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I hope your stepping was casual. And I know This isn't exactly timely but the information about turnbuckles is timeless.

Thread count on typical domestic TBs is standard; National Fine (NF). Some European products use metric sizing so be sure to pay attention to what you're buying.

The open body turnbuckle is chromed bronze and studs are Typically stainless steel. This combination prevents the threads from getting fouled which can happen easily with stainless on stainless. Often TBs are oriented so that a counter clockwise turn adds tension but the important thing is that you orient them all the same.

I'm sure your wires aren't original, but if they are that's more of a risk than old turnbuckles. You should seriously consider new wire if so. You can tell if a wire is shot by how it lays when free of tension. It should lie straight, not wavy. Any damaged or broken strands also warrant retirement of a wire.

Remember, a rigging plan is designed around a safety factor, not simply "ultimate breaking strength". Any compromise to the wire's integrity reduces the safety factor and leaves a vessel more vulnerable to rigging failure.

Typical you tighten the headstay by taking turns on the backstay. Headstay length is a pre determined length based on the rig dimensions. In any case you should be sure you don't pull the masthead forward of plumb. A slight rake, or aft leaning is usually desired.

Here is a more detailed procedure to complement the initial post. Maybe to much detail...

Centering Masthead:

Fore and aft position isn't much of an element as headstay length is a set measurement based on rig design. The backstay should only be under light tension, a few turns past hand tight as to prevent any compression load on the mast which will make this process more difficult and lengthy.

All shrouds should be finger tight; loose but not floppy.

Starting with the cap shrouds you must first determine the relative length of port and starboard caps. A competent Rigging shop will build them at an equal length which makes centering the masthead a simple process. But it is wise to verify if you have any doubts by hoisting a tape on the main halyard and measuring the distance down to port and starboard cap stud tips. On the original Ariel mast the main sheave is center but on most any other mast head the sheaves are off center so you must allow for this difference from one side to the other.

If your Rigging is equal you simply measure from chainplate pin to shroud stud tip and set both sides equal and make hand tight. If not equal take up the difference on the longer one as measured in the TB from chainplate pin to the stud tip then put equal turns to achieve hand tight. From here the masthead is centered and equal turns should be taken on the cap Turnbuckles.

Again, the goal is to avoid any compression load on the mast before the lowers are tensioned as this will encourage an athwartship bend making it difficult to maintain the mast in column as you increase tension.

Once the masthead is centered tensioning begins from the lowers to the uppers. When tensioning you should be checking the mast for deflection. Some masts will have an uncorrectable deflection so don't spend days tweeting and don't try to adjust you cap shrouds as you will be moving the mast head out of center.

Generally the fwd lowers get more tension than the aft for two reasons: to introduce a slight positive pre-bend to the mast and to compensate for the energy produced by the sails which pull the mast forward when under sail.

starting with the fwd lowers make tension even. To ensure the mast is in column you may need to put a few extra turns into one side, but start from equal tension. When the wire is finger tight it's pretty easy to feel how equal the tension is. It's maybe a quarter turn between floppy and loose. Once the fwd lowers are set bring them up until you see the mast belly slightly.

Once the fwd lowers are tensioned some and the mast is in column you can do the same with the aft lowers. Only don't try to pull the belly out unless you prefer no pre-bend.

At this point your ready to put tension on the rig. And since you took the time to get the mast in column all you need to do is put equal turns on the fwd and aft lowers, then the uppers and backstay. But the emphasis is on supporting the midsection with the lowers before introduction any substantial tension up high. Under normal circumstances the headstay is never "tensioned" as this changes the headstay length. It could be semantics butTension is put into the headstay by turning the backstay turnbuckle, not the headstay. The important thing is to know the headstay length eithr by design or familiarity with your rig.  

Mast rake is a product of headstay length, not backstay tension. You can hook the mast with backstay tension, but not all masts will do so without considerable tension.

Ultimate tension or rig tune is best determined under sail, about 10-15 knots of air. At least you can achieve proper rig tune this way as well as with the data provided above.

i hope this is useful.