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Custom head stud torque

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Hi everyone, Im new to the forums and would like to pick your brains for a bit.

Im busy repairing my McLaren M838T which is an 8500 rpm 3.8L V8, during the process of "repairing" it, I got a little carried away and ended up ordering a custom set of 200hp/cyl forged connecting rods and pistons, the big end journals happened to be the same diameter as a Honda K24's big end journals and I now have a set of King race rod bearings aswell, I have a new set of McLaren head gaskets and they came with a full set of head bolts, I would like to swap over to ARP head studs because these engines tend to lift the heads when approaching 27psi of boost, but I can't seem to find anything matching my application, there is no workshop manual available for this engine as McLaren keep it under wraps, the few performance engine building shops that do have access to the torque settings and head studs don't want to share. Is there any way to measure or calculate the amount of torque which I need to apply to the head studs? Also, is there someone with knowledge of the ARP parts catalogue or a contact at ARP who might be able to help me have a custom one off set of head studs made?

Thanx guys! Pics to come if I manage to get the parts!

First, I would be VERY leery about anything labeled with a generic xxx hp, especially for ANY forced induction engine, as how that power is achieved can make a BIG difference to the loadings on the components. And that's without considering the mass of, in this instance, the pistons.

You have the bolts (a little surprised they didn't use studs?), so you know the dimensions you will need and so the first thing to do would be to check that against what they have as normal production items and what alloys they're available in. Failing that, you can enquire directly with them for custom parts and their torque recommendations. The OEM parts are probably torque to yield, or 'stretch', so will be a service part.

Don't forget, the block threads may also be a weak spot, with a risk of pulling the threads if you don't maximise the thread engagement and/or over-torque them. If you have the material around the holes, you may wish to consider a larger diameter (make sure you pick up the threads in the same alignment, and maybe use with waisted studs) or use some inserts that offer greater resistance to being pulled, and wear reduction?

If you haven't done so, you should start here - https://arp-bolts.com/

Thank you Gord.

This motor runs a factory dry sump system, the maincaps the cradle and the sump scavenge section are all a single casting. Its got a 93mm bore and 69.9mm stroke with a cross plane crank configuration, although in stock trim it makes 70-80% of its useable torque from around 2500rpm, it isn't a lot of torque, this motor loves to rev and the car has quite short gearing, if your leaning in to it you'll never be below 5000rpm. Its a cross plane crank and has oil squirters aswell, the valve train doesn't have a conventional lifter system, instead it uses finger followers and the shims sit inside the upper retainer, the valves are factory Del-West items with stainless steel intake valves and I believe the exhaust valves are sodium filled.

So I sent my sample piston and rod off to CP Carillo, the stock piston is forged and it has or had a low friction coating on the skirt, the stock rod is a fractured cap rod, Im sure it's forged aswell, it's an I-beam design and the small end has been machined or ground to a trapezoidal shape. The new piston and rod combo is about 100g heavier than the oem parts. ( I haven't checked or compared the balance of the rods yet.) The new pistons have a similar low friction coating on the skirts and ceramic coating on the crown, we're hoping to achieve less heat transfer to the oil with the ceramic coating. Im stumped as to why McLaren didn't go for a stud instead of a bolt, I have measurements to order a custom set of studs but I can't seem to get through to anyone at ARP, the oem threads are 12x1.75, stepping up to a half inch stud wont work as it's too close to 12mm, there won't be enough material to machine new thread. I can't remember the dimensions for the studs off the top of my head, I'll have to check my notebook. The inserts sound promising but I have been sceptical about how safe this would be, Im afraid of having the block machined because I don't know how the casting looks to the lower side of the thread and if we might go into an oil or water port.

I really appreciate your input and feedback and Im always open to advice or suggestions.

Have a great day.

As Gord has mentioned it's most likely the factory bolts are a TTY design so they would need to be replaced regardless. Personally I'd be inclined to go down the path of having some custom studs made by ARP. I've done this many years ago and found ARP to be a little slow to deal with and my guess is they aren't overly interested in making a custom one off stud set but we got it across the line. I don't know what i can suggest other than to keep trying to get in contact with them.

The other alternative that might be worthwhile would be to bypass Mclaren and search out some of those in the aftermarket servicing or modifying them. This is still not by any means a guaranteed solution but you may find that if you're prepared to purchase the parts from them they may offer the technical specifications you're looking for.

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