Torque vs stretch in rebuilding engines

on rod bolts you should always try to get a bolt stretch spec.. Some manufactueres do not list a torque spec purposly so that you have to use new bolts. Once a rod bolt is used it should not be reused. Once the metal has yield it is no good. I'm a bit crazy so when I rebuild I do bolt stretch but watch the torque too so that I know the torque is a reasonable number relative to the stretch. You know...measure twice cut once mentality.

 

The reason for correctly torquing a bolt/stud is to get the stretch the bolt into the correct area or yield.

As said above in other posts over torquing will over stretch the bolt/stud and thus weaken it. Under torquing simply means it is not going to correct clamp the thing it is holding on to.

Designers specify the bolt/size size to get the correct clamping force ... and could not give a rats arse about the torque, they are interested in the clamping force the bolt is going to maintain ... which comes from the properties of the bolt/stud and changes as it is stretched.

Pete's reminding us that you do a bolt up to a particular torque to get a particular stretch out of the bolt which directly relates to the clamping force ... no other reason

ps: Thus a stretch spec is probably more accurate that using a torque spec ... as the correct stretch is really what you want.

 

There are three ways to tension/stretch a fastener,
1-torque wrench
2-measuring the stretch
3-turning a predetermined amount(angle torque)
number 2 is the most accurate period,only any good on rod bolts though like to see someone use no 2 method on head bolts-you can't do it,
so what we going to use on the mains/head bolts,well you have the data in front of you,if no 3 method you will need a base to work from so its no 1 first which will be a low setting of around 10-20 lbs,ft, there are angle torque gauges on the market which you will need for no 3 method,
torque wrench's have a bit of a problem with FRICTION thats why methods 2 and 3 are now used so much,
FRICTION-NOT GOOD so use the best lube you can on the fastener,not the oil can you have on the shelf,
if you use method 1,2,or 3 use the very best anti friction lube you can on major engine fastener components,
oh,and make sure the work area is clean and enjoy it dont rush it, its not a race,

cheers
Dale.

 

porsche 911 head studs use a combo of 1 and 3:

Step 1: torque to a low 15 Nm setting,
Step 2: turn an additional 90 degrees.

Lubed only at the base of the head nut with Moly lube.

 

There's a good discussion here:

http://forums.pelicanparts.com/showthread.php?s=&threadid=106020&highlight=stretch

-Wayne

 

Wayne, been reading about the locktite bit on rod bolts/studs,
do you use locktite on rod bolts in every type of engine you have built,

cheers
Dale.