How to Torque Rear Prop shaft front bolts
Putting back in my newly rebuilt propshaft today, the workshop manual specifies 35Nm for the 8 bolts front and rear. Although it doesn't specify that new bolts should be used, I went ahead and bought a new set from MB anyway.
Problem is how to apply the correct toque to the front bolts. The rears can kind of be done with a crows foot and wobble bar.. allowing a small adjustment for the slightly longer leverage this gives you. I'm happry that they are OK.
The fronts are completely inaccessible this way though.
Any suggestions, or can I rely on a guestimated approximation here (I have heard it said that people can be fairly consistent if they pay attention when guestimating torque - I'm sure that these are within a shout of the rears so I'm not too worried.. just wondering how they would have been done by a profesional.
Hello fcp,
When you are asking by referring to 'Professionals', how they torque prop shafts. On the assembly line the power tools used are pre-torqued, hence torqueing is done automatically. By training schools and the pedantic mechanic, he would use an appropriate torque wrench. Where a small scale, 1/2" drive or 1/4" drive torque wrench does not do the job, one can use a crows foot fitting torque wrench.
But the 'Professionals' as you put it, have their own tool and torque setting - it is their hand and elbow tool and a setting of 'FT' (F--king Tight).
If the professional mechanic started torqueing everything specified to be torqued, he would not last in the trade, firstly, he would be laughed out by his peers and secondly, he would never make his standard working times, let alone make his bonuses.
Cheers,
Torque Wrenches are over hyped.. they do a job in a controlled situation by de-skilling a job. If you are sensitive to it you can feel the give in a nut/bolt system that tells you when it is close enough to yield .. and hence mechanics do not use torque wrenches. With the prop shafts it is about how much torque you can get into them with out deforming the heads!
I did a fair bit trying to set standard torques for equipment at work.. the differences between applying the correct preload can vary as much as 50% torque by the time you take into account;
1. finish on bolt - chemi blacked, bare steel, rusty, zinc plated etc
2. lubrication - lightly oiled is the norm but lots of folk (myself included) slap black moly loaded grease on which dramatically changes the torque; doing this has caused me to stretch bolts at "factory figures" yet to put just oil on is awaiting a seized bolt IMHO
3. Washer/mounting method e.g. no washer, plain washer, spring washer, swage headed bolt, studs, turning bolt or nuts etc
4. Grade of nut/bolt.... and if into tapped hole then that material's tensile capacity.. and brass/stainless etc.
...at the end of it .. it is a mine field! Something that does work that they use in sky scrapers 1920-1980 was to do nut up snug (say 100 ft.lbs) and then use a ratio of clamp length and diameter to "turn to calculated angle" (like you do modern stretch cylinder head bolts) .. but really this needs a bit of maths and works on 1.25" shanks and above.. fine if you have a whole building but not for a factory manual. No doubt the torque wrench salesmen have got into sky scraper builds by now...
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Hi chaps,
sorry I missed your replies.
Good advice on both accounts I am sure of course like all these things, the degree to which correct torque is critical depends a lot on the application. Cylinder head bolts for example are pretty important in my experience (learned the hard waywhen I was 17).
That said 'f'ing' tight in a less obvious situation (high-tensile 12.9 overstretched for example) could be bad news for everyone too if it sheers. In the end I went for f'ing tight on the vibration dampner end because that was all I could get!
Having been though a number of analog wrenches over the years I bount a snap-on techangle when I got the G and have been very pleased with it, so where I can I'll try to stick to the service manual (no one paying my wage to work on my truck unfortunately!)