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chris_seven chris_seven is offline
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Join Date: Feb 2004
Location: UK
Posts: 2,230
We had a similar issue with Coventry Climax Wet liner engines and using Coopers Rings. The precision of the machining work was essential to ensure sealing and in our case to avoid water leaks.

Have you measured the change in hardness of the heads? I would be interested to know how much change occurs as this would allow an estimate of the stress relaxation to made.

Do you measure the stretch of the head stud with an an Ultrasonic Gauge or have you made a mechanical fixture?

We have made a fixture to measure preload on an individual stud with a load cell and we have used this to determine a suitable 'torque + angle' relationship for preload but I am not sure about how much preload is needed to prevent the head from lifting with a Turbo engine.

Sadly this is a background task for us so it is not something we work on every day.

If you are using standard steel studs then a stretch of 0.008" would result in a preload of about 3700lbs per stud which is about what could be expected when using the standard 32Nm.

This would mean that at a peak cylinder pressure of around 1400psi on a 3.0 litre engine the head would tend lift clear.

I am not sure how much the preload needs to be relaxed before the head would shuffle but I would like to see preloads of about 6000lbs per stud which should allow a peak cylinder pressure of about 2200psi before lift occurs.

The later version of the workshop manual suggests a torque of 15Nm and an angle of 90 degrees.

This would suggest a 'stretch' of around 0.015".

When we make measurements using 15Nm and 90 degrees we see preload values of around 7000lbs.

This is significantly more preload than tightening to 32NNm which does produce a value of around 4000lbs an

Have you considered making studs with an increased thread size on the case end? This could then allow a larger shank diameter and increase the preload for a given stretch.

It would avoid the need to use a case saver or timesert and would remove all of the material that has been highly stressed and suffered from the effects of stress relaxation in the area of the case thread.

Last edited by chris_seven; 08-13-2017 at 03:58 AM..
Old 08-13-2017, 03:25 AM
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