This is the fourth topic in the series about the 100 hour / annual inspection. This series focuses on maintenance performed during an effective 100 hr. inspection on R1340 and R985 engines:
- Oil Change with filter/screen & sump checks.
- Valve adjustment – Positive or compression.
- Ignition timing check – Spark plug servicing.
- Compression check – differential.
- Air filter and carb – heat system check.
- Fuel System Screens.
- Cylinder Head Checks.
Compression checks: These checks are valuable tools in determining the health of an engine or cylinder. However, a mechanic needs to be familiar with larger displacement engines in order to get the most out of it. For example, a reading of 65/80 in a 0-200 may be cause for concern, but in a 1340 it may be that the reading should be noted and the cylinder rechecked at the next opportunity! Those of you familiar with some larger flat-engine cylinders may be aware of a service bulletin dealing with upper cylinder wear and compression checks. The technician was instructed to put a larger orifice in the compression test gage set and compression-check the cylinders. A compression value specific to that orifice was given and the cylinders were allowed to remain in service until the special (lower) compression limit was exceeded.
The piston rings in the radial engines drift around in their respective groves and occasionally the gaps will line up and cause an abnormally low compression reading. I’m afraid I’ve see way too many cylinders pulled on completely normal running engines because the piston ring gaps happened to line up just before a compression check. I have seen many 65/80 cylinders give 76/80 after a 20 minute run-up. If, however, the cylinder continues a downward trend or has an audible exhaust leakage that “staking” doesn’t help, removal and repair may be necessary.
Staking is the practice of carefully hitting an exposed exhaust or intake valve-spring cap with a soft face hammer while compression-test air pressure or shop air pressure is being applied to the cylinder. The sudden opening and closing of the valve while air pressure is applied tends to dislodge any form of contamination between the valve and seat faces.
It is also common to see a compression test reading significantly improve by carefully rocking the propeller with air pressure applied. Piston to cylinder barrel clearances in the 1340 is generally just over .020 inch! Minimizing this clearance is one of the principal reasons it is recommended to conduct the compression test with the engine warmed up. Rocking the propeller may also help seat the compression rings against the lower ring land better reflecting the running position of the rings.
CAUTION: Extreme care should be taken when rocking the propeller while air pressure is applied so as to avoid the air pressure on the cylinder overpowering the person handling the prop and causing injury! It’s best if this person is not the smallest person you have on staff and is experienced with being around propeller equipped aircraft.
Does a reading of 65/80 mean the cylinder should be pulled? Depends! Is the compression check being done as part of a troubleshooting effort to correct a performance issue or as part of required maintenance during a 100 hour inspection? If the engine isn’t performing properly or, has a vibration issue, the cylinder should be pulled. If performance is normal the compression value should be noted and the engine put back into service. Another compression check should be done in 25 to 35 hours to see if the compression has returned or is worsening. If it’s worsening, by all means, pull the cylinder and see why.
By the way, if your R-1340 cylinders all test 80/80 you should repair or replace your tester!
God Bless ya’ll and fly safe!