Coolant out the exhaust

1990 Miata 5speed, 150,000 miles, fuel saver, commuter car.
I've notices a significant reduction in gas mileage, and I see some coolant coming out of the exhaust.
I had 1 plug that was seized, but I was able to get it pulled through it took some of the thread our with it.
The replacement plug seems to go in snug, but I know the cylinder head thread is compromised a bit.

Now deciding how to address the issue of the coolant leak. Looking to get it fixed.

I'm hoping that the leak is the head gasket, and not something like a cylinder wall crack.
I don't see any fouling in the oil.

What shop is most respected for Miata cylinder head machining?
Might as well mill the head flat to get a true surface, and pick up a little compression.
Hoping that the valves are still in good shape and don't need work.
Next time I pull the plugs I'll do a compression and leak-down test.

I'm mulling over the option of a motor convertion to a 5.3L LSx with Glide. I already have the motor and tranny, I just don't have the info on home-converting it.
The bolt-on conversion kits are just too expensive for this low buck commuter car.
Any links to home-conversion info would be appreciated.

Jay Johnson

This is a pretty typical problem in most production engines with cast iron blocks and aluminum heads. The labor will be high, but it's not expensive to fix. You need to replace the head gasket and have the head resurfaced and install new head bolts. 95% sure this is your problem based on your description alone.

I'm going to do the tried and true 5.0L ford swap eventually. I'd love an LSX based swap but its more expensive, and probably too much power for me in a Miata chassis.

I'm not able to find the replacment head bolt set, from Vatozone/OReally's/Fleabay.

Anyone have part numbers for the 1990 head bolt kit?

Jay Johnson

I'd just get head studs. It appears you need ten, so you'd save almost 20 bucks with a superior product.

Head bolts: http://parts.arlingtonmazda.com/prod...2d135A%29.html

Head studs: http://949racing.com/arp-head-studs-miata.aspx