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leak down?

6K views 24 replies 9 participants last post by  StrokeThis 
#1 ·
What is a safe % for a leak down on a nitrous motor for a gen 1 with 20 24 ring gap. Thanks guys.
 
#3 ·
how much spray are you running also? if its a big shot then and if you have too much leakage somewhere the nitrous will find it. Nitrous will find the weak link.
 
#6 ·
Depends on your leak down gauge to if your using a harbor freight one it only pressurizes the cylinder with about 10psi it will so more leak then.there is sometimes not enough pressure to help seat rings try using a motion pro they use about 60 psi get a better reading just my 2 cents and I agree that gap seems a little big. Depending.who put it together if the idea was to spray I know some guys go larger just to be sure the heat doesn't close the ring gap. But still seems to much gap to me.
 
#11 ·
Try this with your HF guage next time and see if there is a better leak down across the board on your motor..


1. With the spark plug adapter installed in the cylinder but not connected to the gage set.
2. Connect the gage set to a suitable source of compressed air. Doesn't have to be 100 PSI. Can be any pressure.
3. Adjust the gage pressure regulator for a reading of 0% leakage on the "Cylinder Leakage" gage.
4. Quickly connect the spark plug hose adapter to the gage set. This is the only part I do not like. Instead of a valve to do this part, you have to be real quick and attach the quick connect fitting from the spark plug adapter hose to the matching hose from the gage set.
5. Read the leakage as a percentage on the "Cylinder Leakage" gage.



Basically how this works is that the pressure regulator attempts to maintain the preset pressure but it can never do so because of the leakage in the cylinder, so the difference in the preset pressure and the cylinder pressure is presented as a percentage. You do not have to have a big compressor that maintains 100 PSI so you can use this tool with a smaller compressor or even a tank of compressed air from the gas station, if you're really quick connecting the hose from the spark plug hole to the gage set.

So, with the HF leakdown tester, it's not the working pressure that you are interested in. It's the percent leakage shown on the "Cylinder Leakage" gage. The "Cylinder Leakage" gage reading (as a percentage) does the math for you.
 
#7 ·
JE recommends 0.0045 per inch of bore for NA, 0.0055 per inch of bore for Nitrous. (81mm=.017")4% is more of a ProStock number for me. I wouldn't pull anything apart until I hit 20% on a nitrous motor. Also, check it warmed up. Cold motors can give false readings.
 
#9 ·
20%? holy $hit. maybe on a car engine, but not a bike. A well built motorcycle engine should have no more than 2-4% leakdown at the most due to how small the displacement is. Anything over 7-8% and I pull them apart and freshen them up. You will see a mph lost if your motor goes from 3% to 8%.
 
#16 ·
I use a leak down test almost every single day at my job on internal combustion engines, maybe more like once every two to three days. Either way my point is that I have alot of experience with using the leak down tester as a diagnostic tool. I almost think that this tread is people over thinking about it. I can promise you that if you have a any valve issues or ring issues you will know it! if it is over 20% there is a problem somewhere period. i have seen cars with 15% leakage with almost 200,000 miles on them and still running fine considering the wear on cylinder walls and rings, so i believe that a fresh motor with even too much ring gap wont even hit 10% on an up too temperature engine, would almost put money its not even to 5%. of course, i dont use cheap tools not that they dont work will cause i dont know that, its just when it comes to any measurement type tool I only use will known name brand tools. your engine will most likely have more than normal crankcase pressure so you may end up need to use a catch can on the crankcase vent. burnt exhaust valves will usually put the percentage around 30-40% and bent valve or damaged seat will be 80% or more. for those that are interested, OTC is the company that makes most of they tools similar to leak down testers for other tool companies like matco, mac, cornwell, and so on. so if you buy OTC brand you cant go wrong.

And the comment about the not tearing into unless its more than 20% i would agree with ONLY if you can hear or feel the are coming from the crankcase breath hole, if its blowing out that exhaust or intake then tear it down and dont spray it but i would put money that if you got a valve seal issue its going to be well over 20%. this much past the rings will only make the engine down on power and excessive crankcase pressure.
 
#17 ·
And the comment about the not tearing into unless its more than 20% i would agree with ONLY if you can hear or feel the are coming from the crankcase breath hole, if its blowing out that exhaust or intake then tear it down and dont spray it but i would put money that if you got a valve seal issue its going to be well over 20%. this much past the rings will only make the engine down on power and excessive crankcase pressure.
I agree 100%. I don't use the tool professionally and I assume I am adding some measure of error. I know the spark plug thread isn't sealed as tight as I'd like. I know when my gauge gets over 20%, I gotta start digging. 10 or 15% past the rings, no big deal for me as long as the valves are good. 20% sounds like a lot, but I don't think it is. That's MY number. It's a compromise with my wallet. I can't afford to keep it much tighter and I can't afford to let it get any worse. If I do, bad things can start happening in the ring lands.

I agree that if the valves need attention, it's fairly obvious and rarely have I caught them leaking 'a little'.

If I were running total seal rings and a vacuum pump, I would log crankcase vacuum data and have a much better idea where I was. I'm just not willing to go that far for where I'm at.
 
#22 ·
the ONLY reason i spoke up on this tread is because i can give someone my actual experience and knowledge and not just a theory which is what you will get from most poeple on here. Good luck.
 
#23 ·
Calm down, Big Guy...no one and especially myself was trying to offend anyone.
I mentioned Steve as I know, as Bob Carpenter's engine guy for years, he had assembled well over 1000 motorcycle engines, and continues doing the same thing today, years later. His motors hold a good number of records of LSR records including the fastest motorcycle in the world, Bill Werner's. He is familiar with NA, Turbo, and nitrous motorcycle motors. He has over 5000 1/4 time slips of his own and has run over 250 at the Bonneville Salt Flats and he's a hell of a nice guy to boot. His credential's speak very well for him, don't you think?
And for another example, a friend had a turbo kit built by Richard at RCC on a streetmotor with miles. The first thing Richard did was leak it down and said if it wasn't tighter than 6%, he wouldn't do the kit. He's got some pretty good motorcycle credentials too.
My point without getting into a pissing match was I though there was a bit of misinterpretation of when a motorcycle needs to be looked at and I only offered Steve as someone I'd qualify as an expert, on motorcycles.
I've built some motors myself and am well familiar with leak down...If I get a motor that leaks more than 10%, it gets fixed period, but that's just me.
Hank
 
#24 ·
Lots of things can affect the readings, basic leakdown tester setup, no 1 ,
from a practical point of view , as mentioned above occasionally, if its not breathing excessively its fine
too tight in the rings will cause major problems as the ends can meet when the piston gets hot, damaging piston and cyls
my turbo motors are set to .020, and i can't tell you what the leakdown is at that,
the landspeed bikes don't get small problems , if i have a leakdown issue i can hear it at cranking and or see evidence of excessive blow by
 
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