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Ask HN: Different size nuts?

5 points by lsiunsuex ↗ HN
I know HN is more computer / IT / programming related but you see the random article / question here and I've always seen people from different backgrounds post, so... There are usually some pretty smart people here.

In replacing spark plugs last night on a 2016 Ford Mustang - I had to remove my strut tower brace to remove the engine cover to gain access to the spark plugs.

The 2 bolts for the strut tower brace closest to the engine are 10mm by 1.25 thread pitch while the 2 farthest from the engine are 10mm by 1.5 thread pitch. (4 bolts on either side).

I got the nuts confused - they weren't marked differently and I couldn't see the difference in pitch - so... I stripped 2 of the 1.25 nuts - 1 on either side.

(I'm told I'm safe to drive it with 3 of the 4 properly torqued to 35lb until I can get a die to recut the bolts, and new nuts)

The question - WHY would a designer / engineer use 2 different kind of bolts / nuts - so closely to each other, providing the same purpose?

I'm curious what the reasoning is behind this. Do the different thread pitches provide different security for the bolt?

With the amount of boost I'm running (tuned) and methanol injection - I'm told I should change the plugs every 10-15k miles so this won't be the first or last time the strut tower brace will need to come off.

8 comments

[ 3.0 ms ] story [ 30.3 ms ] thread
LeoSolaris
It increases the difficulty to repair by adding accidental cost to repair shops. It also makes using aftermarket parts harder.

In the end, it's about revenue for the OEM's. Quicker, more expensive breakdowns leads to buying new cars faster rather than repairing old ones. Those sorts of things are business choices, not engineering ones.

lsiunsuex
Honestly, that's the answer I was expecting, but was hoping there was some cool / logical based reason for it, lol...
m0ck
I was expecting quite different question based on the title and first paragraph.
brudgers
I expect that bolt selection was algorithmic not some individual's deliberate decision...and with a near certainty not some individual's or conspiring group's malicious one. I mean, having to remove the strut brace to replace the spark plugs sets the context: a 2016 Mustang is not grandpa's '66 straight six. Sparkplugs have been chosen with 100k service life. The average 2016 Mustang will have one spark plug replacement during its service life. Expecting ASE mechanic type expertise that one time during normal use is not entirely unreasonable.

In the grand scheme of things, this seems like a pretty cheap DIY hot rodding oops. Fixable with a twenty dollar bill and a trip to Harbor Freight. It's not scored pistons, bent valves, and shrapnel in the oil pan. Good luck.

lsiunsuex
Basically yes - get a bolt die cutter in 10-1.25 size, re-cut the threads on the bolt (their welded in, otherwise I'd just replace the bolt) and buy a new nut. No big deal - just a PITA.

While the OEM spark plugs might be OK for a stock car, it's been tuned by a shop for more boost and soon I'll be running a methanol injection system - so, lifetime of the spark plugs drops to every 10-15k miles, so I've been told.

</afterworkhobbies>

brudgers
Looking at Youtube, there's no mention of different thread pitch for the strut tower brace mounting bolts when discussing the performance pack or upgrading with the Ford kits. The videos depict what looks like the same threads on both bolts. Maybe it's not the first time a die has been applied post factory?
lsiunsuex
2 different nuts based on this video / product purchased

https://www.steeda.com/steeda-mustang-strut-tower-brace-2015...

brudgers
To the best I could determine, the OEM part only uses the two large studs (each side). It does not require use of the strut mounting bolts. Unlike the Steeda part, partial disassembly of the strut system is not involved with the OEM system. For additional rigidity the Ford part has a supplemental brace to the firewall.

Are you sure the engineering surprise is attributable to Ford?