjusted a few thousandths of an inch before it will have worn through the coating. Monitoring the starting valve shim size once the engine has been broken in (or new valves installed) and comparing that size to the shims required the next time the clearances are adjusted is a great way to assess valve health. Normally within the first 3-5 hours of breaking in a new engine the valve shim sizes may change slightly. This is due to the mating of the new valves to the seats and any valve seat creep which may occur. After this occurs and the valves have been shimmed to compensate, usually an adjustment up to around 0.004” (0.10mm) is all that can be done before a valve has worn through its hardened surface. Once this happens the valve face will wear much more quickly and start to wear out the seat as well. This will result in more frequent valve shim intervals and necessitate the need for having the valve seats cut. By paying attention to changes in shim sizes you will be able to approximate when the valves have worn through their hardened surfaces and must be replaced.
Thanks for reading and please leave questions or comments below. I enjoy hearing from you!
Remember you can get 300 pages worth of in-depth dirt bike engine information with The Four Stroke Dirt Bike Engine Building Handbook. Be sure to use discount code tt2016 at checkout to receive 15% off your order!
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