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Neil Harvey Neil Harvey is offline
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Join Date: May 2004
Posts: 217
Regardless of what sealing system you use; proper assembly procedures must be performed.

Head stud stretch and any collapsing of the cylinder head must be known and under control otherwise any sealing system is pointless. The integrity of the threaded bores in the engine case must be of the highest quality and any cleaning or repair work must be performed with care. This area is the foundation of holding the heads down to the cylinders. Any sort of thread reconditioning must be done with a straightening tap and never with a cutting tap.

Typically, the assembly procedure requires a torque on each head nut but if the stretch of whatever head stud used is unknown, the torque value is meaningless. Friction must be considered here and anything that will lower the friction must be performed. When buying any aftermarket stud, the supplier should be supplying the stretch value that stud will achieve to put the stud into its designed “tensile stress”.

The head stud must be put into “tensile stress” so under running conditions when the cylinder pressure tries to push the head and cylinder apart, the tensile stretch or elongation added to the stud must resist any lift. This puts the head and cylinder under compression with each other.

Air-cooled heads often soften over time and many are never checked when they are repaired. This is a quick and important test before any repair is started. Check that the platforms where the head washers sit are not collapsed inwards as this will affect the clamping forces applied to the head studs. We want all the force applied to go into the stud and not be lost in any movement within the case threads, the head collapsing under the washer or absorbed by friction.

If you assemble the heads and only use a torque value to tighten the head nuts, you are not compensating for any of these issues. If you do not want to measure the stretch, then an angle method should be used as it eliminates any friction.

Without any stretch value, you should do a dry test assembly and measure the stretch of the stud at various angles. In our tests with different studs, we have found that the typical stretch was between 0.006” – 0.008” to achieve successful clamping. Make sure in any testing the stud returns to its relaxed length. Stretch can be measured in thousands of an inch or hundreds of a millimeter so measuring can be achieved with simple measuring tools. The same tool (dial indicator) you use to do the cam timing (lift) can be used here. This method can give you an angle to use in assembly that will obtain an average desired stretch falling within the elongation desired. However, the best method is to measure each stud.

Making sure you have equal stretch on each stud will achieve an equal compression load on each corner of each head. This eliminates the probability of any leakage between the head and cylinder from poor assembly. If you continue to have any leakage issues, you will know it’s not from how you are assembling the engine. Now you can consider sealing systems and other solutions.
We recently did our sealing system for a customer and asked for the head stud used to be sent with the heads, so we could test the stretch of the stud for him and give him a stretch and angle value to use in the reassembly.

I tell customers that all this fancy work and fancy parts are worthless unless good assembly procedures are upheld. It is typically the basic’s that let us down.

“Nothing will humble a man more than an engine or an angry wife”.
Old 08-12-2017, 03:41 PM
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