Author Topic: Leak down test  (Read 239 times)

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Offline 1of1044

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Leak down test
« on: September 19, 2018, 09:31:56 pm »
When doing a leak down test I remove rockers, position piston at TDC, and set regulator to 100psi and take reading. Have been told to always use TDC and that using BDC is wrong. With using BDC there will be more volume of air but using same 100 psi so I don’t see the difference. What are your methods of using leak down? Thoughts/comments please.
Thanks

Offline rockhouse66

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Re: Leak down test
« Reply #1 on: September 19, 2018, 10:54:13 pm »
My only comment would be that you don't much care what the leakage is at BDC compared to TDC.
Jim

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Offline redscj

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Re: Leak down test
« Reply #2 on: September 19, 2018, 11:05:39 pm »
     I agree, Tdc is where the most cylinder wear occurs. So why test anywhere else? Ii'm thinking that bdc would give you false good numbers.
Grant
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Offline 1of1044

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Re: Leak down test
« Reply #3 on: September 20, 2018, 05:24:47 am »
Ahhh....makes more sense to me now. Trying to “over think” I guess.
Thanks Jim & Grant!
Jim...It was good to see you at Norwalk! What a great show!! You & the Grande gonna make it to Henderson NC show?

Offline rockhouse66

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Re: Leak down test
« Reply #4 on: September 20, 2018, 07:16:28 am »
Gary
I promised JH I would do my best to be there this year but, once again, I have a schedule conflict that weekend.  So no.  Great show though and I miss going.
Jim

'69 Grande CJ

Offline LightWtJet

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Re: Leak down test
« Reply #5 on: September 20, 2018, 09:44:15 am »
I am curious as how this is done and what results you get.
Ford had a durability test car that would occasionally blow the coolant out at WOT - turned out there was an intermittent head gasket leak - finally found it by over pressurizing cylinders with nitrogen.   
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Offline redscj

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Re: Leak down test
« Reply #6 on: September 20, 2018, 10:47:13 am »
     I'll venture to post my understanding of the process.
     The crank is blocked so that it won't rotate.
     Valves are to be closed. Removing or loosening the rocker arms is the normal way to do this.
     Spark plug is removed & a threaded adapter is installed.
     A known steady air pressure is applied to the cylinder.
     A special (flow) gage will show how much air needs to be added to the cylinder to replace the air that "leaks".
     I think that most people will time how long it takes for the initial air fill to "leak down". i.e. out of the cylinder.
     The more air that the cylinder needs to maintain pressure or the faster the original air leaks out is an indication of how well the cylinder is sealing at the the piston rings, valve seats or heaven forbid the head gaskets.
     You can listen at the oil fill hole. If hissing is heard then that would lead you to know that the valves aren't sealing very well. If you have the ability to listen in the crankcase. Hissing at the piston skirts is air leaking past the rings.
     I apologize if my description isn't quite right. The basic test should help you decide the overall condition of the rings & valve seats. Btw, they all will leak, how fast is the question. Pretty much a standard test amongst the racers. I hope that this helps.
Grant
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Offline 161854

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Re: Leak down test
« Reply #7 on: September 20, 2018, 11:10:35 am »
   Roger,
         A cylinder leak down test is a fairly simple deal using a specific dual gauge tool . It is done with the piston at TDC and both valves closed ( as if the cylinder was firing) Regulated air pressure ( 100 psi on gauge #1) is put into the cylinder  and the difference read on gauge #2 (for the cylinder) is the "leakdown percentage". "Normal" ( piston ring) leakage is from 3 (really good) to 10%. If the leak is higher , the leaking air can often be heard it the tail pipe , carburetor, or valve cover breather , and on very rare occasions , bubbles in the radiator. This is because there are only four potential sources for a leaking cylinder. Many "older " engine builds can have up to 18-20% leakdown and still be strong runners. I use a the leakdown tester as a "relative tuning aid" to determine if a cylinder changes from it's initial number , up or down.
   It is not an "absolute".
      Randy
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