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All of my passengers side arp bolts are in but none except for the very front one on the drivers side will thread in on the header bolts. Every time I try to bolt in the 3/8 1.000” header bolts they stop after 1-2 threads. But the bolts that came with the headers will go right in by finger without a problem. I’ve tried taking the washer off the ARP’s, using a different ARP bolt (as in I have a 6 left over), and tried using a lubricant. No change.
These are the cheap no marking bolts that came with the speed engineering headers. 10mm socket. These are the 3/8 1.000” ARP’s 5/16” socket.
Sounds like the problem of using SAE & metric bolts interchangeably.
SAE 3/8-16 is 16 threads per inch, metric 10mmx1.50 is 16.9 threads per inch.
So im even more confused. They all went in fine both sets on the passengers side. And only 1 (the forward most bolt on the drivers side will accept both. The 5 behind those will not accept the ARP’s.
What is the ARP part number on the package? Also, take those two bolts above, flip one opposite the other and see if the thread pitches match all the way up the bolts. If they don't then there is your problem. Like so...
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ARP bolts will not thread in 
