http://www.kelleyswip.com/1989XJMJ.html
There you go - same specs 1987-1990.
All specs are for "clean, dry" threads unless specifically noted otherwise.
NB: Take one of your old cylinder head screws and make a "thread chaser" to clean out the screw holes in the block. Using a narrow cut-off wheel, cut three or four flutes in the threads of the screw, spaced roughly equally. Dip the screw in oil or grease before you run it into the hole (to catch and hold any crud,) and spray it clean with carburettor cleaner before you go after the next hole (you don't want to put anything nasty into a hole you're trying to clean out, do you?)
It's probably not a bad idea to do this with a 3/8"-16 and a 5/16"-18 screw of a convenient length, since these are common thread sizes in the AMC150 and AMC242. You can even do a 1/4"-20 screw if you like, but that takes a bit of patience - so you don't send the screw flying across the shop!
Eventually, do this with a full set of SAE screws from 1/4" to 1/2" - UNC and UNF - and a full set of ISO screws from M6-1.0 to M14-2.0. Keep them in a box in your toolkit, and you'll find they often come in rather handy.
Often, people will tell you to use a tap, but I don't do it and I don't recommend it. Why? The tap carries a risk of cross-cutting threads, while the screw will simply follow the existing thread (and the flutes you cut into it are what will gather the crud as you turn it in and crank it back out.) No risk of cutting new threads if you're not careful.
You can buy a purpose-built set of "screw thread chasers" from a speed shop if you like, but the price is comparable to a set of actual taps. Save the money and work on steadying your hand with the cut-off wheel anyhow - the confidence you build in making your own tools helps you when you're working...
