the strange thing is when i look in the parts catalogue of my triumph the standard wheel sizes are :
3.5 x18 (rear).
Not strange at all, these were the standard sizes on most non-US-market Triumph twins.
whats the metric equivalent ?
3.50x18 is relatively easy - 100/90x18 is close.
3.25 is difficult - 90/90 will have a similar width but a noticeably-smaller overall diameter, 100/90 will have a similar overall diameter but be noticeably wider. On balance, I'd probably go for 100/90 but the choice is yours.
Buy the inch size AM26. 325-19 and 400-18.
Only if you rebuild the wheels.
Don't. The 4.00x18 has both a much larger overall diameter and width than either the original 3.50x18, a 100/90x18 or the existing 4.25/85x18. If you go for 4.00x18 on the rear, you also need the 3.25x19 at the front because they are similar overall diameters.
They just do not make tires like they used to though. The rubber is not as good, and the side walls softer, depending upon tire pressure to do its job.
On the contrary, the rubber is much better. What you're talking about here is fitting 'V'-rated (up to 150mph/240kph) tyres to bikes that would've been fitted originally with 'S'-rated (up to 112mph/180kph) tyres.
As for softer sidewalls, they give you a better ride. If you want stiffer sidewalls, use Dunlops; they still recommend 1970's tyre pressures.
some new sidewall design that stands up under a blow out.
Hardly new - iirc, Dunlop Denovo(?) car tyres came out in the 1970's. Might be ok after a blow-out but you'd have endured tens of thousands of miles of pi$$-poor ride - more comfortable in a tank!
A blowout with the new wimpy sidewall tires, I think,
Having actually experienced a small number of blowouts on bikes (including one front), the stiffness or otherwise of standard bike tyre sidewalls makes not one iota of difference, they are still one of the best natural laxatives known to Man. If you're really that concerned about what might happen after a blowout, fit a couple of security bolts to each wheel.
There's the metric tire sizes, which are usually 90% of the full height tires.
Too much of an over-simplification, imho; as I say, the 3.25-section (in either 18" or 19") does not have any easy metric equivalent.
Then there is the K81 size. K81 4.10 equated to a standard ~3.50, K81 4.25 equates to a ~4.00 size.
K81 are simply Imperial-sized 90%-aspect-ratio tyres, original Avon Roadrunners are the same. K81 4.10(x19) is a good equivalent of 100%-aspect-ratio 3.50 because Dunlop had the late 1960's production racers' favourite, the Avon GP 3.50x19, in mind when they designed it.
Otoh, the metric equivalent of 4.25(/85x18) is 110/90x18. 4.00x18 has a similar overall diameter to 120/90x18 but is thinner overall, but not as thin as 110/90x18 - more like '115-width'.
18" front tires are hard to find.
Low profile "metric" tires are tiny compared to the standard old K70 size. This can throw off your ground clearance,
As I say, 90/90x18 is a noticeably-smaller overall diameter than 3.25x18 (think daylight between mudguard and tyre) but on a twin doesn't affect the cornering clearance unless you're regularly scraping the bottom curve of the exhausts. Otoh, 100/90x18 is very similar overall diameter to 3.50x18.
Avon 4.00x18 is 795 turns per mile, 100/90x18 is 834 turns per mile (about 5% difference) - given Triumph fitted the same speedo. drive to both US- and non-US-market bikes ...