Ashley Palmer:
My engine, currently undergoing overhaul, turned out to have oversize cylinders. The shop has given me two options:
1. Cerminil the old cylinders to “new” dimensions.
2. Buy new set of 4 cylinders using Continental kit EQ6657 (Service Bulletin SB 00-2).
Which would you do, and why? Everything else is OK, with the exception that the crank is already ground to 0.010 undersize (so I hope it is still OK).
Editor:
The Cerminil process from ECI has received consistently good write-ups in Light Plane Maintenance, and from aircraft owners; considerably better reviews than the more traditional chrome and Cermichrome processes. The Cerminil provides very good corrosion resistance in the cylinders if you don’t fly much, but of course that won’t save your cam, etc. if the plane sits too long.
New Continental cylinders have earned a poor reputation for achieving and holding compression, so much so that Continental simply changed the way they are tested to make the test numbers seem better. When doing a compression test, if you pull the prop through on an 8.5 compression IO520 Continental, and pull it through on a 8.7 compression IO360 or IO540 Lycoming, you can feel a dramatic difference in the effort it takes (much higher compression on the Lyc). In all fairness, the relatively low values seen on Continental leakdown tests and dynamic tests seem to have little bearing on the engine’s performance in flight (assuming there is no valve leakage). Continental has tried to address this negative cylinder history through advertising and warranty services.
If it was a Lycoming, I would say to go with new cylinders for sure. While true for either brand (Lyc or Continental), if you get new Continental cylinder kits, make certain that the shop plans to check them out thoroughly before installing them. LPM and Aviation Consumer have both reported too many out-of-tolerance conditions in brand-new TCM cylinders. You want to make sure that only a set of four good ones ends up on your engine.
As far as a recommendation goes, there is one factor that seems most compelling to me. You said your current cylinders are “oversize”. You did not say whether they are just worn too much, or whether someone actually bored them oversize on an earlier overhaul. You also said that the crank is .010 undersize, so there has been at least one overhaul on the crank, and possibly more; normally a crank would not need grinding to undersize on its first overhaul, unless someone failed to keep the oil clean and changed. The cylinder history is important. If they are “first run cylinders”, with no previous overhauls on them, I would be comfortable with using the Cerminil process to restore them. However, if there is any indication that these cylinders have already been through two or more TBO runs, I think I’d be inclined to buy all new cylinders via the kit.
Unless the IO346 is/was a low compression engine, the cylinders probably see the same stresses in operation as an IO520 cylinder with 8.5 to 1 compression. The older IO520 cylinders don’t have a stellar reputation for a long life. I wouldn’t run those cylinders into a third overhaul. Even though the steel bores can be restored to standard size, the cylinder heads have taken a beating; the aluminum head alloy does have a finite fatigue life. If for some reason you get backed into a corner and simply must have second or third run cylinders restored, have your shop talk to ECI about their stress-relieving process for cylinder heads.
There are other factors (repairability, cost, availability, etc.) you will also consider, but above all you will want reliability and a projected TBO lifespan in the finished engine.