Apart from having a better stronger head gasket, having a stronger cylinder head bolt is essential to providing optimum sealing especially if the engine is going to be making twice or more of its original horsepower.
ARP or Automotive Racing Products is reknown in motorsport for being the preeminent fastener source. ARP customers include Nascar, IRL and Formula 1. Unfortunately ARP does not have any off the shelf cylinder head studs for the Nissan QG16/18DE engine. This means the only way is to get measurements of the stock head bolt and send to ARP for the nearest equivalent. The shipping to send the stock head bolt to the US ain't going to be cheap so I inquired if I could purchase a stock QG18DE head bolt from Nissan US and have it sent to ARP for measurement. The answer is yes but Nissan only sells in minimum quantity of 5 bolts so I would have to purchase all 5. Anyway I did so and a couple weeks later the ARP equivalents were here.
Picture below of the stock cylinder head bolt including part number - 11056-4Z000
Picture of the ARP head stud I received including 12-point nut and washer. Material is ARP2000 alloy which is stronger than the standard 8740 chromoly alloy. ARP2000 is capable of achieving a clamp load of 220,000 psi while 8740 chromoly alloy can be heat treated to deliver a tensile strength of up to 210,000 psi. In contrast regular stainless steel is rated up to 170,000 psi. There are stronger alloys out there than ARP2000 but cost will increase exponentially and ARP2000 already is more than up to the task of holding the power I am expecting to make.
Close up on the 12-point nut
You can clearly see the difference in thread length and overall length. A quick email to my supplier confirmed the mistake.
So I now have to send the incorrect studs back to the US while my supplier sends me the correct ones. This will take a couple weeks.
Stop Press! - Email from my supplier - From ARP: There were only 4 in stock @ ARP when this order was placed. The ETA on the other 6 needed to complete this order is 2-3 Weeks.
This means by the time I get the correct head studs it will be a month or more :(