Which is more of value?


17 replies to this topic
  • CaptainKnobby

Posted May 04, 2009 - 05:30 PM

#1

A compression tester or leakdown tester to determine if you need to do a topend rebuild(Mainly just piston and rings).

I hate puting money in for new parts to just have them swaped out with good parts.

  • 426 NOOB

Posted May 04, 2009 - 05:39 PM

#2

A compression tester or leakdown tester to determine if you need to do a topend rebuild(Mainly just piston and rings).

I hate puting money in for new parts to just have them swaped out with good parts.


from what I have read on the forums, leakdowns seem like the way to go

  • CaptainKnobby

Posted May 04, 2009 - 05:54 PM

#3

from what I have read on the forums, leakdowns seem like the way to go


Do you know which brand or kind people had? I dont want to spend a wole lot on one cause Iam going to get the Motion-Pro soft-jaws vise so i can work on my forks.

Iam determined to learn how to service them and to become good at it.

  • JamesNad

Posted May 04, 2009 - 06:12 PM

#4

if u wanna know if u need new pistons and rings then u wanna do a compression test.

a leak down test will test the seals and valve seats.

compression will test well the compression, if its low then your rings are probably alittle worn out and or your piston, and to make sure your not replacing good parts when u take it apart u can measure the piston with a micrometer, and you can take the rings off the piston and put them in the cylinder and measure the end ring gap with feeler gauges to tell if its out of spec.

hope that makes sense

you can get compression gauges pretty cheap, and there will never be 2 identical gauges, they can very up to 15-20psi, i take a compression reading when new or after rebuild and then check it every so often with the same gauge and when it starts dropping i know i need a rebuild.

something like this would work just fine http://www.sears.com...20070921x00003a

  • 426 NOOB

Posted May 04, 2009 - 06:14 PM

#5

Do you know which brand or kind people had? I dont want to spend a wole lot on one cause Iam going to get the Motion-Pro soft-jaws vise so i can work on my forks.

Iam determined to learn how to service them and to become good at it.


They are fairly expensive which is why most people take the bike to a shop, but motion pro sells one for $150. Pricey but would be an awesome tool to have. The reason it is better than a compression tester is not as good is because they just tell you if its high or low. Leakdowns show exactly how much is leaking, and lets you find out exaly what is leaking as well.
http://www.motionpro.../partno/08-0126

  • CaptainKnobby

Posted May 04, 2009 - 06:56 PM

#6

if u wanna know if u need new pistons and rings then u wanna do a compression test.

a leak down test will test the seals and valve seats.

compression will test well the compression, if its low then your rings are probably alittle worn out and or your piston, and to make sure your not replacing good parts when u take it apart u can measure the piston with a micrometer, and you can take the rings off the piston and put them in the cylinder and measure the end ring gap with feeler gauges to tell if its out of spec.

hope that makes sense

you can get compression gauges pretty cheap, and there will never be 2 identical gauges, they can very up to 15-20psi, i take a compression reading when new or after rebuild and then check it every so often with the same gauge and when it starts dropping i know i need a rebuild.

something like this would work just fine http://www.sears.com...20070921x00003a



I have heard that if you take a leakdown test and if a hissing sound is come from the carb it is an intake valve if hissing sound is comeing from exaust it is the exaust valve and if air comes out of the crank case vent hose on the cylinder it is the rings /piston ........ Is this right?

  • husqy360

Posted May 04, 2009 - 07:24 PM

#7

if u wanna know if u need new pistons and rings then u wanna do a compression test.

a leak down test will test the seals and valve seats.

compression will test well the compression, if its low then your rings are probably alittle worn out and or your piston, and to make sure your not replacing good parts when u take it apart u can measure the piston with a micrometer, and you can take the rings off the piston and put them in the cylinder and measure the end ring gap with feeler gauges to tell if its out of spec.

hope that makes sense

you can get compression gauges pretty cheap, and there will never be 2 identical gauges, they can very up to 15-20psi, i take a compression reading when new or after rebuild and then check it every so often with the same gauge and when it starts dropping i know i need a rebuild.

something like this would work just fine http://www.sears.com...20070921x00003a




i heard that with the auto decompression in the new models, you cant do a compression test and leak down test is the only way to go.

  • CaptainKnobby

Posted May 04, 2009 - 08:09 PM

#8

Iam pretty sure you can do a leakdown test to see if the rings is wore. I believe you just got to listen for the air escaping out from the crankcase vent that runs from the top of the cylinder. Iam looking at this one to get..... What do ya think?

http://www.pitposse....oledoteset.html

  • gdbarr1976

Posted May 04, 2009 - 08:21 PM

#9

Tell me if I have this right. If I have worn piston and or rings then compression does not stay above the piston but would leak down to the crank area and cause pressure go back up to the top of the head and cause oil to be blown out the breather hose?? So oil and or smoke comming out of the breather hose would be a sign of bad piston/rings??

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  • grayracer513

Posted May 04, 2009 - 08:29 PM

#10

It is not possible to perform a meaningful cranking compression test on either an auto decompression engine (for reasons that really should be obvious), or on a YZ426, etc., manual decompression engine. In the second case, no numbers are published, and it is basically impossible to kick a healthy YZ4** through a complete compression stroke. If it's unhealthy, the test will give you no feedback as to the cause.

Leak down tests are far more sophisticated in that they eliminate several variables associated with compression tests, give the total compression loss as a percentage, and isolate the source of any significant leaks.

  • CaptainKnobby

Posted May 05, 2009 - 06:15 AM

#11

So what should be the compression reading rate or percentage be on a stock 450f? (I know the manual has no listing for how much compression it is supposed to hold on a fresh piston and rings).

Like if you hooked up a leakdown tester after getting the piston on TDC and applied the air. BTW how much air you put in (like 6 to 10 psi) Never used one of these before.

  • grayracer513

Posted May 05, 2009 - 09:46 AM

#12

There are no compression specs for the YZF, because, as I said, the test is fundamentally invalid anyway. One could break in their bike, do a compression test, and note the figure reported for reference down the road, but it's still not as useful as a leak down test.

The tester consists of two gauges with an orifice between them to slow the flow of air at that point. The input air pressure is not very important beyond certain limits. What the tester does is to compare the air pressure being supplied to the air pressure that can be maintained in the combustion chamber. As long as rings have an end gap, air will escape to the crankcase. What you look for is what percentage of the input air pressure is lost to the leakage.

You must use enough air to "load" the metering orifice, that is, so that it becomes a significant restriction to the total air flowing through the tester, but after that, it doesn't matter much. The tester instructions should give you a working range, and it is usually between 50~150 psi somewhere. Say that you use 100 psi, because it makes the math really easy, and the line gauge connected to the air supply shows 100 psi. If the engine gauge, the second one in the series, shows 96 psi, you're loosing 4%, which is pretty good for most purposes, and unless you go through the engine constantly, it won't really stay much better than that.

Every engine builder has his own set of standards, and people will argue up and down about these numbers, but in general, any engine loosing less than 6% is pretty serviceable, overall. Start dropping beyond 10%, and you can consider the engine to be getting tired. Competitive pro level racing, particularly like flat track, where power is at a premium, will have much more rigid standards.

One very important thing is that when you do this test, the engine must be locked into position in the upper third of the compression or power stroke. The air will want to drive the piston downward, and as soon as it gets close to BDC going either direction, a valve will open, and there goes the pressure. You should know that it is not easy to hold the engine against the force created by 100 psi of air, either, so watch yourself. Start with lower pressures and work up to get the feel of it. I like to use a 24" long breaker bar/ socket combo on the crank. I start at TDC and slowly turn the engine forward to let the rings sweep over a good area of the cylinder, just in case there's a "bad spot" on the wall.

  • CaptainKnobby

Posted May 05, 2009 - 11:17 AM

#13

There are no compression specs for the YZF, because, as I said, the test is fundamentally invalid anyway. One could break in their bike, do a compression test, and note the figure reported for reference down the road, but it's still not as useful as a leak down test.

The tester consists of two gauges with an orifice between them to slow the flow of air at that point. The input air pressure is not very important beyond certain limits. What the tester does is to compare the air pressure being supplied to the air pressure that can be maintained in the combustion chamber. As long as rings have an end gap, air will escape to the crankcase. What you look for is what percentage of the input air pressure is lost to the leakage.

You must use enough air to "load" the metering orifice, that is, so that it becomes a significant restriction to the total air flowing through the tester, but after that, it doesn't matter much. The tester instructions should give you a working range, and it is usually between 50~150 psi somewhere. Say that you use 100 psi, because it makes the math really easy, and the line gauge connected to the air supply shows 100 psi. If the engine gauge, the second one in the series, shows 96 psi, you're loosing 4%, which is pretty good for most purposes, and unless you go through the engine constantly, it won't really stay much better than that.

Every engine builder has his own set of standards, and people will argue up and down about these numbers, but in general, any engine loosing less than 6% is pretty serviceable, overall. Start dropping beyond 10%, and you can consider the engine to be getting tired. Competitive pro level racing, particularly like flat track, where power is at a premium, will have much more rigid standards.

One very important thing is that when you do this test, the engine must be locked into position in the upper third of the compression or power stroke. The air will want to drive the piston downward, and as soon as it gets close to BDC going either direction, a valve will open, and there goes the pressure. You should know that it is not easy to hold the engine against the force created by 100 psi of air, either, so watch yourself. Start with lower pressures and work up to get the feel of it. [B] I like to use a 24" long breaker bar/ socket combo on the crank. I start at TDC and slowly turn the engine forward to let the rings sweep over a good area of the cylinder, just in case there's a "bad spot" on the wall.

Can you just use the kick starter to turn the crank instead of having to to take the plug on the crankcover off and useing a socket and rachet and turning it counter clockwise?

  • grayracer513

Posted May 05, 2009 - 01:06 PM

#14

Can you just use the kick starter to turn the crank instead of having to to take the plug on the crankcover off and useing a socket and rachet and turning it counter clockwise?

No. Read again:

One very important thing is that when you do this test, the engine must be locked into position in the upper third of the compression or power stroke. The air will want to drive the piston downward, and as soon as it gets close to BDC going either direction, a valve will open, and there goes the pressure. You should know that it is not easy to hold the engine against the force created by 100 psi of air, either, so watch yourself. Start with lower pressures and work up to get the feel of it. I like to use a 24" long breaker bar/ socket combo on the crank.



  • CaptainKnobby

Posted May 05, 2009 - 03:31 PM

#15

How come it locks it to position by turning it with a rachet vs. useing the kickstarter? I dont really understand this. I was thinking whether you used the rachet or kickstarter it was turning the piston either way.

  • grayracer513

Posted May 05, 2009 - 07:53 PM

#16

How come it locks it to position by turning it with a rachet vs. useing the kickstarter?

Read again. When did I say "ratchet"?

A breaker bar with a 2 foot handle on the end of the crank gives you direct, bidirectional control of the shaft, and more leverage than you get with the kick starter. If you put the engine at TDC, and the air decides to blow it backward, you might hold it with the starter. What are you going to do if it goes forward?

  • BC3

Posted May 05, 2009 - 08:37 PM

#17

[COLOR="Blue"]We have liked the leak down tester for our hot rod engines because like mentioned you can pin point your worn or damaged parts
with a little engine oil added to the cylinder after your first test you can determine whether it is rings or valves leaking.......if you have worn rings or bore the oil will create a quick compression increase......if it's a bad valve seal it won't help ...Letting you know you need rings/bore job[/COLOR]


  • CaptainKnobby

Posted May 07, 2009 - 07:07 PM

#18

Read again. When did I say "ratchet"?

A breaker bar with a 2 foot handle on the end of the crank gives you direct, bidirectional control of the shaft, and more leverage than you get with the kick starter. If you put the engine at TDC, and the air decides to blow it backward, you might hold it with the starter. What are you going to do if it goes forward?


You never said "ratchet".... I just said that cause it was shorter to type than breaker bar.. But breaker bar was what I was refering to.

What would I do if it went forward? I dont know:excuseme: Wind up with a dislocated shoulder maybe:smirk:

But anyways Iam begining to think all of this would be a waste of time. For 85 hours or so on it. It doesnt seem to be lacking any power that I can tell.
It starts good,usually first kick if it is warm out. Maybe three if it is cold.

I will just wait till the 100 hour mark and then put a piston/rings and cam chain on and might take a look at the valves and mic them and see if they are still in spec. Ill check the tilt of the valve springs and the LBS.





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