leakdown test

ak · 10-18-2008, 06:01 PM

Hey guys,
I was wondering if anyone could give me a quick run down on how to do a leakdown test,it has been a long while since i have done one,i bought a nice matco tester and i tried it today on my new 509 and when i applied all to the cylinder it would turn the motor over alittle as to move the cylinder off tdc and i would get a 40% leak reading,i know that is not right,also the engine is not warmed up as the engine is now out of the boat,i need some readings for rm builder for a new cam.

stevesxm · 10-18-2008, 06:10 PM

if its out of the boat you have a couple options. best case is you get the cyl dead nuts at tdc and it doesn't rock. if it rocks the numbers don't mean anything.

if you can't seem to accomplish that then you can always swing the valve covers off and pop the rockers off and do it at bottom dead center. the numbers will be slightly less accurate but close enough.

the motor not being warm is more problematic. thats going to give you some higher ring sealing numbers but if its huffing out the valves then that will be definitive.

im not sure why cam selection would have anything at all to do w/ leak numbers unless he just wanted to make sure that the motor wasn't all wore out before he did anything for you.

anything less than 10 % is good. 15 % and i would start thinking about it ... certainly if it was out the valves. between 15 and 20 and i would start thinking it has to be freshend up.

the problem is that if its wheezing out the valve covers and its marginal, i would have to wonder if it got warm if it would seal up.

ak · 10-18-2008, 08:30 PM

Do i remove all the rockers and then still get tdc or bdc i am not sure what you mean,i understand that the valves will not open anymore with out the rockers installed,but not sure about where the piston should be,engine was just built in june,only 20hrs on it,rm builder wanted a compression check and head flow numbers as well,i am going to remove all the rockers tommorrow and try it,any other advice let me know please.

ak · 10-18-2008, 11:13 PM

So does it matter where the piston is in the cylinder or does it still need to be at top dead center?I understand that the valves will now not open but,does it matter where the piston is?

Mr Gadgets · 10-19-2008, 12:16 AM

ak,
Generally the piston is suppose to be at TDC to get a true reading, if you look at a heavly used car motor, the wear will be at the top of the cylinder. They have you use a ridge cutter to get the piston out of the hole. In your case 20hrs, this should not be the case.
Wheather or not you have more leakage at TDC or BDC would be dependant on the condition of the ring seal and cylinder walls. I have a hard time holding the piston at TDC.. a long breaker bar helps and some one to hold it while you mess with the air supply. If the motor spins with the bar on it.. keep your self out of the way!!!
Lets say your valves are sealing good and the cylinder wall is in good shape.. Then it shouldnt make much difference as to TDC or BDC. If there is some scoring or the block isnt sealing due to shape because of temp.. well you may be splitting hairs.
I would always look for small numbers but if I found one that was hissing out the exhaust or intake, I would take the rubber mallet and whack the valve to blow out any junk that may be holding the valve open.
If you dont have some one to help hold the breaker bar at TDC and you cant get it there exactly so it stays there without using a breaker.. I would do BDC numbers and see what you get.
At 20hrs you should see low numbers..
Hope that helps.
Dick