when to replace a piston


8 replies to this topic
  • golybrown

Posted August 02, 2010 - 01:35 AM

#1

i have a08 yz450 and was wondering what the sign of you needing to replace the piston are?
Any help would be appreciated as i am clueless about pistons

  • grayracer513

Posted August 02, 2010 - 07:24 AM

#2

Tear down, inspection, and measurement are the only sure ways to know, unless there are some outward symptoms of trouble.

  • motochebyrashka

Posted August 02, 2010 - 08:24 PM

#3

you can run the comprassion test just to get an idea at first.

  • ttr250dude

Posted August 02, 2010 - 10:18 PM

#4

what should the compression be when cold?

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • grayracer513

Posted August 02, 2010 - 11:14 PM

#5

Compression tests are fairly useless in an engine with automatic decompression. They're fantastically inaccurate in engines without it. Leak down tests are far more reliable. Have one done at a good shop.

  • nwbikerider

Posted August 03, 2010 - 07:40 AM

#6

Tear down, inspection, and measurement are the only sure ways to know, unless there are some outward symptoms of trouble.


So, what would the outward symptoms of trouble be?

  • tech24

Posted August 03, 2010 - 07:44 AM

#7

So, what would the outward symptoms of trouble be?


Burning oil and smoking, poor performance, and possibly some noise.

  • jmik517

Posted August 05, 2010 - 02:39 PM

#8

alot of the old bike repair manuels I've read though list, poor starting as a possible sign of a bad piston or rings. Don't over think your way into a repair that didn't need to be made. I've seen alot of yamaha's with a bunch of hard hours on them, that haven't been touched.

  • Luigic81

Posted August 05, 2010 - 03:05 PM

#9

You can buy a leak down tester for fairly inexpensive, but the percentage of leakage may not mean a whole lot to you unless you have a new engine to compare it against. Once you see a significant decrease in your leak percentage then you can determine if it's time to rebuild. A leak down tester will also help you diagnose if the compression loss is due to a piston/rings that aren't sealing as well as they used to, or if you have a leak somewhere else, like your valves or a bad head gasket.
I did one on my Husky SMR 510 that I used to have, and was leaking 12%, which I was told was ok to keep running for another season, but without a comparison percentage from when the motor was fresh, who knows.....

that said pistons are cheap, if you have 200 hours or so on your bike, just spend the $150 and do it



here's one on ebay http://cgi.ebay.com/...rts_Accessories

not sure why they show a 2 stroke piston in the pic :banana:





Related Content

 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.