Our dear old friend John Haynes recommends 18lbsft (25nm) which is a decent figure for the likes of a M8 bolt to give you some idea.Not quite the same problem, but I cross threaded one a while back. My local man fixed this and charged 1 hour which seems pretty reasonable to me. Torque figure he quoted me was 18 somethings which is not a lot. If they have all been over done and the threads stripped Its a head off job I agree. New plugs havbe compressible washers so trying new plugs might help?
As jrk points out above your average DIY quality 1/2" drive torque wrench will not be accurate at that sort of figure. In fact most traditional torque wrenches are only truly accurate at mid range settings. If you want to buy a device which has a large range but is still accurate near the upper and lower limits of it's scale I can recommend a digital wrench which works via a strain guage rather than a mechanical mechanism which will be affected by friction and cleanliness and/or how it's stored and calibrated. I bought one of these a couple of years ago:https://www.machinemart.co.uk/p/040215238/?awc=3131_1679589983_961cdb4e34a6c8c0d725f9a1bee939b5 and I must say it works very well indeed. It's one failing is that it is quite deep once assembled onto a socket and power bar. Fine for doing heads but not so easy in restricted areas like timing belts. When I first bought it my old mechanical Britool torque wrench had just been calibrated (you do all know that mechanical torque wrenches should be periodically calibrated don't you) and I connected it to the electronic torque adaptor so I could check what the electronic device thought of several setting on the Britool. I did it at roughly a third, half and two thirds full scale and the two agreed exactly so I think the digital adaptor is pretty accurate. Now I use the digital adaptor to periodically check my mechanical torque wrenches against and only send one away for calibration if it disagrees with the electronic device.
Edit. What's quite nice about the electronic adaptor is that you can set it to beep at a set figure, just like a conventional torque wrench which "clicks" but you can also set it to display the torque being applied numerically on it's wee screen. I can see this will be very useful if/when I have to do the water pump/thermostat housing on the Ibiza which has to be held at a set torque figure as the securing bolts are tightened so that the drive belt to it is properly tensioned. A conventional "click type" torque wrench couldn't do this, although the old "spring beam" type favoured by my American friend would.
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