Only 6mm of thread engaged on rear skewer. Enough?

[Globalti]

I have already returned the standard Specialized QRs to the shop because both worked loose so I would not have felt secure with only half the thread engaged, especially as the RH cap was quite wobbly on six threads, telling me the engagement fit was not very good.

I'm taking the 130mm skewer back to the shop this lunch time to swop it for the 135 mm skewer they have got for me.

Edit: many years ago a crappy back-street garage fitted PAS to our Land Rover 90 using parts taken off a scrap Defender. On inspecting the work I was horrified to see that the junior mechanic had used too short bolts to attach the steering box to the main strut that braced it against the chassis and the bolts were only 2/3 engaged in the nuts. I am normally a placid bloke but I absolutely played hell with the owner of the garage who jacked the vehicle up himself and replaced the bolts with longer, without a word.

 

[Smurfy]

Yet the ISO standard for a M5 nut is 4mm thick

A fair point, but it all goes wrong if you have a skewer of steel and a nut of aluminium (which is the usual situation in a QR). When the materials are not equal, insufficient engagement in the weaker material (the aluminium) is a very bad thing.

 

[Smurfy]

many years ago a crappy back-street garage fitted PAS to our Land Rover 90 using parts taken off a scrap Defender. On inspecting the work I was horrified to see that the junior mechanic had used too short bolts to attach the steering box to the main strut that braced it against the chassis and the bolts were only 2/3 engaged in the nuts. I am normally a placid bloke but I absolutely played hell with the owner of the garage who jacked the vehicle up himself and replaced the bolts with longer, without a word.

In some situations this is OK, although I can well understand your refusal to accept what they'd done. If I remember correctly the amount of thread engagement required is a function of the ratio of shear strength to tensile strength. Shear stress is what you get in the threads, and tensile stress is what you get in the shaft of the bolt. For a properly designed fastener, the breaking/stripping force for the threads and the tensile strength of the shaft of the bolt should be about the same. For some thread designs, materials or combinations where the nut and bolt are different material specifications, what looks like insufficient engagement may be perfectly acceptable. However, I wouldn't trust a back street garage to make that decision as it requires detailed knowledge of what your nut and bolt spec is, and what the properties of those materials are!

If you want to have a play around you could try this:

http://www.tribology-abc.com/calculators/e3_6e.htm