Basic Engine Question

[supercut469]

Hey all, I'm an amateur and am looking for a junk saw to rebuild. (I have 2 working saws now but CAD is setting in).

I have a general question about top end rebuilds:

Why does it matter if the piston is scored when you look in through the exhaust port? Isn't the compression because of the seal between the rings and the cylinder? Why does the piston itself matter?

 

[Fish]

Well, the initial loss of compression is from the molten aluminum pasting the rings down, and cleaning the grooves out may help a bit and get some more run time, but if you have a lot of free time, you can get some more life out of a saw.

But first, you need to find out what caused the meltdown, and doing so, do you really want to rig up a temp repair. Normally the pasted on aluminum can be buffed out of a cylinder, but the grooves in the piston remain, and will likely cause future trouble. so a new piston/rings is
normally the cheapest {good} repair...

 

[Snyper438]

Also, if you have scoring on the piston, you want to make sure there isn't any on the cylinder wall to cause compression issues with the rings. This is why you're looking in the first place right? The scoring had to come from somewhere.
IMO, it depends how bad it is as to how extreme the rebuild needs to be. Minor scoring with a little transfer could just need a good cleaning and buffing. Deep groves most likely need a bit more work.

Good luck to ya buddy,
Ed

Edit;
Of course, if you have it apart and can go with new at resonable price (even with minor damage) why not go ahead and give it that much more life.