newbie question top end vs bottom end
 

I have been reading posts and classifieds on craigslist. When a person says it has a new top end or has a new bottom end.....what are they referring to? What parts of an atv belong to these 2 catagories?

well simply put, the top end is the top half of the motor containing the piston, valve or valves, cam if its an over head cam and the head. the bottom end is the bottom half of the motor containing the transmission. im not a mechanic but i got the basics. usually when a motor blows up in my experience either the top or bottom locks up. ive never had them both go out at once. thats just an easy way of putting it

thanks for the quick reply. i am kind of new to all of this but trying to learn as much as possible.

pretty much what's said above- but to get a little deeper into your excellent question...

the top end of a motor refers to everything from the crankcase up- cylinder "jug" piston, piston rings (which consist of multiple pressure rings and an oil ring), piston boss pin & circlips, upper connecting rod needle bearings, cylinder head (where the spark plug screws into), cam-shaft (if it's an OHC "over-head cam" engine) & cam bearings, valves & valve seals (if it's a 4 stroke with OHV "over-head valves") and any seals, gaskets and o-rings that go with it. Usually, reed valves & carbs are not part of a "top-end" overhaul.

the bottom-end crankcase (extensive work which involves in "splitting the case") is where you'll find the connecting rod, crankshaft, transmission gears, clutch, main crank bearings and all that internal stuff (too much to list.)

typically, a top-end is fairly cheap & easy to restore- (relatively speaking when compared to servicing the bottom end) when people say they have replaced the "top-end" what they usually mean is that they replaced the piston, piston rings, honed (or bored) the jug (cylinder) and replaced the gaskets and spark plug.

It's very common to see 2-stroke machines with newer top-ends... not so common to see 4-strokes with redone top-ends unless there was some mechanical failure or the motor had high mileage.

I used to know guys who'd replace their top end on their 2-stroke dirt bikes every few hundred hours regardless if it needed it or not.

one of the best diagnostics to use when checking out the condition of the top-end is a compression test; a simple gauge that screws into the spark plug hole that measures the combustion chambers ability to hold the pressure when the piston is at the top of the cylinder (TDC)- it also gives the mechanic a good indication if there's an o-ring, gasket, piston ring, or cylinder score issue. low compression = top-end issues. (that's a whole 'nuther thread ;) )

most seasoned riders can tell how good the compression is simply by kicking the motor over, and feeling the pressure through the muffler.
Poor compression or a badly worn top-end can cause a list of engine characteristics like: rough idle, power loss, excessive dirty motor oil, black smoke from the pipe, spark plug fouling, poor gas mileage, bogging down, oil in the coolant, coolant in the crank oil, white smoke, overheating... :blahblah:


Hope this helped ya out :)

beergut is my new friend.:yup::thanks: