I have a 360 Magnum in my 37 Plymouth Coupe and have an issue with the header bolts coming loose over time. Have to tighten them every few hundred miles. Using ARP stainless bolts and the washers that came with them.

This is my first Magnum all other Mopar V8s have been big blocks or LAs. Is this a common issue with the Magnums?

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You might wanna go with OE shouldered studs & you can trim the outer fine threaded section so just enough remains for the nut to fit flush when tightened OR try another set of fasteners

I thought that was normal. My car has eddy heads with headers and mr gasket ultra seal gasket, and 5/16" head ARP bolts. I have to tighten them every few hundred miles as well. I don't put a lot of mileage on but they could use a resnugging about once a year. Some of the bolts don't seem to need much, others seem to get looser.

May I suggest Stage 8 locking header bolts?

Had this same problem in my Coronet until one came completely out in the water box and it sounds like the engine grenaded.

Switched to studs, don't recall the brand, and I never had another issue. Those Stage 8 units are nice, but pricey. Studs are cheap and work.

another option you can try with studs. there are several versions of steel locking nuts, some with deformed threads near the top of the nut, and some with indents on three sides. those might work.
do not use nylocks or those with nylon buttons for obvious reasons.
i call these things "mash nuts" because they grab the female threads.
there are correct names for these critters, but i can't remember them. [CRS]