you always bring things up in steps... like on head bolts. what steps are should be used is often specified. the waiting overnight to torque 928 headbolts , could have been about letting the head gasket settle.. and not about the bolts so much. were they stretch head bolts, or non-stretch type ? but yes..always in steps, that's only logical and natural. especially when it's multiple fasteners in a group. like on a cylinder head say.
----- Original Message ----- From: "Don Hanson" <dhanson928@GMAIL.COM> To: <vanagon@GERRY.VANAGON.COM> Sent: Tuesday, September 27, 2011 9:33 PM Subject: Re: Torque Specs - Lug bolts and nuts
> On Tue, Sep 27, 2011 at 8:51 PM, Gary Bawden > <goldfieldgary@gmail.com>wrote: > >> Snipped. >> >> I believe the engineers know this, and set torque values to a number that >> they believe will do the job, without over-stressing the part. And as has >> been pointed out by others, it's the uniformity of torquing fasteners (at >> least on parts having more than one) that is probably more important than >> the actual torque value. >> > > One thing to keep in mind about torquing something to specs....Depending > on the parts connected by the torqued nuts, bolts, studs, whatever....they > should often times not be taken right to the specified torque, all in one > tightening. When I do alloy wheels up to 100lb/ft....I go round to about > 80 in a star pattern, then I re-set the wrench and do it up the final > 20lbs. > > My engine mechanic used to take two full days to torque the heads on the > 928 motor....he would (I don't have exact numbers) torque em up to about > 50% > and leave em all day, then come back and go to like 80% and leave em > again. > then again with a 'resting, settling period' and finally go to full > torque... > > Castings seem like they are pretty brittle, somewhat fragile...If you are > to torque say a water pump to (out of thin air, this figure) 30ft/lbs....I > wouldn't feel right just doing it up, one fastener at a time, right to > 30lbs...I'd probably snug it up, then torque to 20, then go back and go at > it with 30ft/lbs....minimize and spread the stress. |
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