Conrod Bolts - Measuring Stretch

[Si.G]

Hi Everyone,

You will probably be familiar with using a torque wrench when tightening conrod bolts, however some manufacturers like carrillo specify that the bolt should be stretched a particalar amount. For example, on my rods the bolt stretch should be 0.040-0.060in. So how do you measure that?

Tools Required
-- bolt stretch gauge, basically a dial guage on a fancy chassis so it can be used for bolts (see below)
-- the correct spanner for your rod bolts



Step
1 - get your conrod, piston and bearings all assembled. Tighten the conrod bolts so the dowels locate and the gap between the rod and end cap closes, but then back off the bolt so that it is finger tight.

2 - Set up the dial gauge so that there is some pre-load on it - fasten the dial gauge in place on the chassis and then loosen the shaft at the other end. Put on of the rod bolts between the dial gauge and the shaft, now push the shaft in so that the dial gauge needle moves about 0.020in. Now tighten the shaft in place. You now have some sprung pre-load on the bolt and the gauge is ready to use.

3 - Now going back to your assembled conrod, put the appropriately sized ring spanner on the rod bolt, now put the stretch gauge on the bolt, it is easiest to start with the end with the spanner on and push the dial gauge in so the other end can go over the rod. Ensure both ends of the gauge are located correctly on each end of the bolt.

4 - Now zero the gauge, mine has a moveable scale that rotates. Double check that the gauge is zero'd by moving it slightly and letting it settle back to zero. If it is not zero - repeat the exercise.



5 - Work out how far on the scale achieves the stretch you need. Each mark on my gauge is 0.005in. My gauge has some handy markers that you can move so you set the end point that you want. I am aiming for 0.050in stretch (see above).

6 - You are all set, tighten up the rod bolt until the gauge needle moves the desired stretch.


(unfortunately this image is poor, but need has moved).

7 - You are all done, repeat steps 3 - 7 for each bolt - ensure you zero the gauge for each bolt, because you will find that they vary in length.

Si.

[NitroDann]

Very nice writeup, im sure many people will greatly appreciate these articles. Should help the noobs to get their spanners out I hope.

Dann

[mazdatenfive]

Si, Am I right in thinking that this tool can be used in lieu of a torque wrench.

[NitroDann]

This is correct assuming that the bolt manufacturer specifies stretch rather than torque. Its a similar system to the torque to below yield, then angle tightening.

Dann

[Si.G]

Si, Am I right in thinking that this tool can be used in lieu of a torque wrench.

Dann, is correct with his reply, it depends on the manufacture, but in general stretch should be more accurate, because things like friction on the thread and bolt surface does not come into play, but not all manufactures will specify stretch. So, if a manufacture specifies a torque setting that is what you should use.

I put this together because torque is a common method and stretch is not so commonly known or obvious , so I thought it would be good to have here for info. I should have been clear about the torque vs stretch up front. Thanks for your valuable input, the question has probably crossed other people's minds.

Si.