The corvair has been parked for a little while. It started running badly, then wouldn't run at all. Compression was awful (actually 0psi in one cylinder).
I bought some tools and pulled the engine. It's been a fun project. I currently have the engine mostly disassembled on an engine stand, and I thought I'd post some pictures since this is a such an unusual motor.
Dropping the engine from the car using an ATV jack. At this point, there's a lot of metal shrouding and trim used to route air over the heads and cylinders for cooling, as well as keep the enginebay separated from the underbody. The engine is also coming down with the transaxle attached in this case.
This is the motor with all accessories, trim, and shrouding removed. You can see the air-cooling fins, and it's pretty easy to spot the transition from crankcase to cylinder to head in this picture. The large blower normally has a shroud over it to pull air through the engine cover louvers and blow down through the cooling fins and out through rear thermostatically controlled doors under the rear bumper.
Here's a view up from the bottom. Those tubes house the pushrods, whose camshaft is driven directly from a gear on the crankshaft (no chain or belt). The tubes are notorious for leaking oil with the original oil o-rings where they meet the crankcase and head, but newer materials have solved that issue.
I've got the passenger-side head pulled in this shot. The head studs run past the cooling fins to the crankcase, sandwiching the individual cylinder cans between. The missing cylinder had some severe blowby burn wear, so it's getting replaced. Two other cylinders from the opposite bank also have burn damage, though not as bad. I'll be replacing all three cylinders and pistons before I put everything back together. The cylinder with the worst blowby damage was the one with 0psi of compression, so that would appear to be the culprit.
Overhead view with the blower and top crankcase pan removed. Also shows the oil cooler at the bottom left. Bottom center is where the mechanical fuel pump (pushrod operated from a cam on the crankshaft), oil filter fitting, and distributor (again directly crankshaft driven) all attach on a big cast aluminum piece.
A close-up of the crankcase insides. You can see the boxer crankshaft. Not all flat/horizontally opposed engines are boxers. There are really two types of flat engines: the boxer, and the 180-degree V. Boxers have 180-degree offset crankshafts, meaning each opposing pair of cylinders is at the same stroke position at all times (see how cylinders 1 and 2 at the bottom of the picture are both at TDC). To do this, each has to have its own crank journal. A 180-degree V is exactly what it sounds like: a V6 with a vee-angle of 180 degrees rather than the standard 60 degrees. This means each pair of opposing cyilnders SHARES a crank journal, and as such, they are always 180-degrees opposite each-other in terms of stroke position.
Boxers tend to be much smoother running, because the lateral reciprocating forces largely counter each-other, where with a 180-degree V they compliment each-other.
There are tradeoffs to both, but the vast majority of flat motors are boxers. Ferrari has used both, but the most famous Ferrari flat 12 was a 180-degree V (the one in the 512 Testarossa).
Here are the 3 bad cylinders. You can clearly see the damage on the worst cylinder, highlighted by some orange that I assume is thermal oxidation in the iron (the cylinders are iron, while the crankcase and heads are aluminum). The others aren't as bad, but the second has some clear wear in a similar pattern, and third has some notable scoring that's going to end up the same way if left un-checked. All 3 pistons have visible erosion at the same position as well. Basically, all 3 were suffering from blowby past their headgaskets, causing low compression.