After a lot of discussion I've noticed on here lately about machining and rebuilding 6.0's, I thought I'd post a write up and some pics about the process. I hope it sheds some light on the subject to those who aren't familiar with the machining process in general, and to the greater intricacies of the 6.0 in general...or at least helps explain why it costs so much. If anything else, perhaps you will see pictures of parts of your engine we all hope to never have to see.
I work for a company that rebuilds roughly 200 6.0's a year, along with the 6.4, 7.3 some cummins, duramx, cat and others. This is not by any means a sales pitch, I won't mention my employers name... just hopefully helpful to those looking at having their 6.0 heads machined (which happens to be most of us given enough time).
Basically the process starts with the engine core being taken apart, evaluated to make sure it's rebuildable, and cleaned. The cleaning entails two chemical spray tanks ($40k), a Goff Shot Blasting Machine ($90k), an Ultra Sonic machine ($10k), and various other manual cleaning methods (glass bead machine, parts washers, ect) Then blocks, cranks, rods and heads magnafluxed (checked for cracks through magnetic particle inspection), and evaluated. I've never seen heads crack as badly as they do on 6.0 heads. I would say that out of 100 heads, 98 of them would have cracks, and out of the 160 seats in those heads, and possibly 100 -120 seats would be cracked. Navistar calls these cracks Micro-Fizures and says they're ok. I think they're kind of covering their butts, but I will say I personally haven't seen any cracks that were bad enough to go into a water jacket.
My job is to cut the valve seats, valves, mill them, and to set the valve height, and valve recession, so that's what we'll focus on first. And, oh yeah... we also remove the injector cups and the glow plug tubes...
Here is some pics of some of the tooling required to properly check the valve guides... called a dial bore gauge. Typically we look for valve to guide clearance to be .0020-.0022"... any more and they'll have to be drilled out and have new guides pressed in.
Note the clearance of the guide shown.
And some of the tooling to drill out the factory integral guide, and then ream the new guide to the correct id.
Now, after all the guides are taken care of... it's time to tend to the seats. The guide is what centers the valve to the valve seat in the head, so the guide work has to be done prior to the seats being tended to... if you had to put a guide in after cutting the seat, the valve might not be concentric to the seat.
Since 6.0's crack the seats so badly, we press a hardened stelite seat in almost all 6.0 exhaust seats, and in the intake seats that need it. This is how all of the big rig motors come, like the bigger cummins, and cat motors.
Aside from the cracks, there are other reasons for doing a valve job, either seats will be out of round, or the surface finish of a seat with many miles is less than desirable. Like this one...kinda hard to see - but generally it looks very "textured"
To put a hardened seat in, the factory seat that's an integral part of the head (non-removable) must be cut out. Here is how they look after being cleaned, magnafluxed, but not yet cut.
A cutter bit is centered to the valve guide via a pilot, which sticks out of the guide. Pilots are made from carbide, and run $180-250 each. We have a few of them...
They range from 5.5mm to 3/8" and increase by .0004", or 4 tenthousandths of an inch. One for every size guide
The cutter bit is set to cut a hole .008-.010" smaller than the outside diameter of the new seat, which by the way looks like this.
And here's some of the bit being set with a digital micrometer, accurate to .0001", or about 1/30th of a human hair.
The cutter bit will cut to the depth (thickness) of the seat down into the head, and then the new seat is pressed in. Since the seat is bigger than the hole it's going into, press fit, or interference fit is what holds it it (and yes, that's all that holds it there, and no... none of mine have fallen out, yet - knock on wood) Here's some of how that process goes...
The reason this is an acceptable repair is that if a crack only goes .200" deep into the head, and you cut .250" to put a seat in, then you've effectively cut the crack out.
After that, the valve seat angles (typically 3 - hence 3 angle valve job) are cut into seat. This is achieved by setting yet another cutter bit, one for intake and one for exhaust (intake angle is 30* and exhaust is 37.5*)
If you look very carefully, you can actually see all 3 angles of the cutter bit, 15*, 30* - which is the same angle as the valve, and 45*. Three angles are used to make the 90* transition down into the cylinder less harsh...so it transitions gradually.
This is again lowered onto the pilot, which centers the valve to the seat, and a cut is made to the proper depth, to achieve a valve recession of .035".
The depth is checked either with a depth micrometer or a depth dial indicator. I set the recession of the valve to .035 so that way when it's milled, the recession will be around .030-.032". The specs are .013"-.027" but there's been reports by many of piston to valve clearance issues on heads with valve recession well into the specifications. Setting them higher than the specs doesn't hurt anything, other than maybe lowering compression by .02-.03, which is offset by having been bored over anyway.
After that, the rest of the seats are cut, and then it's off to the mill.
If you look closely you can make out the 3 angles on the seat.
Among the many great reasons for milling heads is to remove any imperfections left by the head gasket, namely the fire ring (and, that's the fire ring that all head gaskets have, not high performance o-ringed heads, that's much further down the line). Second to removing gasket impressions, would be to insure that the deck surface is indeed flat, and also to insure that the surface finish is smooth, particularly after being shot blasted and scrubbed on. Here's a picture of the head gasket impressions left on the head prior to machine work.
Here's a rather severe case of a duramax block that had already been milled .005" and still hadn't removed all of the fire ring marks out of the deck surface. Note the slight ring around the cylinder...and the funny looking blotches inside around the top of the cylinder. Isuzu (who makes the duramax) hardens the tops of the cylinders to promote better cylinder wall wear.
Here's a picture of the Rottler SG8 machine that does all of the seat and guide work.
Tomorrow I'll get some more pics up in regards to milling them, and setting valve height, and re-facing the valves. It's late for now, but, I hope this is worth the read. Thanks, Jared.