Will compression test indicate need for top end?

[Psalm31.24]

I own an 02 yz250, assuming I know the compression of a freshly rebuilt engine, will a simple compression test tell me if I need a new top end or not? Or do I need to actually check the wear on the rings and piston with a caliper?

Thanks in advance.

 

[bclapham]

Troy: its best to measure that piston since bikes have been know to blow up with an older piston that shows good comp. however, you didnt do the top end that long ago so i would say youve got ages yet. in hindsight on my yz, i was changing pistons way too often when i could have got away with just new rings to freshen things up. the wiseco you have last well also.

that said, you should see the piston from my 144- it looks worse after 5 rides than the yz did after 5 months.

btw- well done for getting to those races, i am ashamed i still havent been out there yet....but it will happen soon!

cheers

Bruce

 

[Jaybird]

I agree with Bruce. You may not actually need to replace all the goodies just because you have a low comp. reading. It could just be a worn ring or too large a ring end gap.
Your manual should give specifications on the sizes of all your parts. When you take it apart, measure everything to confirm if they are indeed out of spec before replacing.
Be very careful and measure many places on your piston and jug. Take pains in making sure ring end gap is proper.
Always replace bearing and rings when you are there.

 

[Psalm31.24]

Bruce & jaybird,

Thanks for the advice! After 50 hours of riding everything is still in spec. Guess that's cause I ride like a girly-man. I'm working on that too tho. ;-)

BTW: Do I need to break it in nothing is new? I assume not.

Troy

 

[Jaybird]

I would scoth-brite the jug walls a bit. The rings are gonna conform to the jug walls and hatching right off the bat, so I would heat and cool the motor a couple of times for good measure.