Before I tear into my 73 L82 engine to do a intake, head and cam swap I want to do a health check on the bottom end. A compression test is easy enough but I also want to perform cylinder leakdown test and have a good tester for that as well. The process is pretty straight forward perform on heated engine, bring cylinder to TDC than add 100 psi air and read gauges for % leakdown. There seems to be a number of opinions out there on how to do this, one plug at a time, all plugs out, crank engine to find TDC on each cylinder etc. Some even go as far as saying remove all the rocker arms to ensure valves are closed, I'm not going there.

Bringing #1 to TDC and testing is no big deal and by turning the engine 90 degrees for each cylinder TDC I walk the process through the firing order 1-8-4-3-6-5-7-2. All good in theory but I don't have a degree tape on the harmonic balancer and while I can get a socket and ratchet on the balancer bolt there is not a lot of room to ratchet the motor around. Seems this might also cause error readings if the cylinder is not at true TDC and valves are closed.

For those of you who have done this with the engine in the car what was your process for checking all cylinders?

Thanks
Dennis

 

Ok.....start at #1 of course. Now put three marks 90 degrees apart from TDC mark on the balancer. Each mark as you turn to it is the next TDC in the firing order. Nothing else really matters.....but I take all of the plugs out just so that they are out and you can continue the process seven more times without dicking with taking a plug out each time.
Lars and I disagree somewhat as he says that you can do the leakdown with the piston anywhere......as 100psi will push a piston down the bore. My argument was on a wore out engine there is taper at the top of the bore...the "ridge".....his argument is that if it is wore out that bad.....the numbers even at the bottom of the bore will be low too.....we are both right to an extent.

Jebby

 

Thanks Jebby, that was pretty much my plan for dealing with the 90 degree marks. Lot of people have the same thoughts on testing with the piston at the bottom vice top more for keeping the engine from rotating when the air pressure is applied. I'm not going to make this any more difficult than it needs to be. It's not rocket science.

 

Let me add something interesting here.
I have to do the leak down test every 100 hrs of operation or at each inspection on aircraft engines.
Ours is 80 psi and we start at TDC. If below 60 PSI on second gage we move piston down a bit and yes it wants to go to the bottom, then back up to TDC. This allows the air to get behind the rings as the combustion would and push the ring against the cylinder wall and will get a more correct reading. this is almost impossible on cars.
We had a engine that was using a bunch of oil and also blowing air out of the crank case. Leak down was in the range when at TDC. We slowly moved the piston down the bore and about half way down the reading went to 0. It was the wrist pin that was a full floater that lost its pad, which would be equal to the teflon pads used years ago that were replaced with clips to hold the wrist pin centered.
Our pistons are press fit so that isn't a problem unless you have full floating wrist pins, then the pin can score the cylinder lower travel if the pin comes out, or forgotten.
In this case the pin ate the lower half of the cylinder wall and when at the top (TDC) there was a good reading, but half way down the bore the reading went to 0 and caused the engine to create crankcase blow by and blow oil out the breather.
So, thats one reason to check the complete travel if there is a suspected cylinder problem.
Dom

Dom

 

Arn't airplane engine cylinders tapered? Larger at the bottom than the top?

 

Yes, some are and are called choke cylinders. Bore is about .002 smaller at the top. Then as you already know, when the cylinder heats up, bingo, the bore is the same top to bottom.

Dom

 

No , I didn't know that part. My son has 2 planes that he flys. He mentioned something one time about the cylinders being tapered. About the closest I got to a plane engine is my corvairs.