Hello All,

I just wanted to pick everyones brain a bit. I just finished lowering down my cylinder head (89 300 CE 3.0). My manual states the following torque specs:

1- 70nm (52 lbs/ft) Stage-1

2- Angle-tighten through a further 90 degrees - Stage 2

3- Angle-tighten through a further 90 degrees - Stage 3


Some specs for my Jag seem to be the similar. I'm I understanding correctly?

First torque to 70nm / 52 lbsft through the proper sequence. Then Go through the sequence again 90 degrees further. Then Go through the Sequence the 3rd time another 90 degrees further?

Seems straight forward but the wording seems a little "funny" to me. (Why not just list additional torque specs like most other engines?) - for my own understanding.

Thanks

That is the correct procedure and it needs to be done carefully. You don't want any interruptions in one of the sequences such that you lose your place. To be safe you can mark each bolt with a daub of paint. Then if you do get interrupted you can see which bolts you've already done.

Also, you need to measure the length of each bolt if they are used bolts and see that they are no longer than specs. If so they are stretched too far and should not be used.

The REASON they do it this way instead of another torque reading is that the idea is to stretch the bolts such that they maintain clamping pressure. Once you get to the stretching point, the torque does not change much. You could not achieve this with a torque measurement.

Get in a comfortable position with a good stout breakover bar and get ready for a workout. You will feel like you are just breaking the bolts in half.

Good luck,

Thanks Larry,

We must think alike. Whenever I do any headgasket, or intake torque / retorque, I always make sure to dab the bolts with a little touch up paint or grease pencil through the process just incase an interuption occurs.

After doing quite a few GM cars with torque to yield bolts, I just decided to order a new set for the Benz. (They did all measure within the 108mm I beleive it was.) but there's nothing worse than breaking a bolt half way through the job and having to pull it back apart again. I figured the new bolts were a good idea. (especially since I did'nt buy the car new and did'nt know if the head had been off before).

Thanks Again,

Brian

As long as they measure within spec, they won't break. That said, it never hurts to be careful.

Good luck,

I like the fact that Mercedes specifies a stretch limit for the bolts. Some people think of reusing torque-to-yield bolts the same as putting on dirty socks. It's not necessarily so, especially if you understand the stress-strain curve.

Torque values rely on the elastic part of the stress-strain curve. Clamp loads based on these values are prone to error, such as thread lubrication, grit, and burrs. Torque-to-yield puts you past the elastic curve, and into a somewhat flat zone where the stress doesn't change much with strain. The clamp load is much more predictable and consistent in that region, thus less chance of blowing a head gasket.

The head bolts are rather long for MB heads, which gives more usable yield. The torque-to-yield recipe (52 ft-lbs + 90° + 90°) uses only a small part of the usable flat zone. The specified limits on bolt length allow for reuse, and at the same time weed out any bolts that may have had a "problem".... such as torqued twice because of interruption or other loss of concentration, or excessive thread lubrication.

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95 E320 Cabriolet, 169K