By Justin James
With Harley-Davidson unveiling new bikes equipped with overhead cam engines, I feel this is a good time to jump into this topic. Metric bikes have been running overhead cams for decades and today only a few manufacturers even bother to produce pushrod engines at all. In motorcycle applications, overhead cams are superior to pushrod engines in virtually every way imaginable. They have fewer moving parts that can wear out or fail. Less moving parts also means less power loss and a higher RPM range. Positioning the cams over the valves allows for more variation in valve angle and chamber shape which in turn increases the power and efficiency potential of an engine.
Modern overhead cam motorcycle engine designs only come with one real drawback, which in my opinion, is a small price to pay. Overhead cam engines require periodic maintenance, which can be time consuming, tedious and expensive. The most notable being valve lash adjustment. Valve lash is a clearance that is set in place to compensate for the thermal expansion and wear of the valvetrain. Street motorcycle inspection and adjustment intervals can range anywhere from 5,000 to 26,600 miles depending on make and model. This edition of Tinker Talk will touch base on the different configurations and valve adjustment procedures for overhead cam engines.
Set Screw and Lock Nut (Photo 1): This style allows for the simplest valve lash adjustments and is commonly found on single overhead cam (SOHC) engines. You will see this configuration on early inline engines, air cooled singles, many metric cruisers and H-D XG’s. Some engines with this design will have an access port in the valve cover and others will require removal of the valve cover for adjustments. To adjust the valve lash, simply break the lock nut loose, rotate the set screw until the desired clearance is achieved and torque the lock nut.
Shim Over Bucket (Photo 2): This configuration can be found on early dual overhead cam (DOHC) model motorcycles such as 70’s/80’s Japanese performance bikes. Performing an adjustment on this configuration will be slightly more difficult and time consuming. You will need to remove the valve cover and anything preventing you from doing so. Disassembly of the valvetrain is typically not necessary. A special tool (Photo 3) will be required to compress the bucket while you swap out the shims (Photo 4).
Shim Under Bucket: This has been the standard for most performance-oriented bikes over the last few decades. Tackling a valve adjustment on this setup is not something that should be taken on by anyone lacking a good degree of mechanical knowledge. This configuration will require disassembly of the valvetrain to adjust the clearance. Not only that but many later model engines will require you to remove things such as the throttle bodies, exhaust and radiator. Some even require that you unbolt the engine and rock it forward or downward. Don’t even get me started on performing this service to a Ducati.
Now that we have broken down the differences in adjusting valves for the common overhead cam engine configurations, let’s go over a few key points that will be universal when performing a valve adjustment.
• Valve adjustments should be performed when the engine is completely cold.
• Consider removing the spark plugs when performing a valve adjustment. This will keep you from working against the engine’s compression while rotating the crank. Be sure not to let a bolt or shim fall into the spark plug bore.
• Prior to performing a valve adjustment, you should inspect the timing chain, set the tension in accordance with manufacturer specifications and verify the tensioner is functioning properly. If any of the timing components are questionable or at the end of their recommended service life, they should be replaced. Neglect these crucial parts and you may end up turning your entire engine into a pile of scrap metal.
• Always turn the crank in the rotation direction of the engine. If you have any difficulties or are unsure of which direction to rotate you can utilize the rear wheel. Simply elevate the rear of the bike, put the bike in high gear and turn the rear wheel forward to rotate the engine.
• Clearance for a given valve is normally checked at the precise point when it is fully seated and completely out of contact with the camshaft. A good service manual should provide a detailed sequence on how to position the crankshaft and check the valves for a given engine.
• When working with a shim and bucket style, it may be helpful to make a diagram to record your clearance and shim measurements (Photo 5).
• Aftermarket or performance camshafts may call for a different clearance than what the manufacturer of a given engine recommends. When faced with this dilemma it is best to follow the guidelines of the camshaft manufacturer.
Keep in mind, this is merely a crash course on this subject. There is far more to adjusting valves than what I can condense into a couple of pages. Anyone who wrenches on motorcycles should get comfortable with performing valve lash adjustments. This is something that is only going to become more and more common. I imagine it will be quite some time before overhead cam motorcycle engines won’t require valve adjustments, like most of today’s automotive engines. Overhead cam engines will continue to gradually replace pushrod engines. Look at most upper level racing sanctions or classes and you will notice that pushrod engines are almost nonexistent. In motorcycle applications, pushrod engines have yet to match the range, power or efficiency per displacement that overhead cam engines offer. Get with the times or get left at the line. Spend any amount of time at a track and you will learn this quickly.
Tinker, Shred, Destroy, Repeat
-Justin James (follow more of my Tinker
shenanigans on Instagram @justinjamesmoto)