Head machining for head gasket, a must?

 I am probably looking at head gasket replacement soon. On my 97 Miata. It does not overheat, it does not consume coolant, but when I put coolant in it, it does get a little brown after a while.

So I turn to you guys for this answer. I could probably search this on the Internet of things, but I simply do not trust the back-and-forth between some people, and I know you guys will lead me to a general consensus of intelligence and wisdom and prior experience.

So, if the engine has not overheated, does the head need to be machined flat to install a head gasket? 

Only if it’s not flat now. Get a good straight edge and some feeler gauges 

Brown as in rusty coolant, or oily sludge?

Yeah, check for flatness, go from there.

Not a Miata, but I've done head gaskets on a 22R motor that had a BHG, bad BHG, and never bothered to get it skimmed.  No problems.

But, if you're not losing coolant and the oil isn't turning to chocolate milk, yeah, are you sure it's not rust?  Try flushing the system.  Try one of those combustion gases in the coolant test kits.

Yeah the oil isn’t chocolate milk. It might be rust. This car has been neglected. I’ve changed coolant twice, once when I first got it, because it had just water in it (and maybe the water wetter stuff, which I’ve heard looks like scum in the radiator), and again just last week (leaky radiator). It doesn’t seem to be oily. I am leaking oil externally though, so it’s hard to watch for oil consumption. 

Ill say this-I get a bit of smoke or steam on start up, but it goes away. This thing has no cat. It does not appear blue smoke, but white, and a bit of condensation gets spit out on start up. But goes away when warm. 

You can always do it the old fashioned way. Get a big piece of glass, use some spray adhesive on it and put some sandpaper on it. Then you go to town wet sanding on it. I recommend 320 grit as a good compromise between cutting ability and how smooth the finish is. You may have to lay two sheets edge to edge with normal length cylinder heads. It only took mine maybe 5 or 6 sheets of paper per head and an hour or two each. Of course you'll want to very thoroughly clean everything out when you're done.

I actually just did this on my Subaru heads when changing the head gaskets. They were really bad when I started due to the previous head gasket mechanic using a really aggressive abrasive disk randomly all over the surface.

In reply to freetors :

Nice! I’m just cheap enough to try that. I saw a YouTube video of some guy doing that, but questioned his knowledge and experience. 

So how do you know when you are done?

do you just keep checking with a flat edge and gauge feelers? 

On a related topic, I'm interested in learning what people generally pay to get an aluminum head freshened up at the machine shop.  I guess that would be a disassemble & clean, check the valves for bends and bad sealing, check the guides for centering/stability, repair if needed, pop in new valve guides, maybe spring seats or valve locks if they're getting ratty, and reassemble.

$500 a head?

For the bonus round, shops in DistMarVa?  Sorry if that's too greedy.    

I'm not a machinist or an engine building expert, but:

The Miata 1.8 uses a multi-layer steel head gasket from the factory.  These tend not to fail in normal use, which suggests to me that it's much more likely to be rust in your coolant than oil.  The fact that it only had water in it at one point also suggests there's probably a bunch of rust floating around in there.

Also, as I understand it, MLS head gaskets are very demanding of the surface prep for installation.  Sand paper does not provide a finish that is good enough to seal properly, it needs to be cut with a machine with the right kind of tooling.  I'm not sure exactly what that is, but every time I've built mine I've had it machined to a finish like this:

Have you done a compression and/or leakdown test?  Without more evidence of a failure, I don't think I'd be pulling the head off, personally.

I've always sent the heads out to the machine shop as a just-in-case.  For a hundo, they'll clean it up, make sure it's flat, and put in new valve seals.  I figure that's worth not having to do it twice.  You can get a feeler gauge and something that is really really straight and check it yourself.  I've used shorter metal rulers bought brand-new for that particular event and made sure things check out.

Also, when I did my '97 Miata's head gasket, I let the machine shop do their magic on the head but I hit the block with acetone, brake cleaner, WD-40, a honing stone, and some sandpaper.  Not all at the same time.  Once I was able to drop the head straight down on the block (had a little whoopise the first time thanks to a missing dowel pin), it's sealed up just fine.

Also don't forget that when you skim a head or deck a block, you change the valve timing. 

Buy or rent this before you pull the head:

All super good advice. The more I think about it, the more I’m starting to believe it’s rust or scum. There’s no telling what kind of water the previous kid owner used, or how long it’s been in there. The coolant has never felt oily, just the usual coolant slickness. Also, no sheen or oil slick. 

If it was running with straight water, it was lacking the corrosion inhibitors that glycol (and other additives) bring with it.   I'll 2nd (6th?) doing a thorough flush.

 

Also, as I understand it, MLS head gaskets are very demanding of the surface prep for installation.  Sand paper does not provide a finish that is good enough to seal properly, it needs to be cut with a machine with the right kind of tooling.  I'm not sure exactly what that is, but every time I've built mine I've had it machined to a finish like 

 

Felpro recommends a 50-60 Ra finish in aluminum for their mls head gaskets. This is actually right in line with what you'll get out of 320 grit sandpaper. It just doesn't have that cool looking machined highly reflective finish.

In reply to freetors :

I’ve had at least a dozen heads off motors with mls gaskets over the last few years. Never had them machines , gave them a light sanding and have never had a failure , even running relatively high boost on a few of those subjects.

That being said, I may have just used up all my luck and not know it.