Sure. Actually, the intenet is not a bad medium for talking about these things.
On Allisons, there is a mod that some people run that was started by John Sandberg. The Allison has an olil ring on the bottom half of the pistion, and three rings in the top of the piston. JRS (John Samdberg Racing) came up with a mod by which they cut the bottom of the topside rings wider and put the oil ring there, with two compression rings on top of it, and eliminated the bottom ring. When you do that, the piston slaps around a lot, particularly at around idle and just over idle and you typically must then overhaul the Allison at 250 - 350 hours when the slapping cracks a cylinder liner or trashes one or more pistons.
The solution is to run stock pistons instead of the JRS mod. The reason the owners liked the JRS piston mod is that they could change a cylinder bank on the aircraft, without removing the engine.
If you use stock pistons, you must remove the engine to change a cylinder bank becuase the ring compressors slide into the engine crankcase when you lower a cylinder bank on, and most aircraft mounts do not allow you to remove the oil pan when the engine is in the mount.
Also, if you run the Allison hot, you'll crack a cylinder liner and start venting coolant into the cylinder and out the exhaust. If you continue to fly it, it will fail eventually. Mostly, if properly built and run, the Allison is pretty bulletproof.
The failures I have seen are mostly in tractor and boat engines that are run well past aircraft rpm levels (3,000 rpm). The main indicator is shrapnel. When a tractor guy blows an Allison at 4,200 rpm, pieces fly. In aircraft, I have seen coolant issues due to water pump cavitation (caused by owner mods to the system), one failed distributor (landed OK on one cylinder bank), and cracked cylinder liners due to overheating by the pilot. Otherwise, mostly the odd failed part that was discovered on the ground before flight. Magnetos, improper routing of electrical or oil / coolant, and fouled plugs that the owner is just too cheap to replace!
As for Merlins, while they aren't nearly as strong as Allisons, if you run them in stiock form, they are just fine with stock power output. The center crank bearing retainer is scarce and sometimes gets broken, meaning you sometimes need to find one or make one, but most often not. The biggest issue with Merlins is the about 40 years ago, they used to throw away engine parts when they started wearing out and put new parts in. That was VERY shrt sighted and today they are flying parts they would have thrown away 40 years ago. The bottom line is Merlin spares are drying up and that is a big disadvantage to owning and flying a Merlin. I don't think anyone has done a JRS piston mod on the Merlin (but am not altogether sure), and that is good becasue it would also not be very long-lived.
When they actually break, say in racing or in service when a failure is experienced, I have seen rods and piston go, holes in cases and, in one case, the engine stopped because the camshaft drive failed and the distributor simply stopped. To be fair, I heard that happened once in an Allison in the last 10 years, too, so its not a unique weakness to the Merlin.
One weakness common to both Allisons and Merlins is the radiator. Good radiators are hard to find and are expensive!
Generally, both of these engines are very reliable in operation and last well past typical wartime TBO if built and operated correctly. Neither one is especially suited to 3,000 HP but, of the two, the Allison is better suirted to handle 2,500+ HP than the Merlin die to the strength of the cases, the case studs, and the rods and cranks. Most "
failures" I have seen are from racning, not from typical warbird operations. In warbird operations, the typical issue is overhaul when it is indicated, not engine failures.
The rub is that the people who know how to properly build an Allison or a Merlin correctly are getting fewer. The people qualified to modify one for racing are even fewer yet, and getting less as they get older and retire or worse. The supply of propellers for North American P-51 Mustangs is depleted and they are searching for new blades they can adapt to the existing P-51 prop hubs.
The R-3350 engines are, by far, the worst off, They are simply out of main crankshaft bearings. When the current crop of mains is used up, they will become museum pieces unless someone starts producing new main bearings for a very small market. Fortunately, there is still s pretty good supply of R-2800 mains avaiable, so the R-2800 crowd is doiong the best of any of the bif radials in that regard.
Tell me, what failures have YOU seen while turning wrenches on Merlins and Allisons? Donlt tell me it is too technical and reply with a 2-line post, give us a summary of failure types you have seen and the reasons you uncovered like I posted. C'mon, it ain't that hard to write a paragraph.
Something in there makes you feel the V-12's are lacking something and I have been impressed the other way, with the reliabiolity of these engines. At the Planes of Fame last year, we flew 31 warbirds for two days, anywhere from 2 to 5 flights per day each ... and we experienced zero mechanical failures and not one single down aircraft due to engine or any other system. For what is generally a 60+ year old bunch of aircraft, that's not bad!