Author Topic: 87 lt500r compression  (Read 333 times)

Offline bigbird337

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87 lt500r compression
« on: February 14, 2019, 09:15:51 am »
I have a all stock 500. It has never been torn down. I have 150psi compression and going to tear it down to do thrust washers. Is 150 good if everyhting looks ok

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Offline MotorGeek - Jerry Hall

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Re: 87 lt500r compression
« Reply #1 on: February 14, 2019, 12:59:56 pm »

Checking the compression on a 4 stroke typically is used to determine how well the valves are sealing ..... not to determine how well the rings are sealing.  Worn out cylinder walls and rings typically seal well, as well as having cranking compression that is within specifications, but will smoke and have high oil consumption. 

Cranking compression on a two stroke does not give an indication of the condition of the piston clearance, fatigue in the piston or general condition of the rings and cylinder walls.  If the engine has never been torn down, the piston probably needs to be replaced even if there is not any ware on the cylinder wall. 

The OEM pistons are thin-walled cast pistons and shatter unannounced.  The pistons fatigue very quickly when they get a little loose in the bore, even though the rings are ok and sealing very well

I check the compression when I am trying to diagnose a non running engine.  If it has above about 80 psi on most gages it has enough compression to start.  The cranking speed, check valve location, length of the hose, and length of the threaded fitting that screws into the spark plug hole will affect the pressure registered on a compression gage. 

My advice is to tear down the top end and take the cylinder to a competent machine shop and have them measure the bore with a bore gage.  If they use a feeler gage between the piston and cylinder wall to attempt to determine the bore condition, take it to another shop that is properly equipped to do the job correctly.


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