Leakdown tester

[Grady Clay]

Chris,

Yes, you are correct. A cranking compression gauge has two valves in it. One is a one-way check valve closest to the engine that lets the compression into the gauge but not back out. The other is a release valve so you can release the pressure in the gauge prior to the next measurement.

I have several cranking compression gauges. The best and most accurate is one that screws into the sparkplug hole and the inlet check valve is in the part that threads in the sparkplug hole. The gauge is fixed and just clears the valve cover enough to reach the release valve. It is made up of a conglomeration of Mac, SnapOn, and homemade parts. It uses a rubber O-ring to seal against the head. However, it only works well on the dyno because the gauge face may end up in any direction.

The one I most commonly used has a taper rubber piece that fits the sparkplug hole, a substantial metal piece that extends about 2” past the valve cover, and a rubber hose to the gauge. This has the inlet check valve in the rubber piece and the release valve at the gauge. This one takes more than twice the revolutions to get to peak pressure because of the larger internal volume. It also has a fitting (with check valve) that connects it to the cylinder leakage gauge adaptor shown above.

I have several of the common gauges with the tapered plug and swivel attached to the gauge. There is also a box of assorted pieces that I used when nothing else would work. As you can tell, I’m not shy about making my own tools or modifying commercial tools to suit some specific purpose.

When you are doing this just for your own 911, get or make the exactly right tool. That will pay off for you regularly.





OK, back to cylinder leak test.

We are going to do a technical review of the article titled “Take a Leak – How to Use a Cylinder Leakdown Tester” by Jeff Smith in the June 2004 issue of Car Craft © PRIMEDIA Specialty Group, Inc.

I have e-mailed Primedia and requested the appropriate permission to post the article here on Pelican. It is a reasonably good article and hopefully they respond today.

Most of what Smith says echoes what has been posted on this thread.

For safety purposes I’ll take exception to one thing Smith says; “… put air to the cylinder and then gently rock the piston around TDC to insure the rings are seated.” While the idea is OK, a 911 can suddenly rotate. Never leave a wrench on the crank or fan when you put air to a cylinder. If it rotates, there is enough power to break your hand or un-screw the pulley bolt. Perhaps a SB Chevy with 7 plugs won’t move far but I wouldn’t take a chance with my hand.


Smith feels that 15-18% leakage on all is not cause for alarm. That may be OK for a SB but a proper running 911 just off the track from a DE event typically is 2-4%. One that is a daily driver around town and has 10%, I would want to re-test after a stint on the highway.

Smith reports; “The consensus among performance engine tuners for the leakage past the exhaust valves that it would have to be massive – over 30% or more (sic) – before it should be of concern.” In my opinion, that is an invitation to a seriously burned exhaust valve and the risk of the valve head ending up rattling around the combustion chamber.

Best,
Grady