SOHC.co.uk Forums > CB350/400
Head + Rocker cover torques...
Laverda Dave:
Lobo, the Haynes Manual I have gives a generic torque figure for 6mm threads of 7-9lbs. I played safe and went to 8lb and a couple stripped (now helicoiled due to lack of any metal around the damaged thread). But going on what Nigel mentioned in his earlier post it could have been caused by me adding copperslip to the thread coupled with the wrench calibration being out and likely thread damage caused by previous P/O's.
Strangely the 6mm threads on the CB250RSA are also torque set at 8lb none have stripped!
K2-K6:
I've come back to this as it links into a question I was asked about the figures given for torque settings on manuals.
Question was "Why do the torque figures range over such a larger percentage difference given that some of the tolerancing is so very very fine and precise?" I couldn't answer this with credible logic.
Now, if we apply it to these bolts only, 7 to 9lb per foot, has the top figure something in the order of 9 being 33% increase on the 7lb setting!
If you add in "marketer's" psychology of giving a price range, the middle one being what you sell most of, the lower figure to make the middle purchasers feel they've got something a little bit better for their informed choice, and the highest cost, is just pure additional margin for those foolish enough to shell out for it :)
Comfortable we are in choosing the centre point, and it just seems so logic doesn't it. So, if you take the 8lb and work through possible errors/ potential for unquantified effects, you can get 8 + 10% torque wrench accuracy (depends which way it defaults) + 15% for grease/coppaslip change to torque measurements, you can end up with 10.1lb measured torque.
Or for those making a decision that you should use the highest torque figure, something like 11.4lb :o
It's easy to see why they are often stripped. Also why they gave us just screws on many of the covers on these engines so that generally people can't load too much torque onto them with just a screwdriver.
If you look at the joint on these cam covers, they are metal to metal with no gasket to compress (once touching with the o-ring compressed) so have no yeald built into the torque interface.
If you had to specify it, once the casting faces touch, then you'd only need something like 2 to 3lb of torque to prevent the bolt backing out in use. So a safety margin of double that, 6lb, would be a good set point to make sure they reliably exceeded a practical minimum during engine use.
Then if you set your torque wrench to 5lb, lubricated the threads ONLY, made sure the underside of the bolt head was clear of lubricant, along with the casting face. You should get a true torque of between 4.5lb (if torque wrench was 10%error in minus) to 6.3lb with all errors maximised to plus figures in above illustrations.
This would minimise potential of stripping them, and make sure they don't release in use.
ka-ja:
After a lifetime using spanners etc. I still trust my hands and fingers on small items, never stripped one yet.Things like cyl. head and crank bolts are more critical.
Bryanj:
+1 on that last one.
Lobo:
Great food for thought Nigel, I’m a coward and by default always take the lower figure.
You mentioned lubricating the threads only - one turn of a 6mm thread would be approx 18mm, so (eg) a dozen turns would be getting on for 20cm worth of minimised thread friction? Would this not be very significant in terms of falsified torque readings?
Ka-ja / Bryan... you guys have the experience / skills of a lifetime fettling such things, us mere mortals can’t hope to accurately set torques using fingers!
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