I am looking to purchase a die grinder for head porting and want to know what I should look for. Would like a good quality piece that will last. I have an electric grinder but would like to try an air version with adjustable speed. What do you guys recommend.

There will be lots out there to choose from, but which ever one you get, be sure to buy a speed control. The bits are very hard to control without it.
I got mine from HF, it's listed for a router. Very handy. I even use it on my drill to slow it down for larger drills.
FWIW, I got my grinder from HF too. If you cheap out like me, be sure to buy the extended warranty. This is my second one since the switch only lasted 3 starts on the first and then " no clickie."

http://www.dotco-tools.com/tools

Doesn't get better than dotco

You need a big compressor to run an air grinder. The hose can be a nuisance. You want a grinder you can get two hands around - these are not common features of an air grinder. Speed adjustment on air tools is controlled by air volume.

The downside to electric grinders is they get so damn hot you need a welders glove to hold them. Cheap grinders will go through bearings quickly - this seems to be their weakest point. They're also considerably heavier than an air grinder.

I have an old Florida or Chicago Pneumatic(?) 1/4" collet with a "front" exhaust and use a Harbor Freight swivel adjustable air controller.

I don't port for a living but have done my fair share and it has worked out well for me. I like the "front" exhaust models because it don't blow air/oil back in my face and instead blows the chips out of my work area.

You don't have to spend a lot just buy something decent and keep it oiled

Nick @compuflow extended the male side of the air coupler, the one in the grinder, with a pc of black pipe. Gave you something to hang onto , not just the small grinder

Are you porting iron or aluminum heads or both? There is a world of difference between grinding on iron and aluminum
The bits can make a world of difference on how it goes also Buy good bits and use plenty of lube on them when grinding on aluminum

I have an old Makita GEO600 grinder, made in Japan. They may not be made there anymore. It has done a lot of headwork as well as seen plenty of general use in the shop. It has been a great tool and still works well. Also have a Snap-On PT200L air die grinder which I like and use a great deal. Variable speed paddle and not too big. It has been replaced by the PT200AL. Both tools are large enough to be used 2 handed for control.

John C. turned me on to these type of grinders. They are also known as rotary or flex shaft tools and are often used by jewelers. They are electric and pretty quiet though they can spin up pretty high. You hang the motor close to your work and use the flex shaft and tool holder to do the shaping. You can control them with a foot pedal.

The shank on the burr in the pic would be too short to port heads with but you can use most any burr you want, they attach like any other grinder with a collet.

The tool holder makes it easy to use, you have relatively finite control.

Mine is a Dumore but there are other brands out there that do the job.

I buy the cheap die grinders from harbor freight. A guy down the street from me claims to port heads and has 1 expensive die grinder. I usually have 13 cheap grinders in rotation setup with different bits. What I can get done in 15 hours takes him 80 hours. He wastes time changing bits all the time. I basically screwed his world up and no one goes to him any more.

the skinny body cheap die grinders from HF seem to work better and I have both straight and 90* grinders. 90* grinders seem to help with leverage and I can cut more material faster.

need an air dryer close to your work station and I have a ball valve to control air on the tool.

when my 5/3 hp motor went south I bought a real 5hp motor. I also have fans blowing on my compressor when ever I'm hacking

I cut a round sponge and put it in a heavy plastic cup and have enough wd40 in cup to keep the bits cool

I have one like that I've also used, made by Dremel, set up for 1/4" shank burrs. I don't think it's even made anymore. Very handy. It would be worth picking up if you can find one.

I want something that is comfortable to use, I would imagine a longer grinder would be better. I will be using it on aluminium and iron. Who makes good quality bits? Thanks for the replys guys.

You can't beat the Dotco die grinders, short neck, long neck, right angle and pencil grinder, for good bits, look at Aircraft Spruce or one of the commercial machine cutter companies. Carbide for steel, high speed steel for aluminum.

An air grinder can keep a 5hp compressor very busy making lots of heat and noise. Very inefficient.
I bought a flexshaft setup and love it. I was too cheap to buy a complete FOREDOM setup (look 'em up). I bought the $50 HF flex shaft grinder with throttle pedal and 1/4hp motor and swapped the 3/16" die grinder for the Foredom 44T 1/4" grinder (another $50). The 44T is small and light and will fit in tight places while the HF motor makes plenty of torque/power. No ear protection required and the thing just hangs there, always ready to use. It's not as convenient as an air grinder for moving around the shop but any time there's more than a little to grind and I can take the work to the bench that's what I'll do.

The Foredom grinders can and will spoil you real quick. Light, quiet, small, and lightweight. I haven't touched my big electric or air grinders in years