stretch spec.

[tcramer]

Hi guys,has anyone used the stretch method on the verbus 12.9 rod bolts for 1975 2.7 engine.No one seems to have this info. stretch wasn't used in 1975 just torque.Thanks,T.C.

[chris_seven]

Stretch is a much more reliable and repeatable method of tightening bolts than torque as it eliminates the majority of the measurement errors due to variations in friction.

ARP typically suggest loading a bolt to 75% of its yield stress which means it will operate in the elastic regime and calculating the correct amount of stretch is straightforward.

The yield point of most high strength steels is quite difficult to determine as not many steels of this type exhibit a 'discontinuous yield point' and theoretically the point at which the material yields can vary depending of strain measurement accuracy.

Because of instrumentation issues it is routine practice to determine a property known as a 0.2% offset proof stress.

This is the stress at which a material permanently deforms by 0.2%. This is most easily shown on the following sketch



The 0.2% Offset Proof stress is shown as (σ1)

Steels in the elastic regime generally obey Hooke's Law where the loading is linear so the strain at 75% of yield is 0.15%.

Engineering Strain (e) is defined as e=(final length-original length)/(original length) x 100

Basically the stretch should be 0.15% of the length of the fastener.

The strength of the bolt will have no influence on the amount of stretch, it will just change the torque needed to achieve the correct stretch but there is no need to measure torque when using a stretch gauge.

[304065]

Chris, don't the VERBUS bolts go over the top and into the plastic deformation range?

[tcramer]

Dear moderator,do you mean by "over the top into plastic deformation" that if I stretch the verbus 12.9 rod bolts 0.15% of their overall length it would not be a good thing?T.C.

[chris_seven]

Originally Posted by tcramer

Dear moderator,do you mean by "over the top into plastic deformation" that if I stretch the verbus 12.9 rod bolts 0.15% of their overall length it would not be a good thing?T.C.

A stretch of 0.15% will not cause any yielding of a fastener regardless of its grade. All steels have a yield stress that is about the same as a 0.2% Offset Proof Stress.

0.2% has been chosen for being equivalent to yield stress for 3 practical reasons - it was measureable using the standard 'extensometry' available in the sixties and seventies, it proved to be repeatable for a range of materials and heat treatment conditions and finally it is about the same strain as the yield point in lower strength steels that exhibit a 'yield discontinuity' which clearly defines the yield point.

Modern test systems have more accurate measuring systems but the body of data available will keep the 0.2% proof stress valid as a measure of a materials yield strength for the foreseeable future.

When you measure stretch you are measuring strain rather than stress and this is related via Young's Modulus (E) which is the slope of the stress/ strain curve shown in the earlier diagram.

E = Stress/Strain. E doesn't really change for any steel regarless of its strength or heat treatment condition. It is always 207GPa +/- about 2%.

Because you are using an 'Offset' Proof Strength of 0.2% you draw a line parallel to the load line and when the material deforms permanently by 0.2% you consider this to be the yield point. By loading to 75% of this value the fastener will remain in the elastic regime and should not yield.

I think the point that is being made is that the early Rod Bolts were of a 'use once' type but this was back in the early Sixties and bolt technology was not very advanced at this time.

The early 911 Workshop Manual in Volume I stated quite clearly that con rod bolts should only be used once because they were 'malleable' but this statement disappeared in later editions. I don't believe that this is beacuse they were designed to stretch in the same way as a modern head bolt.

The early bolts were only tightened to a level of 5mkp (kilopond metres) which is around 36 lbsft. This value remained in use until the early eighties (I think)

In theory this should not be sufficient to yield an M10 x 1.25 fastener if it were Grade 12.9.

I would think that the useable torque rating of an M10 fastener to produce a clamp load of 9500 lbs should be about 45 lbsft for a K factor of 0.15 (Lubricated thread) and about 60 lbsft for a K factor of 0.2 (Dry thread). DIN 267 permits a max torque for an M10 fastener of about 60lbft.

These figures are based on using the fastener to about 75% of yield.

If the early bolts did yield at 36 lbsft they must have been a lower grade than 12.9 or the errors in torque measurement which could be as much as +/- 25% in clamp force terms could have resulted in a few bolts being permanently stretched.

The final version of the Workshop manual gives a revised procedure of a 2-stage process.

Stage 1: 14 lbsft
Stage 2: 90 degrees

This procedure will give a more consistent clamping load than just torque but stretch will be better than either torque method.

A final point the 0.2% proof stress of a Grade 12.9 fastener is 1100 MPa and the UTS is 1220Mpa. (Once the UTS of a bolt is exceeded it will start to unload.)

This is a difference of only 10%. If a Grade 12.9 bolts was deliberately tightened past its yield point there is the possibility that using torque as a measure could cause some failures during initial tightening - allowing for typical torque measurement errors.

I am confident that tightening to a stretch of 0.15% will give enough preload and will not cause yielding.

[tcramer]

Thank You so much!