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By WWI standards the Liberty 12 was an advanced and sound design, but the engine suffered from a short operating life. The Liberty was expected to have a TBO (time before overhaul) of only 100 hours, which probably made some sense in wartime as aircraft often did not survive very long. But apparently many engines did not manage to run that long. The first parts to fail were the connecting rods and the main bearings.
The Liberty 12 came in high-compression and low-compression versions, with and without various types of reduction gear, and was built by a number of manufacturers; this must have resulted in a lot of variation in reliability.
By peacetime standards the Liberty 12 was considered dangerous and unreliable, so in the late 1920s Allison was given a contract to modernize some (2000 - 3000) of the mass of Liberty engines that were still in stock. By replacing the plain bronze connecting rod bearings by steel-backed lead-bronze bearings, and some other modifications, the time between failures was increased from about 75 hours to 600 - 800 hours, according to Allison.
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