Top End Rebuild Questions

So i have the gist of how a top end rebuild works. Piston and seals and bla bla get replaced. So say my bike blows up, and i open up the engine, how do i know that it is the top end that has gone bad. I know one sign is obviously that it wont start and is not seized, because a seized engine is bottom end right?

I am asking all of this because i am about to buy this kx60 and use it as a pit bike, but my friend doesn't know the last time it was rebuilt. So it occurred to me that since top ends blow up more than bottom ends that it would most likely blow on me some time after i get it, so i want to be able to rebuild it myself for the experience and quality of working on my own bike, and to save a hell of a lot of money. I have been looking at tutorials etc, cant you tell by the rod bearings or some thing? Im very new to this and just looking for some help, probably a noob question.
 
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Welcome to the forum Jake.
You will know what happened and what needs to be replaced upon top end removal. You can check crank play by removing sides of engine on stator side and clutch side also. But believe me, if she blows, you'll know what it needs when you open it up.
 

SRAD97750

Moderator
Staff member
Im very new to this and just looking for some help, probably a noob question.
Number one, No motor should ever "blow-up" on you.
That is a clear indication of outright neglect.
A motor can get worn out, and once it is wrongfully expected to perform like new, it may blow up.
Piston rings have the shortest life in the engine, and need replacing more often than any other properly maintained component. All components wear over time.

I like to check a few main measurements against the manual to see the status of an engine.
-Visible Damage
-Piston-Cylinder Clearance
-Piston Ring End Gap
-Crank bearing play
-Crank rod Play Radial/side to side

These will give you an excellent indication of whether then engine is worn or not.
Also, a great indicator of engine life is DOCUMENTING a "change in compression over time."
-BIG DAN:thumb:
 
Mike is right. A seize means the piston is basically now welded (melted) to the cylinder wall.

Number one, No motor should ever "blow-up" on you.
That is a clear indication of outright neglect.
A motor can get worn out, and once it is wrongfully expected to perform like new, it may blow up.
Piston rings have the shortest life in the engine, and need replacing more often than any other properly maintained component. All components wear over time.

I like to check a few main measurements against the manual to see the status of an engine.
-Visible Damage
-Piston-Cylinder Clearance
-Piston Ring End Gap
-Crank bearing play
-Crank rod Play Radial/side to side

These will give you an excellent indication of whether then engine is worn or not.
Also, a great indicator of engine life is DOCUMENTING a "change in compression over time."
-BIG DAN:thumb:
Could you explain further on the "change in compression" do you mean just keeping track of compression lose over time?
 

SRAD97750

Moderator
Staff member
Could you explain further on the "change in compression" do you mean just keeping track of compression lose over time?
Exactly, Pretty much just writing it down.
A compression reading is guessing if you don't know what compression this motor was making before. It's the actual change in compression over time that shows engine health.
As the compression changes, it will be a consistent trend. Any readings significantly lower than that trend, (even within service limits) should be considered a warning sign.
-BIG DAN:thumb:
 
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