This is worth your time to read:
https://www.lycoming.com/sites/default/files/Cylinder Compression.pdf
The short version is:
1) Less than 65 on once cylinder, Lycoming says check it again in 100 hours;
2) less than 60 on a cylinder, Lycoming states indicates a top overhaul is in order; and
3) a 10-15 psi difference between cylinders indicates an investigation as to the cause should be made.
----
That's Lycoming. Many good mechanics will tell you to just go fly it a couple hours and then retest it with the cylinders still at operating temperatures. Some mechanics just warm it up on the ramp before a compression test and that can skew the results. Also, if it has sat for any length of time, a couple hours in flight may well get a low cylinder back in the normal range.
If it's till in the 60-65 range after a couple flight hours and a retest, I'd borescope it before I pulled the cylinder and look for an obvious issue. Low compression just says something might be amiss, it doesn't really tell you what is wrong. And there may be nothing wrong a couple hours in the air might fix.
----
If it's a prebuy and it's out of annual, and you don't want / can't get a proper retest, I'd still bore scope it and look for obvious issues:
a) If you see rust or (worse) pitting on the cylinder walls then it's got corrosion issues causing excessive blow by;
b) If there is rust on the cylinder walls, I'd be even more worried about corrosion on the cam lobes and lifters and the potential for excessive wear and galling than the low compression;
c) the valves should not have a lot of deposits and the exhaust valve should have reddish and gray coloring with a symmetrical bullseye pattern. If you see greenish hues, its an indicator of high temps. Asymmetric patterns indicate a hot spot on the edge of a valve and a warped valve currently or in the near future. That hot spot can be caused by excessive deposits on the seat that prevent proper heat transfer from the valve, or a worn valve guide (caused by excessive deposits between the valve stem and valve guide); and
d) if you see none of the above, a couple hours in flight might resolve it. If not, you probably have a ring problem.
----
The big concern with 7 hours between 2016 and 2019, with no annuals in 2017 and 2018 is that it wasn't flown for an extended period. That can result in corrosion on the cam lobes and lifters. Those parts are face hardened for wear resistance. However, if that surface is compromised by corrosion and pitting, then the exposed softer metal underneath will quickly start to gall, and you'll need a new cam and that involve a very expensive tear down to replace.
*If* the engine was properly pickled with storage oil and desiccant plugs in the exhaust and the cylinders, it might not be an issue, (However the preferred storage method is to do the above, plus invert the engine so the cam and lifters are submerged in oil.)
If the aircraft was stored in a dry climate the odds improve a bit. Similarly, if the aircraft was stored in a hangar, the day to night temperature differentials are lower. That results in both less air exchange in the engine and less condensation in the engine. If it's both hangared and in a dry climate it's less risk than if it was parked outside in a wet climate.
Straight weight mineral oils adhere better to the cam than multiweight and synthetics, and Cam Guard or Shell W100 Plus oil also helps reduce corrosion in infrequently flown engines.
----
Look for all of the above. My major concern would be that it runs fine now, but starts making metal due to corrosion in the engine. That'll be evident in an oil analysis but you also need some basis for comparison.
----
Also be aware that the FAA has been mulling over a wing spar AD for all PA-28s with more than 5000 "factored service hours.
https://www.federalregister.gov/doc...iness-directives-piper-aircraft-inc-airplanes
The short version is that there was a wing spar failures on a PA-28-200R operated by ERAU with >7000 hours, but the FAA is concerned about all PA-28s.
Given the effects of how the aircraft is used in service the threshold is proposed to be 5000 "factored service hours". The number of 100 hour inspections is used as a measure of how intensively the aircraft has been used. The formula is (N × 100) + [T-(N × 100)]/17 = Factored Service Hours, where N = number of 100 hour inspections and T = total hours in service.
This is pretty good news for PA-28 owners with complete logs and no or few 100 hour inspections.
An aircraft with 14,000 hours but no 100 hour inspections:
[0 x 100] + [14,000 - (0 x 100)]/17 = 823 factored service hours.
The same 14,000 hour aircraft with 45 100 hour inspections:
[45 x 100] + [14,000 - (45 x 100)/17 = 5056 factored service hours.
Some commenters objected to the use of 100 hour inspections as it's more complicated and would require complete logs. I don't really get that reasoning as if you are missing logs and can't prove no, or a low number of 100 hour inspections and have 5000 hours total time on the aircraft, you are no worse off than you would be with a mandatory inspection at 5000 hours, which is where the FAA is setting the bar.
Other suggestions were made that the only cracks found in the lower spar cap on PA-28s and 32s were in the PA-28-235 PA-28R models and the PA-32-260 and PA-32-300 models. However, the is applying it to all PA-28 aircraft. The FAA did the same thing with the Champion 7 series. They found failed wing spars in a few 8 GCBC aircraft that had accident histories where the spars were most likely cracked, but then applied the AD to all 7 and 8 series aircraft with 85 or more horsepower.
At best it's an expensive annual inspection requiring eddy current inspection and replacement of the wing bolts (probably $500 per year) and if a crack is suspected it's a very expensive process to remove the wing to confirm, and if it is indeed cracked it'll be around $8-$10K to replace the cracked spar on that wing (and you'd still have to annually inspect the other wing spar.)
This is the long way of saying, check the logs carefully for complete logs and little or no intensive use with 100 hour inspections.
-----
The important thing with a pre-buy is to be fully prepared to walk away. There are a lot of Warriors out there, so don't feel you need to buy this one.
