Compression problems? Here’s a tool you shouldn’t be without


I got a message last week from my friend Craig Fitzgerald, asking for help with his 1965 Corvair Monza. As president of the New England Motor Press Association and editor-in-chief at BestRide.com, Craig has his hands in a number of automotive-related pies. But not surprisingly, we have the most fun when we’re just two regular car guys. This was one of those times.

This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2018/04/16/solving-compression-problems-with-a-leakdown-tester


Leakdown testing is a standard test in the required annual inspection of a piston aircraft engine. It is an easy and excellent way to find intake and exhaust valve leaks. Another tool used in aircraft engine inspections is a borescope. Once an expensive tool, excellent borescopes are now available for under $200 that connect to notepads and smart phones.

Google “mike busch aopa borescope” to find several articles describing cylinder inspections. Much of this carries over to car and motorcycle engines. I now scope all of my engines when doing tune-ups. It is easy and effective way to spot and head off problems.


One important thing to remember about finding TDC using the crank pulley, especially with American cars – when the TDC line is marked on a rubber-coupled harmonic “balancer” (in quotes because they don’t balance anything), if the balancer is more than a few years old it has probably broken loose and no longer indicates TDC.

It’s easy to test for this – with the #1 plug removed and a safe object* inserted in the hole, touching the top of the piston, turn the crank so the line on the balancer lines up with the pointer on the timing cover.

  • A “safe object” is anything that won’t jam in the cylinder or scratch the piston crown. I usually use a #2 Phillips screwdriver because it’s blunt-ended and big enough not to bind up, and easy to watch and remove when necessary.

Is the piston at TDC? It better be. If the balancer’s line doesn’t line up with the pointer at true TDC, you need to replace the balancer before you go much farther, and absolutely before you can call the job “done.”

Otherwise you’ll waste a whole lot of your time, much more time-value than the cost of a new balancer.

I’ve been the victim of this before, and I’ve seen far too many others fall victim, too.

Re-marking a failed balancer is a waste of your time and your customer’s money, because the outer ring of the balancer will rotate against the inner hub every time the engine starts, stops, or is revved up.

You can usually tell whether the “balancer” has broken loose by checking the timing at idle with a timing light. If you don’t see the line on the balancer, replace it before proceeding.

A broken balancer is usually accompanied by an unnecessarily rough idle, especially if you can decrease the idle (older carbureted cars only, please) below 500 RPM. A decently tuned, mild V-8 should idle down to 350 RPM before stalling from low air flow. Anything that can’t idle below 1000? Suspect the “balancer” – and know that if it’s broken, you won’t be able to tune the car anyway. much less do more involved diagnostics.