Why preasure testing an engine ?

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JanBros
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Why preasure testing an engine ?

#1 Post by JanBros » Wed Oct 10, 2018 6:32 pm

I reed it a lot here, not on other fora. I wonder why you people do that, as to me it makes no sence at all.

Putting preasure in the crankase makes the crank seals seal harder as the preasure presses the rubber onto the shafts. But leaking crank seals appear 99.99% of the time when there is vacuum in the crankcase. To me that means you are leak-testing a situation that is not relevant to the real world.

If not for the crank seals, what else would you be testing ? If the crankcase halfs are air-tight ? I certainly never have had leaks between the metal of the cases, nor have I heard from anyone they had them.

If not that either, am I missing something ?
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Re: Why preasure testing an engine ?

#2 Post by evan_calgary » Wed Oct 10, 2018 6:51 pm

Probably correct it is not a perfect situation on crank seals but will show any major areas of leakage. Have also not seen the actual cases leak but have seen leaks at intakes (both sides of reeds), PVs, base gaskets, head gaskets etc. If you were dealing with all brand new factory parts may not be a huge reason to do it but mostly dealing with old parts.

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Re: Why preasure testing an engine ?

#3 Post by JonW » Wed Oct 10, 2018 7:22 pm

As said its usually the red block area that leaks and you really do want to know about that.

ALWAYS PRESSURE TEST. It will save your engine.
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Re: Why preasure testing an engine ?

#4 Post by Smoker » Wed Oct 10, 2018 8:28 pm

JanBros wrote:Putting preasure in the crankase makes the crank seals seal harder as the preasure presses the rubber onto the shafts. But leaking crank seals appear 99.99% of the time when there is vacuum in the crankcase.
I thought a proper test was done with both pressure and vacuum. :smt017

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kobra
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Re: Why preasure testing an engine ?

#5 Post by kobra » Wed Oct 10, 2018 8:42 pm

Maybe it's best to perform a pressure test, as well as an atmospheric pressure test :D

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JonW
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Re: Why preasure testing an engine ?

#6 Post by JonW » Wed Oct 10, 2018 8:54 pm

Pressure and Vac would be ideal, but if all you have is pressure (and everyone has that with a bike pump) then do that. Any leaks will kill your engine very quickly and its very simple to locate them on the outside (reeds, cyls etc) with pressure and a bit of liquid soap in water, or use window cleaner which works well and leaves your engine streak free, lol.
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85 RZ350 F1 Resto
84 RZ500 Resto
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Re: Why preasure testing an engine ?

#7 Post by RuZty » Wed Oct 10, 2018 11:22 pm

I just use a MityVac to pull a vacuum and a small bike pump for pressure. Most important is an accurate low pressure gauge, trying to see change on 6 psi on a 2" diameter 120psi gauge is pointless.

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Re: Why preasure testing an engine ?

#8 Post by JanBros » Thu Oct 11, 2018 5:59 pm

JonW wrote:As said its usually the red block area that leaks and you really do want to know about that.

ALWAYS PRESSURE TEST. It will save your engine.
never done it, and never will. If you make sure all your surfaces are clean and straight, you will not have a leak.

just out of curiosity, how much preasure do you put on it ?
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Re: Why preasure testing an engine ?

#9 Post by silverstrom » Thu Oct 11, 2018 6:49 pm

So, your belief is that only members of this forum do pressure testing?

It is a proven fact that the ONLY way to test cranks seals is to pressure and vacuum test them. Well, I suppose there is one other way, and that is to just keep living in denial and wait for a meltdown.
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Re: Why preasure testing an engine ?

#10 Post by JonW » Thu Oct 11, 2018 7:42 pm

Indeed John.

For the minimal cost and effort to prove the engine is airtight its a no brainer.

You can lead a horse to water... etc.
My Projects:
see them all at www.2smoked.com

80 XT500 Supermoto!
81 RD350LC Resto
82 RD 421 LC Hybrid
82 RD 250 '51L' YPVS LC
83 RZ350 LC2 Resto
85 RZ350 F1 Resto
84 RZ500 Resto
86 RZ350 F2 Hybrid

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Questo vecchio rz
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Re: Why preasure testing an engine ?

#11 Post by Questo vecchio rz » Thu Oct 11, 2018 8:14 pm

JanBros wrote:
never done it, and never will. If you make sure all your surfaces are clean and straight, you will not have a leak.

just out of curiosity, how much preasure do you put on it ?
Here was your basic answer:
RuZty wrote:I just use a MityVac to pull a vacuum and a small bike pump for pressure. Most important is an accurate low pressure gauge, trying to see change on 6 psi on a 2" diameter 120psi gauge is pointless.
Never Too late to learn something.
It's not always clean nor straight, you can have wear, warpage, incorrect tq settings, brand new parts that are out of tolerance.
I can say it's quite likley, if you've never checked your motors, you probably have had this issue, your just lucky...that's all.

I personally know quite a few AMA
& 3 Former Motorcycle
GP race team mechanics and a leak down is just standard even in trade schools it's taught as basic. Many times a fresh rebuild is leaking, and many, many motors won't hold pressure/vac..but they will still run, some for a long time... others not so lucky, you sieze. A air leak is hands down the most common cause of 2 stroke failure, with many incorrectly blaming ,fuel/oil, ignition,jetting etc as the cause...

Here's a good read which can shed some light on the subject for you. :smt023. As the saying goes, Harry Klemm has forgotten more than most of us know...lol
read his tech article under Air leaks.

http://www.klemmvintage.com/" onclick="window.open(this.href);return false;
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Re: Why preasure testing an engine ?

#12 Post by RuZty » Thu Oct 11, 2018 8:33 pm

JanBros wrote: never done it, and never will. If you make sure all your surfaces are clean and straight, you will not have a leak.
I would agree with your principle except that after assembling an engine with meticulous care and still finding small leaks, you don't know if you don't check. I will also always measure the piston and bore before I assemble an engine rather than trust the machine shop to have done it right. Luck is not a measurable quantity.
It is also a good metric for the state of your motor through it's life, just as a compression test is. If I have the carbs or exhaust off I will take the time to do a test to make sure I don't have any surprises looming.

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Re: Why preasure testing an engine ?

#13 Post by brrrappp » Thu Oct 11, 2018 11:28 pm

Smoker is right! You need both Vacuum and pressure test.
Pressure tests make it easier to find the leak. And, by the way, your crank is pressurized briefly on every stroke so those seals need to work in both directions. For a REAL mechanic, there is no substitute for a good leak down test after a rebuild. I blew up an IT175 dirt bike 4 times under warranty (90 days). The Yamaha dealer never did a leak down until the area-rep came asking about all the claims on my bike (slightly mismatched case-halves). That's when the dealer learned the value of a leak-down or vacuum test.

You can buy a brake vacuum pump with a gauge attached at Amazon or any auto-parts store for about $20 or $30. I see leak/compression style gauges on Amazon starting at $30. You can get some freeze plugs at the hardware store.

On the other hand, you could just spray a little starter fluid around your ignition rotor with the engine running at idle or around the base gaskets and even near the reeds if you have an air-box. The idle will go up noticeably if there is an air leak.
A modest "high" can be obtained by listening carefully to the sound of a 2-stroke hitting the fat part of the powerband while inhaling cool fall air tainted by the exhaust scent of VP-C12 and Golden Spectro, mixed at 24:1.

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Re: Why preasure testing an engine ?

#14 Post by JonW » Fri Oct 12, 2018 2:15 am

My issue with leaks and spraying starter fluid is the danger of a run away engine. I had this happen to me with an MT250. It was a total rebuild and everything was up spec'd, new of refurbished. I didnt own a tester back then, I wish I had. The bike started and idled ok. I revved it a bit and it was ok too. ticked over some more and then the idle rose a bit. I blipped the throttle and suddenly it took off. keyed it off and it kept going, faster and faster. At this point its revving waaay thru the redline. I couldnt get behind the carb so sat on it, yanked the brakes on and dumped it into gear and let the clutch out. it left a 4ft skid mark across my garage floor but finally it stalled. Scary as hell and im sure it ruined the crank as it never ran quiet after that. Such a shame, and it could have been avoided, there was a leak around where the carb bolted to the CR250 cylinder, that's all.
My Projects:
see them all at www.2smoked.com

80 XT500 Supermoto!
81 RD350LC Resto
82 RD 421 LC Hybrid
82 RD 250 '51L' YPVS LC
83 RZ350 LC2 Resto
85 RZ350 F1 Resto
84 RZ500 Resto
86 RZ350 F2 Hybrid

Like Watches? Try www.PloProf.com and www.DeskDivers.com

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Re: Why preasure testing an engine ?

#15 Post by brrrappp » Fri Oct 12, 2018 10:09 am

JonW wrote:My issue with leaks and spraying starter fluid is the danger of a run away engine. I had this happen to me with an MT250. It was a total rebuild and everything was up spec'd, new of refurbished. I didnt own a tester back then, I wish I had. The bike started and idled ok. I revved it a bit and it was ok too. ticked over some more and then the idle rose a bit. I blipped the throttle and suddenly it took off. keyed it off and it kept going, faster and faster. At this point its revving waaay thru the redline. I couldnt get behind the carb so sat on it, yanked the brakes on and dumped it into gear and let the clutch out. it left a 4ft skid mark across my garage floor but finally it stalled. Scary as hell and im sure it ruined the crank as it never ran quiet after that. Such a shame, and it could have been avoided, there was a leak around where the carb bolted to the CR250 cylinder, that's all.

The Spraying starter fluid thing is really one of those tests that I only do to confirm the suspicion of a leak when the bike/sled/lawnmower is all together and starts acting weird. I would not recommend it as a reliable test. Leak testing is a great way to check your work BEFORE you put everything back together and give it a rip.
A modest "high" can be obtained by listening carefully to the sound of a 2-stroke hitting the fat part of the powerband while inhaling cool fall air tainted by the exhaust scent of VP-C12 and Golden Spectro, mixed at 24:1.

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