hey guys i have searched everywhere here and on google and everyone says you need to replace ca18det headbolts but noone says why? someone must know thanks

They are designed to stretch when tensioned, but it's a one time deal, they can't be stretched twice.
Just another 'technological advancement' that seems to do nothing more than the old non-stretchy types other than force you to spend more money on parts that were not required on older designs.
I'll stick with my ARP studs.

Now go & spend spend spend.

Out of interest - with ARP studs replacing head bolts is it necessary to tighten the studs into the block to a specified torque?

As I wonder how you would measure the torque (as you would have to use a spanner to tighten them us - and not a torque wrench)

No you do not have to torque the studs going into head just make sure they are fitted to the correct depth.The studs have hex holes in the top so could be tensioned but no need as they would only be bottoming out in hole. Make sure to torque the nuts to the correct tension.
Cheers Andrew

I went & grabbed the box of main bearing studs & it has a poor quality photocopied instruction sheet with it. This sheet seems to be specific to the main bearing studs but basically it says ......

"Install main studs into block hand tight using oil or lube. NOTE Loctite may be used if a permanant mounting of the studs is preferred. The fasteners must be torqued prior to the Loctite setting up. Lubricate the threads, nuts & washers with "ARP MOLLY ASSEMBLY LUBRICANT" [this is included in the box]

In other words, they do not want these main studs to be in the block tightly at all. I presume that tightening them causes some distortion, however slight, around the hole. With full depth engagement of the thread, the tightening of the nut exerts a [presumed] even pressure on the full length of thread in a straight pull without 'screwing' anything into the threads in the block under high stress as it's only the high tensile nut that is rotating on the equally high tensile stud. This end of the stud uses a fine pitch thread for the nut to exert a gentle but powerful tensioning instead of a high tensile capscrew screwing its way into a comparatively soft block via a coarse pitch thread.

It goes on to stress that only a high quality moly lube, such as the one in the box, should be used on the nuts, washers & studs when torquing up. It also says ....

"Torque values are based on75% of the fasteners yeild strength. Use manufacturers torque sequence but do not use the engine manufacturers torque specs."

These particular studs are rated at 190,000 PSI & are 10mm in diameter. The torque spec is 60ft lb's, but only with the recomended lube. It goes on to say "Results may vary with any other lubricant."

Clearly there is more to fitting them that just bungin' 'em in the hole & screwin' 'em down.

EDIT
On the base of the box is a chart showing various sizes of studs in three tensile ratings & the torque values for each.

They give two columns, one with 30 weight oil as a lube & the other with their moly assembly lube. The column with the 30 weight oil has a small rider under it with the words "NOT RECOMENDED" & the torque value for these studs is 70ft lbs with the oil.

It's almost rocket science.

Cool, thanks for that - also Andrew I used a few pics of your coupe in an interview I went for last week. I had pics of my dattos, and had a couple of pics of yours (well your "old" coupe) as the "this is what it will look like when it's all finished".

I think the interviewers liked it.