Sending this note out again, as I don't think it made it out last
time. :)
Here's my 2 cents on the re-torquing question.
Torque specifications are made by the manufacturer assuming clean
threads, and either "dry" or with a specified lubricant (oil,
assembly grease, etc.). The purpose of torquing bolts is to stretch
them to a specific preload. If you have access to both ends of the
bolt (like a connecting rod bolt), the ideal method is to measure the
overall length of the bolt, and recheck the length with a dial
indicator as you gradually tighen. This was the prescribed method of
tightening the SPS-CARR bolts used on the Carrillo rods I used on my
race engine. Now if you don't have access to both ends of the bolted
connection, as in a cylinder head bolt, then measuring applied torque
is about the best thing, unless you can measure the actual angle of
rotation (not easy to do accurately).
If the manufacturer specified a dry nut and bolt when you torque
everything down, and you used oil or assembly lube, then you get more
stretch than what the connection was designed to handle. The
lubricant reduces friction, and you'll be able to tighten everything
a lot more with a given amount of torque. Even different types of
lubricant can give you different amounts of bolt stretch for a give
torque.
A bolt that's been tightened, and has been sitting for a while will
have some residual "sticktion" in it. It's possible that this bolt
is under-stretched, and if you don't overcome the residual torque in
the bolt when re-torquing, you'll miss the spec. In a critical
application, I will always loosen the bolts, and then re-torque,
normally one at a time. Now that being said, on a normally street
application, I'm must admit I usually just throw on the torque wrench
and tighten away. And I always use a beam-type torque wrench. They
are cheaper, more accurate, and you get a better "feel" for how
things are tightening up.
When using a beam-type (or clicker type wrench), it's very important
to make sure you are pulling exactly perpendicular to the wrench
handle. If you're pulling towards or away the bolted connection, you
can throw off your readings.
ARP bolts has a great website, and lot's of good information about
bolt design, tightening methods, etc.
http://www.arp-bolts.com/Tech/TechInstall.html
Happy Roadstering!!
---
Chris Coker
Washington Twp, MI
1969 1600 Street Car (SPL311-24529)
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