With Wings As Eagles: Craig P. Steffen's Blog

all I want is a new head

2008 October 01 08:34

For those of you keeping track at home, I discovered in September that the long-running problem with my Beetle is that the right side cylinder head in the engine had been machined too much, reducing the combustion chamber volume and causing those two cylinders to have too high compression ratio, and thus running too hot. A secondary issue is that the valve guides seemd to not have been tight as they should have; the valve stems were sooty.

So I ordered a new cylinder head, from one of the big VW parts vendors. Unfortunately, this was a lesson in who not to order parts from. I've ordered lots of stuff from them before. Some of it fit and worked, some of it didn't. I shouldn't have taken the chance with something as important as the cylinder heads, which is after all a critical component of the engine.

[I currently have 4 cylinder heads in the garage that fit the engine. To give them names: head A: head that came off right side of engine. Machined down too far, valve guides leaking head B: head from left side of engine. Working just fine; will put back on head C: new head from big VW parts dealer. Not enough venting, valve job poor head D: just got this one, venting good, valves good.]

I realized as soon as I got it that head C didn't have as many vent holes above the combustion chamber as heads A and B (photos below). This concerned me, but I was determined to get the engine together, so I went ahead and started measuring the combustion chamber volume. I discovered that one of the exhaust valves sealed so badly that the water I was using the measure the chamber volume leaked out fast enough that I couldn't get an accurate reading. I took the valve out and discovered that the valve seat wasn't ground correctly; it was more than twice as wide at certain points than others.

At that point I decided that it wasn't worth dealing with it, and I should just get yet another head. I'm sure that if I ordered a head from Gene Berg, that it would be perfect, but their heads are very expensive (almost $400). So last week, I ordered a head from VW Parts, which came yesterday. This is head D.

I'm happy to say that the venting on D is the same as my original heads. I put spark plugs in the holes and put some water in the chambers last night. One exhaust valve leaked very slightly, but probably not enough to disrupt measuring chamber volumes.

Now for some pictures. Here's a photo of head B with light behind it:

The light shows where the venting is. This is looking at the head from "below", if the head was on the engine. The airflow in the assembled engine would be coming at the camera. If you divide the head into three regions vertically, the top region is the combustion chambers and their fins, the middle part is the valves and the intake and exhaust tubes, and their fins, and the bottom part is the valve gallery where the rockers live.

Here's the same photo with lines showing where some of the things are. The red box at the top is roughly where the combustion chamber is. The green lines coming up from the bottom to the combustion chamber is roughly where the intake passage goes. The blue lines define the exhaust passage; at the left end of the head you can see the two studs sticking out where the exahust bolts on. The fat purple line is about where the exhaust valve stem is, and the fat yellow line is where the intake valve stem is. The areas circled in cyan (light-blue-ish) are where air flows through the heads to cool the exhaust valve and the exhaust exit area. I have read that these are very important, and eliminating them is bad.

Here's head C, the "new" one that I've decided not to use. You can see the cyan areas here, the venting near the exhaust valve and passages has been greatly reduced.

And here's head D, that I got from VW Parts a couple of days ago. The venting is equivalent to the original head, so that's what I'll be going with.