Valve Seats

About 8 years ago, I was cruising along in the Bus, heading home from a Bible study. I pulled into the driveway and turned off the key and immediately felt and heard something I had never felt or heard before. Instead of winding down to a stop, the engine came to an immediate halt. It was not a loud sound, nor an abrupt feel, but something just didn’t seem right. I tried to start it up again, but there was nothing, not even a click. I tried to turn the engine over by hand but it was frozen. My mind quickly flashed through several articles I had read about a weak point in the VW Type 4 engine; valve seats.


The air-cooled Type 4 engine was used in the VW bus from 1972 through mid 1983. It was also used in the Type 4 car (a large station wagon), the Porsche 914, and a one-year-only 1976 Porsche 912e. It is a very sturdy engine, and in fact much sturdier than the Type 1 engines used in buses prior to 1972, and in all VW bugs. The main difference is in the alloy of the engine case (that’s engine block for all you Honda owners). Other differences include beefier internal parts such as crankshaft, camshaft, rods, bearings, etc. The engine case on a Type 4 engine is made of aluminum. The Type 1 engine case is an alloy of aluminum and magnesium. Magnesium is softer, more brittle, and actually burns.

(Type 4 Engine Case)

Along with the beefier parts mentioned, the Type 4 cylinder heads are noticeably sturdier. With more metal in the areas needed.

(Inside of Type 4 Cylinder Head)

Small valve is for fuel in.

Large valve is for exhaust out.

Notice the leaks evident at the bottom of each cylinder opening (top of photo)

(outside of Type 4 Cylinder Head)

Unfortunately, the Type 4 heads have a weak point in the valve seats. This is what happens (and in fact what happened in my situation in the driveway): A valve seat is a metal ring that is pressed into the intake and exhaust valve openings of the head. The metal alloy of the valve seat is steel. Heat causes most matter to expand, while cold (the absence of heat) causes that same matter to contract. A soft aluminum cylinder head expands more than the harder steel alloy of a valve seat. As the aluminum valve hole in the cylinder head expands greater than the seat that is pressed into that hole, the seat becomes loose and can actually fall out (into the cylinder – in the direction of a piston) as the valve opens to let fuel in or exhaust out. The seat causes the valve to get stuck in an open position where it is then struck by a piston, immediately seizing the engine. This can cause all sorts of other problems such as a hole in the piston and/or bent connecting rods and crankshaft.

When I tore my engine apart, I found exactly what I just described. It was the same thing I imagined while sitting in the driveway that fateful evening. The number 4 exhaust valve seat had come loose, causing the valve to be struck by the piston, causing a good sized dent in the top of the piston, as well as a bent connecting rod. The resulting repair consisted of a complete tear down of the engine to check the crankshaft and rods for straightness. It also included replacement of a rod, pistons, cylinders, and rings, bearings, and two new Spanish made AMC heads. The repair was very expensive and time consuming.

I eventually got the bus back on the road, but read somewhere (I believe it was Jake Raby) that the AMC heads are good, but that the valve seat problems remain. Jake said he has a process of installing new valve seats that are supposed to stay put. I have logged many miles since with the AMC’s with no problems, but occasionally have it in the back of my mind that someday I may be revisiting this situation, in which case I may give Jake a call. Occasionally, I turn off the engine and listen intently for that sound and feel. So far, so good.

Happy Bus Motoring!
Ron