Every niche, sport, or hobby has its terminology and this can make it very confusing, to say the least.
This can be one of the most confusing things to a beginner or even someone just curious about something let alone motorcycles.
Full disclosure I also have trouble keeping up with all the terminology and I’ve been at this for over 20 years. In this article, I will explain these 5 terms below about motorcycle rebuild and maintenance.
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What does a top end rebuild on a motorcycle mean?
A top-end rebuild on a 4-stroke motorcycle is a replacement of some or all of the piston, piston rings, cylinder, valves, camshaft, cylinder head, or any associated parts to bring the compression back to the optimal range.
A 4-stroke motorcycle has more moving parts, especially in the top end than a 2-stroke motorcycle.
They have valves and springs, either a camshaft or pushrods, shims and buckets or rocker arms, and even chains, all of which reside in the top end of a 4-stroke motorcycle.
A top-end rebuild on a 2-stroke motorcycle is a replacement of some or all of the piston, piston rings, cylinder, or any associated parts to bring the compression back to an optimal range.
A 2-stroke motorcycle does not have any pieces associated with the valves like 4-stroke motorcycles making it cheaper and easier to install.
The valves are very different from their 4-stroke counterparts, and their location makes them removable from the top end without disturbing the cylinder or even part of the bottom end so no removal may be necessary.
Because they can be serviced or replaced without removing the so-called top end of a 2-stroke motorcycle, they are not typically considered part of a top-end rebuild.
What does a motorcycle bottom end rebuild mean?
A bottom-end rebuild on a motorcycle is the replacement of some or all of the crankshaft, piston rod, counterbalance, and associated bearings or gears to bring the bottom-end back to its original state.
2-stroke and 4-stroke motorcycle bottom-ends are virtually identical with the exception that on a 2-stroke the clutch and transmission are separated from the crankshaft so no oil flows between the top and bottom-end.
This is unlike a 4-stroke motorcycle which the same oil that lubricates the bottom-end also serves this process for the top-end as well
Although a motorcycle’s bottom-end houses the clutch and transmission, when someone is referring to the bottom-end on a motorcycle they are typically not talking about these parts.
This is because automotive engines have separate transmissions and clutches and the term bottom-end rebuild has simply transferred over to the motorcycle industry causing all kinds of confusion.
This is something to watch out for when buying a used motorcycle. If someone mentions the bottom-end has been rebuilt it does not mean the transmission and clutch have been rebuilt as well.
What is a motorcycle engine rebuild?
A motorcycle engine rebuild is a replacement of some or all of the engine components, bearings, gaskets, gears, or any other internal part to bring it back to the original specifications after a full engine teardown.
This would include both the top and bottom-end of a motorcycle however many people believe that a top-end rebuild is the same as an engine rebuild.
I believe this is the biggest contributor to all the confusion around the terms engine rebuild and a top-end rebuild.
Almost all other engines, other than most motorcycles, ATVs, and side by sides, typically do not have built-in transmissions and clutches leading to more confusion.
If someone says the engine has been rebuilt it does not necessarily mean the bottom-end has been rebuilt as well because of this confusion.
I would caution you to be careful if someone says they had a transmission, or bottom-end rebuild on their motorcycle but did not mention the top-end. It is impossible to do either of these on a motorcycle without removing the top-end.
What does motorcycle valve clearance mean?
Valve clearance on a motorcycle is referring to the space needed to ensure proper operation of the valves on a 4-stroke motorcycle.
There are 2 main types of valve systems used in 4-stroke motorcycles one is a rocker valve system and the other is an overhead cam system.
In a rocker valve system, valve clearance is the space between the adjustment mechanism on the rocker arm and the top of the valve shaft or “valve stem”.
In an overhead cam system, valve clearance is the space between the bottom of a camshaft lobe and a cover or “bucket” that is over top of the valve stem.
Regardless of the type of mechanism used to operate the valves having the right clearance is crucial to the operation of a 4-stroke motorcycle.
What happens if the valve clearance on a motorcycle is incorrect?
There are 2 main issues caused by an incorrect valve clearance setting on a 4-stroke motorcycle, one is poor performance and hard starting, the other is damage to the piston, valves, cylinder, or cylinder head.
One of the first signs of incorrect valve clearance on a 4-stroke motorcycle is hard starting, whether there is too much clearance or not enough.
If the valve clearance is too big the valves will not open correctly which will cause performance issues but perhaps most importantly there will be starting issues.
If the valve clearance is too small the valves will not close affecting compression or worse the valves may make contact with the top of the piston causing catastrophic damage to the piston, valves, cylinder, or cylinder head.
What is a valve adjustment motorcycle?
A motorcycle valve adjustment is either the replacement of a shim or the setting of and bolt and nut adjuster to provide sufficient space for the valves to open and close properly.
There are two main types, a rocker arm system, and an overhead cam system.
In a rocker arm system, the space or clearance being created is between the adjustment mechanism and the valve stem using a nut and bolt adjuster.
For an overhead cam system, the clearance is between the bottom of a cam lobe and a bucket that covers the valve stem. To adjust it you place a special metal disc or shim underneath the bucket.
What is a rocker arm valve system on a motorcycle?
A rocker arm valve system on a 4-stroke motorcycle is a camshaft-driven system that uses pushrods on the bottom side of a pinned lever to open the valve and with help of the valve springs close the valves in a rocking motion.
This style of valve system works best on lower RPM (revolutions per minute) engines and usually requires less servicing than an overhead valve system.
What is a overhead cam valve system on a motorcycle?
An overhead valve system on a 4-stroke motorcycle is a chain and camshaft-driven system that uses the camshaft, with the help of valve springs and a cover called a bucket, to directly control the opening and closing of a valve.
These valve systems can run at a much higher RPM than a rocker arm system however they usually need a lot more maintenance.
As the motorcycle industry separates itself more and more from the automotive industry much of the confusion around these and other terms should be more clear as time goes on.
Once most people realize that motorcycle engines differ greatly from their automotive counterparts a term like an engine rebuild will only be associated with automobiles and not motorcycles.
In the motorcycle industry, we prefer the terms top-end rebuild and bottom-end rebuild to define what has been repaired.
Hopefully, this article helped clear up some of the confusion around motorcycle rebuild and maintenance terms and as always feel free to contact me with any questions, concerns, or topics you want to be covered.