Due to a variety of reasons my annual has almost become a year duration rather than a year periodicity, but the end is finally near (I think). To be exact, just coming up on the 9 month mark.
My engine is an IO-360 with 950 hours on it, last OH in 2007 and all compressions were above 76 when they were measured at the start of the annual all those months ago and the filter was clean and it runs with Aeroshell W100 Plus. The last flight before the grounding was over an hour which I believe is somewhat important in terms of expelling water vapour from the oil, and I’m not sure what, if any steps the engineer has taken to minimise the effects of the downtime. I do know that the plane has lived in a heated hangar the whole time.
I suppose my question is what can I expect as a result. Am I likely to fall short of TBO by some hundreds of hours, or can such a lay-up cause enough damage so the engine could be in trouble right away?
This is worth a read, especially the PDF here.
You may be lucky, and I would be surprised if anyone can say anything more definite.
You could get a measure by poking a borescope into the cylinders and looking for rust. Also look under the rocker covers; they can accumulate corrosive sludge – especially with Exxon Elite. If it is clean, the next and better measure would be to run it for a few hours and do an oil analysis (and of course cut open the oil filter etc). If you see rust in the cylinders, it is very likely the camshaft is rusty too and then it will get trashed fast, together with the cam followers. Then you will need to get it opened up because these parts “make metal” really fast.
This sort of check should be standard on any plane you buy, before you buy it, but some sellers defeat it by changing the oil immediately before it goes on sale 
And a lot of buyers would buy the plane anyway…
You are not the only one to be in this situation… another guy here is looking at 1 year. Disputes are a common thing.
@zuutroy, I agree with Peter’s advice, but would be less pessimistic about the camshaft.
I bought my Maule from a soggy barn in Ireland, where it had been “resting” for many years. The cylinder walls were quite rusty, but we fired it up, flew it to a repair facility, honed the jugs to the limit and bedded the new rings in for a few hours with straight mineral oil. Now, more than 500 hours later, that O-360 seems to be in rude good health according to oil analysis and filter inspections.
By all means change the oil after a few hours and have a peek at the cylinder walls with a borescope, you probably do that anyway whenever you remove a spark plug, but don’t despair at what you see.
1/ use Camguard. For oil added between changes, open all your oil bottles and add the correct ratio in so each bottle is primed correctly.
2/ in future, don’t accept a mechanic not preserving the engine correctly. Totally unacceptable.
3/ you can open the oil vent after flying for a few minutes to let some moisture out.
4/ use limited prime when starting to try keep RPM below 1,000
5/ send you oil for analysis at Blackstone or similar.
6/ perhaps do an early oil change next to wash out the rust from the engine – maybe at 10-15 hours.
Take charge, don’t just go along with it.
