I've seen several people talk about head studs for the x308 XJR (mine is as 2000 specifically) but it seems like most threads I've found do not lead to them actually being able to locate them. If I understand correctly ARP used to make them, but I'm guessing that was in a limited quantity. I plan on adding a notable amount of boost down the line and I'm a fan of building up a strong motor ahead of that to provide the best chances of not blowing it up! That being said, I have two questions:

1. While I understand a lot of high performance builds (of all brands and types) use head studs, I do still have trouble wrapping my head around how they are stronger than a bolt torqued to the same spec. If anyone has any insight into this I would appreciate it. Obviously if bolts perform the same, and I could just source new ones, that would make a significantly easier job of this rather than having to pull the whole motor.

2. If the XJR specific bolts are no longer available, does anyone possibly have thread and length specs that could be referenced so that I may track down similar ones? I feel as though there is a decent chance that an existing head stud for a different application may work in these Jags and it could help future builders if I could track down ones that are readily available and known to have the same specs. Theoretically I could even work with the specs from a stock head bolt then make the necessary adjustments to calculate what the stud should be.

Thanks for any input on this, and I'm sorry if I have missed this information elsewhere. I promise I did search the forums before posting this and hope I am not beating a dead horse!

 

1. I also thought of using ARP head studs for my SC engine rebuild but they don't seem to exist. The advantage of studs over bolts is that they can be torqued more accurately and more evenly (between themselves). Everything affecting the accuracy of the torquing is right at the top of the stud - the thread, the nut and washer and the friction during torquing can easily be made equal between the studs by using moly grease (always provided by ARP in their stud kits). In the case of bolts, the friction is at two separate places - the thread deep down in the block (difficult to assess its condition, it may have some corrosion) and at the top under the bolt head. Due to the friction in the threads, the bolt shank, especially the long bolts like Jag's, will twist forward a bit during torquing effectively making the bolt "shorter". The twist will relax back (the bolt gets back to its static length) when the torquing is stopped causing some loss of clamping force. However, as the Jag head bolts are, in the final stages, torqued to angle (Torqued To Yield), most of the negative effects I mentioned are eliminated.

2. There are no SC specific head bolts, all engines use the same p/n - originally NCA2650AA (Hex head, NLA), later superseded by AJ89986 (Torx head). AJ89986 are easily available and relatively cheap.

 

I have head studs in stroker, fully rebuilt TS engine. You can buy a custom made set from Raceware ( Thanks Avos) . Just ask studs for Jag 4.0/4.2 V8 engine. Very reasonable price. Lead time 4-6 weeks.

 

This somewhat confirms my suspicion, which is that as long as the head bolts are installed carefully and correctly, they should theoretically perform just fine. The biggest thing would be using brand new and ideally the later style bolts, with some of that grease under the lip of the head to get the most accurate torque possible. I appreciate the detailed answer!

 

What would be the best way to contact them? Do you have a link or contact details? I would be curious to see what the price is.

 

Call or email. I talked with Randy.

Edit: $540 including removal tool ( 2019)

 

At least Jaguar got the head bolts correct. Bolts in the Mercedes 6.3L motor did not have enough meat where the head meets the shank, and after a few years the head would fall off, followed by a blown gasket. If you were "fortunate", it happened under warranty.