Excellent advice from you all. Just I thought, it’s the finer details which can be easily overlooked. So I have purchased for £5 each both an 80mm & 90mm stem. Fitted both & the 80mm flipped up is perfect with no twitchy steering. What a difference. However, it’s a standard stem without the insert (the original Specialized has an insert as the bike is carbon with a composite steerer tube - just as you said). I have looked on line for a second hand stem with insert, but can’t find one at 80mm😕 Here’s the question - if I leave the standard one on but tighten it to say 3Nm, will it be OK? Do I really need to spend £90 on a new S Works Specialised stem with the insert? Or are there any other compatible stems with inserts?
Good work
I think there are several things at play here. I'm not familiar with that specific Specialized stem / the insert, but it would figure that they've taken some steps to mitigate the dangers associated with composite steerers.
I'd be very wary of under-torquing the clamp; I can see where you're coming from but equally if it's too loose you run the risk of it twisting under load / if you hit a bump; the potential consequences of which don't bear thinking about! I do generally go easy on tightening torques, but only usually go as low as 10 or 20% below the stated figure... for what that's worth. You could potentially apply some "carbon grip paste" which is basically abrasive paste that's intended to stop parts moving relative to each other, but I'm a bit wary of this too tbh.
If you do proceed with the one of the stems you have, I'd be inclined to give them a really good once-over at their contact faces / edges, de-burring as appropriate. Might be worth removing it after some miles and checking for damage / witness marks on the steerer tube too.
Tbh really we're into murky waters now; outside the bounds of my generalised advice.. really sounds like a question for someone more experienced with those products / the brand themselves.. although I suspect they'd just use it as an excuse to sell you an expensive stem or whole new bike..