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CP CARRILLO CON ROD BOLT STRETCH

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Hello,

I am in the middle of my 2j build and I am using Carrilo rods (CP 3/8 WMC H6).

CP recommended in their manual a rod stretch between 0.0050-0.0065 inches and that the torque should not exceed 54NM.

Now when I am checked the exact stretch point I found 1-6 con rods' bolts are stretching in a different torque.

** I used the recommended CP lubricant

1st-rod bolts stretched 0.0052 at 54 NM A bolt and B bolt

2nd -rod bolts stretched 0.0050 at 54 NM A bolt and B bolt at 60 NM

3rd -rod bolts stretched 0.0050 at 80NM A bolt and B bolt at 90NM

4th-rod bolts stretched 0.0050 at 90NM A bolt and B bolt at 70NM

5th bolts stretched 0.0050 at 100NM A bolt and B bolt at 70NM

6th -rod bolts stretched 0.0055 at 54NM A bolt and B bolt at 65NM

Then I contacted CP about the matter and they said -

The torque number is not that important, do not rely on the torque value. Use that 54NM as a starting point then check the stretch, tighten bolt until the required stretch is achieved. For your bolt should be 0.0050-0.0065 inches, 0.130-0.160 metric.

But when I discussed the same issue with another renowned engine builder (USA), he suggested something is wrong there. In this regard, what can I do? Am I making any mistake or doing anything seriously wrong here?

Thanks

Hello,

Installing to stretch is the correct method but since you're getting such variance in the torque values I would suggest doing further investigation. Of course there will be some differences but at almost double the recommended torque limit seems excessive.

Is you torque wrench calibrated and repeatable?

How many torque cycles have the bolts been through - could they have been tightened beyond the yield point? This can be determined by measure the free length creep after tightening to a specific torque a number of times.

I'd go back to CP again with these figures after measuring the free length creep and see if they have anymore feedback as 100Nm is what I'd expect to be upper limit for a 3/8 bolt (M10 closest ref)

As Scott said, that is a rather high torque value. This is speculation but...

If it is your first actual full stretch/torque on the bolts part of the problem may be the bolt and/or rod thread was still a little rough - some installation guides recommend taking the bolts to around 90% nominal torque to help burnish (smooth) the rubbing surfaces. You could try backing them off and doing several 90% tightens.

Just how confident are you that you are correctly measuring the stretch? There are specific tools that allow measuring stretch during the torquing down.

While most bolts are intended to be re-used indefinitely, the alloy used in this case has a 'life' of ~15 torques, but you're still well within that.

The Carrillo site specifically states a MAXIMUM torque to be used of 54 Nm (40 lbft) and if they have been overtorqued it's quite possible they have been stretched - while it's unlikely as there is some reserve for the additional tensile loading from the piston/rod mass around TDC, it's still something to check. Because of manufacturing tolerances you can't just compare them to each other as they may vary slightly, so it's a good practice to measure, and make a note of in the engine build file, the individual lengths of each bolt. That said, the most obvious sign of stretch is a 'necking', or narrowing, of the bolt shank or threaded section - if you see ANY indications they should be discarded. Did you use the lubricant under the bolt head as well, as failing to that would also be responsible for a higher than expected torque for the stretch.

This is actually a little confusing, as normally the important thing is the actual stretch, as this is approximately proportional to the tensile load on the bolt, whereas simply using a torque value can be inaccurate due to thread and head friction variations, and inaccuracies in the torque wrench used. The usual warning is when the stretch is achieved at a lower than expected value, as it indicates it's yielding (stretching) and is about to fail.

I really don't want to worry you, but unless under the heads was dry, I'd be inclined to buy some replacement bolts for those that took more than 70Nm, make sure they're burnished, and make sure they're well lubricated on the thread and under the head.

Oh, just to confirm - the bolt material and treatment is marked on the head, they are definitely CWM H6? The H61 max' is 79Nm (58 lbft).