Note to students:

In the article the example engine is an air-cooled Corvair engine for an aircraft application. It is unlikey you will see too many of these, but the principles and practices explained here can apply to the engines you do work on.

The heads and case that came with my "Wisconsin" engine were pretty corroded on the outside (thanks to salty roads), so I bought another engine from a guy in Birmingham (again, for the outrageous sum of $100!). It had a "fresh" valve job on it, but I thought I should take the valves out and take a look at the quality of workmanship.

Get a look at this valve port corrosion that I found underneath! This is from water sitting behind the valve for DECADES until somebody decided to do the valve job on it! And ALL of the ports looked that way! So it turns out my externally corroded engine is in far better shape than the clean one with the heads freshly "rebuilt". It's this kind of stuff that makes me glad I'm skeptical! The guides had been reamed out to the next size and oversize valves installed, but the ports were so corroded I could never trust them not to loose a chunk of aluminum, not to mention flow problems, so these are junk. I wasn't real impressed with the quality of the "valve job" either. The stem/guide clearance was actually way too tight as well. So I used the "externally" corroded heads, which cleaned up very nicely with a little walnut hull blasting work.

A large C-type valve spring compressor can be used to remove the valves and springs. A good whack on the top of the compressor when you get close to compressing it all the way will help break the keepers loose. No, I don't normally use a claw hammer to work on engines, but my wife was using the ballpein. What a woman!

Here's the setup for measuring valve guide play. The spec is .001"-.0027" for intakes, .0014"-.0029" for exhausts, according to GM's manual. Up to .005" is OK for "used" guides, but since all of mine are more than .005", out they come. This is not something that you want to do "just to be thorough". If your guides are not out of spec, don't mess with 'em! For aircraft use, we want to be on the high side of the tolerance anyway, due to the long duration of high heat usage. And there's the matter of valve guide concentricity that we'll talk about in a minute.

Removing guides is done with the official tool from Clarks', but you could make one from a piece of hardened steel with a lathe if you really wanted to "save" $10. Heat the head to 475 degrees in your oven for 20 minutes and drive them out into the chamber side.

If you have been living "right", this is what your valve guide bores will look like.

If you haven't, you might get one like this, which shows some galling as the guide was driven out. I worried about the fit of the new guide, but it was just as hard to drive in as the rest, so I'm convinced that it's just fine.

Installation is reverse, again by heating the head to 475 degrees, but they come in from the spring side, and the guides should come directly from the freezer and be lubricated with grease. That's the black stuff that you'll see around the guides throughout the rest of this page. Also, I marked each bore with an I or an X to show me where the intakes would go and where the exhausts belonged. When that head is cooling (and too hot to handle!) you don't have time to be wondering which one goes where. And I wouldn't have wanted to drive one of these back OUT again!

I chose to use the racing quality Otto valve guides, which are beefier on the top end and tapered for better flow on the intakes. Regular ones are just fine, but I like to build in all the reliability and performance that I can, since I'm going to this much trouble anyway. One other difference is that Otto guides have no provision for mounting valve guide seals, which aren't necessary or desired as long as your stem/guide clearance is up to spec.

Initial reaming is done with a hand drill and a .3420" reamer, working up to my eventual .3435", on .0005" increments. My valve stems were .3415", so that's .002" of clearance, right in the middle of the "new" range.

Before you put any of your new valves into your freshly reamed guides, make sure that there are no burrs on any of the valves' keeper grooves. I used a worn out "point" file for that job, but didn't find anything to worry about.

After reaming to the smallest diameter that you think you'll need, clean out the bore and insert a valve and check it's stem to valve clearance (like I did originally to determine if I needed new guides). Mark this valve so that it always goes back in the same guide. Tolerances on new valves are pretty close, but they are still not identical. You can use this to your advantage by swapping them around until you get a perfect fit. All of my intakes were slightly larger than my exhaust valves, which I think is a deliberate attempt to give larger clearances on the exhausts, since they run much hotter than the intakes.

An 11/32" pilot shaft is inserted into the valve guide for a grinding stone holder to ride on. The blue stuff is Dykem machining dye, which makes it easier to see what you're doing. This seat has already had the seat cut. Astute readers will notice that this head's not been bored yet. That's because this picture was made when I was doing Mark Jones' valve job. I should have made a "before and after" shot, showing the little pits of seat erosion that would have eventually promoted "burning" a valve face.

The grinding stone is spun by a special right angle drill made for the job. Here you can see the sparks flying as the seat is reground. Let the record show that this is definitely "old school" valve grinding equipment, bought cheap at a "real" garage sale, where the mechanic was retiring and closing up shop. This works fine, but is more work than the new method, the "Neway Valve Seat Cutter". See more on that at the update down at the bottom of this page.

Sometimes you'll discover that your guide is not concentric with the valve seat. See how only the left half has been resurfaced, but the right side is untouched? Don't worry, it's nothing that you did wrong...

...it's something GM did wrong! At the time, I didn't know how GM managed to make a valve guide as screwed up as this one is, but I found about four of them in my head that any bozo could see were not drilled coaxial to the outer surface. But after a little extra grinding, all is well with the seat but now your valve sits lower in the head and this chamber now has more volume than the rest. Somebody later told me that GM drilled the guides out at the same time they faced the valve seats, so I guess that explains the lack of concentricity with the guides. It makes replacing the valve guides more of a chore though.

Now that the seat is refaced, you need to return it to the proper width, along with all the others.

After the seat is refaced, it's painted with dye again, and narrowed to the specs (.030"-.050" for intakes, .060"-.070" for exhausts, according to Fisher's "How to Hotrod Corvair Engines) using 60 degree and 30 degree stones. It is also best to raise the contact area on the intakes so it's on the outer 1/3" of the valve face, while the exhaust face's contact area should be centered. This is what's known as a "three angle" valve job, and costs extra at the machine shop. If you didn't ask for it, you didn't get it. And you might not have gotten it even if you DID ask for it. You could argue that any flow work done won't contribute much at the low RPMs that we run at, but I can do it, so I am. But I'll be the first to admit that most CorvAircrafters would be wasting their money to go to this extreme.

Also notice in this picture how all sharp edges of the combution chamber have been removed with a dremel tool or die grinder and then sanded smooth with sandpaper. This helps prevent pre-ignition from originating at the red hot glowing sharp edges, which is very important to avoid in an engine that's going to run most of it's life near full power. Later I'll polish the whole thing to reflect heat back into the chamber and to minimize carbon buildup. I've also used the die grinder to clean up all the gross casting flaws in the ports, and radius the ports in places where it's obviously needed. I also used 600 grit sandpaper to round the edges of the valve seat cuts to improve flow there. But unlike the three angle valve job, anybody can and should round off the sharp edges in the chamber to prevent pre-ignition. A couple of sheets of sand paper is all you need. Also note just below the right valve there's a little area of casting that wasn't quite removed when the head was bored for the cylinders at the factory.

The valve facing work is to "lap" the valves using valve grinding paste. A little of this abrasive concoction is spread on the valve seat and then...

a short length of rubber hose attached and then...

pulled upward and rotated to "lap" the valve to the seat, making a perfect seal.

The resulting indication of where the valve seals is clearly visible on a new valve, but Prussian blue dye (Dykem) is helpful if the valves are used or previously lapped. Check to ensure that the contact area is the proper width and stays that way all around the face, although it almost HAS to at this point! If it's not, it's because your valve is bent! It looks like this one can stand a little seat narrowing still, but the contact area is centered, as it should be for an exhaust valve.

These valves are Corvair Underground's stainless steel swirl polished valves, with chromed stems and stellite faces. They are only slightly more expensive that the other stainless valves, at around $10 each.

Next you need to "cc" the combustion chambers so that you can set up your desired compression ratio. First install a sparkplug and torque it to specification. This is a good time to find out if you need to Timesert a plug hole, rather than AFTER the engine is assembled. Then install the two valves with a thin coat of grease on the faces to prevent leakage. Next I cut a disk out of 1/8" plexiglass, and drilled two holes in it, one the size of my pipette and the other for an air escape hole. A thin film of grease is smeared around the edges to seal the chamber, and the plate is inserted into the chamber and rotated a little to seal the grease. Level the head with a torpedo level across the machined fin surface. I wrote "94mm Corvair" on it to identify it from one of several others I've cut over the years, mostly for VW work.

I used kerosene for this, since it flows easily and is visibly red. Some people use ATF but it's far more prone to having bubbles get stuck under the plexiglas and mess up your readings. I sucked the kerosene up into the 10cc pipette and quickly held my finger over the top end and gradually released pressure to drop the level to where the miniscus (the lower edge of the U shaped top surface of the column) read exactly 10cc (or ml, same thing), making sure that I was viewing the pipette directly from the side, while the end was still submerged in fluid. Then I transfered the pipette over to the plexiglas disk and released my finger to empty the contents into the chamber.

Do this four times, watching the bubble of air and making absolutely sure that the fluid doesn't escape out one of the holes, just the air. This is where previously leveling the head comes in handy. I also keep a wooden "builder's shim" (for shimming doors and windows) under one edge of the head for minor corrections to keep the bubble headed up toward the air escape hole. Be careful not to get carried away though, because when you remove the pipette to reload it it might come out THAT hole, so you want it just slightly out of level biased toward the air escape hole.

When the air bubble disappeared completely, I had released 48.3 cc's into the chamber. The second one was 48.7 ccs and the third one (number 5) is 49.1. That's in line with my theory that GM didn't bore this head the same depth on all three cylinders. Maybe the valve cover surface isn't in the proper plane assuming that's the flat surface they use when milling the chambers for the cylinder bores. The other head is 47.5,47.8, and 47.8. Now all I have to do is find the largest one, and remove a little aluminum from the five smaller ones so that they all match within .1cc.

I wanted to get an idea of how much 1cc of volume looks like, and this is a picture of it. Right now I have a spread of 1.6 ccs, so I have some work to do. I may only do one more iteration, but the result will be combustion chambers of similar volume, which will give me similar power from all six cylinders. And more importantly, I will KNOW what my compression ratio is, rather than guessing. This is part of the "blueprinting" process of eliminating manufacturing tolerance stackup. You might have noticed that my cylinder number 5 (about 10 pictures ago) either wasn't machined to the same depth (by the factory) or the casting wasn't quite as tall, so there is a definite need to see which is the case.

But if for no other reason, you need to cc at least ONE chamber (the middle one) from each head so that you can average them and calculate your compression ratio and adjust it with shims to whatever your desired CR is. I'll make mine 9.25, just like the stocker,which is rumored to have actually been closer to 8.25 by Fisher, despite the factory claim. [Mine eventually ended up at 9.02:1, which is fine with me]. Even the GM specs listed on my web page list the CR as being in the 8.7:1 range. Keep in mind that even IF the factory CR claim was correct, boring out to a larger diameter piston automatically raises the CR somewhat.