Rod, torque wrench or "stretch"?

[Truckedup]

This is a continuation of the motorcycle broken rod bolt....You guys that build racing engines..Do you use a torque wrench only on the rod bolts or do you measure bolt stretch?

[David Redszus]

By far the best method is to measure the stretch...if you know what that should be.

The next best is the angle torque method...if you know what the angle should be.

Friction torque can be used if the proper value is first determined in a fixture designed to measure stretch for a specific bolt.

The worst method is the simple friction torque method using some target torque value.

[engineguyBill]

Bolt stretch is by far, the best procedure for tightening rod bolts, but you need to know what amount of stretch is recommended by the bolt manufacturer. The amount of stretch is dependent upon several factors, including bolt material, length, diameter, etc., etc.

[FC-Pilot]

This may be overkill, but I like to cycle the bolts five or six times with a torque wrench and verify what they are stretching to. Often times the first three or four times does not provide adequate stretch. By the fifth or sixth time the torque value tents to put the stretch right in spec with the manufacturers numbers. I do it that way as I can see that the bolts are meeting what they were designed to do. I saw one bolt once that reached a stretch value well under what the recommended torque value was. We replaced that bolt and had no other issues after that. We may not have had an issue, but I would rather be safe than sorry.

Paul

[learner1]

This may be overkill, but I like to cycle the bolts five or six times with a torque wrench and verify what they are stretching to. Often times the first three or four times does not provide adequate stretch. By the fifth or sixth time the torque value tents to put the stretch right in spec with the manufacturers numbers. I do it that way as I can see that the bolts are meeting what they were designed to do. I saw one bolt once that reached a stretch value well under what the recommended torque value was. We replaced that bolt and had no other issues after that. We may not have had an issue, but I would rather be safe than sorry.

That is the best way. I do it before resizing and during assembly.

[gramps]

If I do the rod bolts using the angle torque method and then check them for stretch will I see the correct amount of stretch?

[peejay]

Angle in theory is just stretch, divided by threads per inch, no?

[rebelrouser]

for what it is worth, I check the rod bolts for length and then torque them, and then check the stretch, more often than not I find the given torque spec does not stretch the rod bolt to the spec given. I then torque in small steps until the stretch is correct. I find most of the time 5 lbs over the given torque spec is pretty close to the correct stretch. so then the rest of the bolts are torqued to the new spec, and I find it works pretty well. I like to cycle the bolts as little as possible. Just the way I do it.

[David Redszus]

for what it is worth, I check the rod bolts for length and then torque them, and then check the stretch, more often than not I find the given torque spec does not stretch the rod bolt to the spec given. I then torque in small steps until the stretch is correct. I find most of the time 5 lbs over the given torque spec is pretty close to the correct stretch. so then the rest of the bolts are torqued to the new spec, and I find it works pretty well. I like to cycle the bolts as little as possible. Just the way I do it.

Bolt torque is a measure of friction only. A given bolt can be stretched at various torques depending on condition of the bolt (rusty or clean, dry or oiled, cut threads or rolled, base metal or plated), washer hardness and threaded material.

A torque wrench is accurate to within about +- 30%. Some mechanics have calibrated hands that are as accurate as a torque wrench. Sometimes.

[modok]

torque vs stretch vs angle....which to use?

ALL Of them. You can watch the degrees it takes to go from snug to torque, see what torque that is, and check the stretch.
If anything is different you will catch it.

[digger]

torque vs stretch vs angle....which to use?

ALL Of them. You can watch the degrees it takes to go from snug to torque, see what torque that is, and check the stretch.
If anything is different you will catch it.

if you can check the stretch why would you not use stretch method and only the stretch method ?

the other methods should probably only be used when stretch cant be done or you are mass producing something

[Truckedup]

Do manufacturers use bolt stretch on production high performance engines? What about bolts were measuring stretch isn't practical, like main bearing caps...

[Warp Speed]

Torque/angle to proper stretch of each bolt is paramount in big effort deals, I think most normal people here shouldn't fret too much over it in most all basic performance builds. As much as this pains me to say, find a torque that gives the proper, manufacturer stretch specifications with the recomended lube (mid spec) for a bolt or two, and torque the rest up.
Spend more time on proper clearances and cleanliness!

[MadBill]

...
if you can check the stretch why would you not use stretch method and only the stretch method ?...

The answer is a few posts back. If 15 of the bolts require 76 ± 2 lb-ft. to achieve nominal stretch but one needs only 60...

[modok]

torque vs stretch vs angle....which to use?

ALL Of them. You can watch the degrees it takes to go from snug to torque, see what torque that is, and check the stretch.
If anything is different you will catch it.

if you can check the stretch why would you not use stretch method and only the stretch method ?

the other methods should probably only be used when stretch cant be done or you are mass producing something

Why would you ignore information that's right in front of you? it takes virtually no additional time to look at the torque wrench, and notice the angles of the torque wrench travels, only a little bit of brain power is required.