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How do I stop cam from moving during leak down test?

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Hi-

I've been trying to do a leak down test and when I pressurize the cylinder to 100 psi I can't get enough torque on the allen screw on the crank to pull the piston to TDC. How does anyone do these test without having the crank (piston) move?

Thanks

Bruce

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I was going to use the motion pro tool to stop the cam gear. But you're right way too much torque for the chain and sprocket to endure. This weekend I removed the chain from the sprocket and let the piston go to BDC.

Why does the piston need to be TDC for this test? I put in 100 psi and lost 12 psi. That sounds pretty good to me.

Thanks

Bruce

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I was going to use the motion pro tool to stop the cam gear. But you're right way too much torque for the chain and sprocket to endure. This weekend I removed the chain from the sprocket and let the piston go to BDC.

Why does the piston need to be TDC for this test? I put in 100 psi and lost 12 psi. That sounds pretty good to me.

Thanks

Bruce

If you performed the test correctly, a 12% loss is significant. Think about it as losing 12% of your power.

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If you performed the test correctly, a 12% loss is significant. Think about it as losing 12% of your power.

i wasn't sure when he said "lost 12psi" .. but that was kind of what i thought, 100psi in, 12psi loss, 12%... On inline 4 roadrace bikes we like to see 2% when fresh, 5% is good but top riders would be buying parts to get ready for refresh, 10% leak down is way time to do, unless your just streeting or track days.. Listen in the pipe while performing the test, if you hear hissing there.. it's getting by the valves. Im not familiar with where the crf motor would come out the bottom, but on an inline4 rr bike, you take the timing cover off the right side, and can hear hissing from there it's getting past the piston/rings.

Im not sure how the decomp lever plays into this though, your going to hear air in the pipe due to the valve being held open a tad right? This is why the compression test may be more relevant? The manual gives a specific min service limit on the compression .. ? would love to know. What about removing the decomp mechanism for the test? The valve cover does not need to be on (don't think)

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Why does the piston need to be TDC for this test? I put in 100 psi and lost 12 psi.

Because you will be losing pressure much faster from a small volume (combustion chamber) than from a large volume (displacement + combustion chamber) of compressed air. You could do a leak down test in BDC but the acceptable loss guidelines would have to be adjusted for that.

Another reason used to be because of unplated cylinders where cylinder wear was worse at the top of the stroke than at the bottom. But I don't think with plated cylinders it makes any difference, they wear very little compared to the wear on the piston and rings.

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I guess if you took the cam chain off and were certain both the valves are closed you could do it with it at BDC instead of TDC.

One of the things you look for during a leak-down is the leak location. You do this by listening.

A 20% leak in a single cylinder engine makes me think there's a problem with the test.

And a true leak-down is done with a leak down tester. One gauge has the amount of air available to the tester, the other gauge shows a percentage of the leak.

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i just got a leak down test and my piston it is at 20% right now... yes time for a change asap!!

maybe, but as Shawn indicated.. where is it leaking ? (most) Past piston/rings or past the valves? If you hear the hissing sound of leaking air in the muffler, it's getting past the valves. At 20% i don't know how it would run at all, or with any power.. that is likely the decomp mechanism holding the valve open a bit?? I have never done one on a crf450 or single..

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we actually didn't really listen as to where the air was coming but the guy who did it suspects it's getting past the valves and piston since that's where the main area where air sounded like it was coming out of... he said i can actually still use the bike but i'm losing 20% of my power so i need to get my top end changed and broken in asap.

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I guess if you took the cam chain off and were certain both the valves are closed you could do it with it at BDC instead of TDC.

One of the things you look for during a leak-down is the leak location. You do this by listening.

A 20% leak in a single cylinder engine makes me think there's a problem with the test.

And a true leak-down is done with a leak down tester. One gauge has the amount of air available to the tester, the other gauge shows a percentage of the leak.

I'm not sure whether this is a fair (correct) test, I had the carb off the head

the left side case off and the header off. Air was being pushed out of the intake of the head and air was being pushed through the crank and coming out of a tiny hole in the spindle of the flywheel. There was no oil because I had just pulled the cylinder off and checked it and the piston for wear. I imagine if I did the test after the engine was on for a few minutes I would have probably had a better seal. I think that's what you do for a compression check. I'm not sure if a dry leak down is accurate.

I replaced the rings since I was down there. The cylinder still looked good cross hatch still visible.

I would have done the test at TDC if there was a way to stop the piston. I buggered up the allen screw on the crank trying to force it to TDC. So how do you hold it at TDC?

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we actually didn't really listen as to where the air was coming but the guy who did it suspects it's getting past the valves and piston since that's where the main area where air sounded like it was coming out of... he said i can actually still use the bike but i'm losing 20% of my power so i need to get my top end changed and broken in asap.

i don't understand why you would not determine where the leak would come from.. im guessing one of the timing check screws could be removed to hear air escaping past the rings, and listening in the muffler is well.. so easy to do i again don't know why it wasn't done. Isn't it possible the test was done at the part of the stroke where the decomp slightly opens the valve? thus you could (emphasis on could) have way less than 20% actual leak. I just have never heard of a motor that was actually running worth a crap that had that kind of leak down.. as SHawn already pointed out, something isn't "adding up". Get a proper leak down test done by someone that knows enough to listen for leak from valves and/or piston..and has the correct test gauges (plural-2 gauges)

I would do a compression test, again.. when you do a leak down, it is pretty much standard procedure to do a comp test. Takes like, 2 minutes?? Almost no cost.. 78psi is hte min spec on an o4, bit lower for 03 and earlier that had the lower comp piston.

I'm not sure whether this is a fair (correct) test, I had the carb off the head

the left side case off and the header off. Air was being pushed out of the intake of the head and air was being pushed through the crank and coming out of a tiny hole in the spindle of the flywheel. There was no oil because I had just pulled the cylinder off and checked it and the piston for wear. I imagine if I did the test after the engine was on for a few minutes I would have probably had a better seal. I think that's what you do for a compression check. I'm not sure if a dry leak down is accurate.

I replaced the rings since I was down there. The cylinder still looked good cross hatch still visible.

I would have done the test at TDC if there was a way to stop the piston. I buggered up the allen screw on the crank trying to force it to TDC. So how do you hold it at TDC?

I agree with you, it wasn't a really fair test for your motor. You need to do a leak down test on a warm motor, that has oil. Oil is held in the cross hatches of the cylinder walls (that's what cross hatching is for..) thus your not getting as good a seal on the rings allowing more leak than normal. Another reason i think the "dry build" method is bullpucky.. The ring will simply scrape ANY excess oil up or down upon priming the oil pump. You are at least left with the protective oil within the cross hatching as the engine is designed to provide. The rings will break in just fine on the 'high spots' of cross hatching. THis is why its so important to "deglaze" or hone when rebuilding.. so sufficient oil can be held in the crosshatching.

I'm not sure how to hold it at tdc on this motor, i have yet to do a leak down on a crf450.. :smirk: only compression tests and im throwing piston/rings at it every 150-200hrs or so. would be easier to get it ther with the spark plug out though..

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our leak down tester was a mac so i'm quite positive it was a sufficient tool... the hissing we could hear was very loud so we didn't have to put our ears against anything to hear.. but no we did not do a compression test at the same time either.. i already have my new top end parts so regardless if my old stuff is in good condition i'll keep them as spare parts.. or is that a no no?

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our leak down tester was a mac so i'm quite positive it was a sufficient tool... the hissing we could hear was very loud so we didn't have to put our ears against anything to hear.. but no we did not do a compression test at the same time either.. i already have my new top end parts so regardless if my old stuff is in good condition i'll keep them as spare parts.. or is that a no no?

Once you have them off, a good mechanic with mics will be able to determine if the cyl, head and piston was in need of changing, or if it would have saved you money to have done a compression test (still not sure why this was not done). And just because you had an expensive leak down kit, it is still suspect if the leak down test was done right.. (just because you have a fender stratocaster doesn't mean ya can play like Stevie Ray Vaughn.. :smirk: .

Not sure why your not looking at stuff before ordering parts either.. When you say top end parts.. from the crank up? or just a piston/rings..

i sold the 04 head ($85) and the high hour cylinder ($80- posted a pic of it in a thread recently about Rotella oil). Both work fine, i just felt they had sufficient hours to warrant replacing. The head was purchased and they did the seats, works great. The cyl was still in spec .. I would never have put these parts back on my bikes, but im not worried about spending $$ on new parts. You need to decide if it is worth more to you as a spare, or if it is in decent engough condition to sell to someone as a used but working part..or just throw away.. your budget will determine this.

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Am I missing something here but why isnt the piston placed at TDC before even applying pressurized air into the combustion chamber to do a leakdown test?

I dont know if the CRF's have an offset crank centerline and how that would effect the ability of the piston to stay at TDC when a pressurized source is applied to it. In the many leakdown tests I have done on engines I have always sought TDC and applied the air in increments and never had a tough time adjusting the crank to get the piston to hold at TDC without any type of stop.

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Hi-

I've been trying to do a leak down test and when I pressurize the cylinder to 100 psi I can't get enough torque on the allen screw on the crank to pull the piston to TDC. How does anyone do these test without having the crank (piston) move?

Thanks

Bruce

Your piston, rod and crank pin need to be in complete alignment. Off even slightly at it will turn on you. You also need to do your test at TDC in my opinion. That is where the greatest wear tends to be on the top half of the cylinder wall as the most pressure is excerted on the wall by the rings due to the greatest amount of combustion pressures.

Hissing at the Exhaust pipe (Exhaust Valves Leaking) Hissing at Carb (Intake Valves Leaking) Hissing at Crank Breather (Rings/Cylinder Wear). You can confirm the latter by squirting some oil in there and see what happens to your leakdown rate as the oil will help seal the rings and wall but there really is no need to.

Joe

Joe

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we didn't compression test because we didn't have one on hand.. keep in mind i didn't take my bike to anybody to get this done just a guy at my shop bought one off the mac truck and was trying it out.. even if it is off just a bit.. better to be safe than sorry. by top end i mean piston, rings and pin, when i bought the motor last year the guy said it was all rebuilt with top of the line components but i don't know if he was lying about the hours on it or not... so when i get the motor torn apart i will know for sure and report back.. i will also post up pics of my old stuff to make sure it is keepable or sellable... regardless of condition i want a fresh top end into my motor so i personally know hows many hours is on it and can take proper care for it from the start

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