As title.

I'm thinking of upgrading to STi v.3 or 4 cyl heads (to make the most of a 20g+supp mods to be fitted at the same time), so was thinking which are the best (strongest) uprated cyl head bolts to buy? Cosworth?

I'm only going for the OEM-sized 11mm diameter ones, as I'm sure they're more than adequate for <400bhp.

 

The high tensile Cosworth bolts seem to do the trick. Not cheap though, but make sure you get the right ones and not the normal Cosworth Head Bolts I used them on my recent build, which is a tad more than 400bhp.

 

Cool. I take it APi are likely to sell them, or at least be able to order them in? Do you know what the cost is?

 

Circa £350..... maybe a little cheaper dependant on the reseller.

 

Sorry circa £330.

 

Blimey lol.

Do you know what the regular, er, lower-tensile lol Cosworth ones are rated to bhp-wise? Indeed are these ones actually any better than Subaru OEM ones anyway?

 

I have no idea how the regular Cosworth studs relate to the OEM ones. The regular Cosworth studs are the same as the ARP's, which come in at around £200.

 

Do you know what the ARPs are rated to?

EDIT: Ignore that - you would've said I'll have to investigate...

Cheers for the advice and stuff

 

ARP's would be fine on a 2ltr with the level of power you are looking at, but so may OE bolts. You need to speak to an engine builder.

 

Roger Clark Motorsport - RCM Product Detail

Apparently, the regular Cosworth 11mm ones have a tensile strength of 180,000psi or 12,400bar!!! I'm sure they'll be fine

 

That is the tensile strength of the material.

Normal ARP ones will be sufficient, but for 400hp the stock bolts should be fine too!

Paul

 

Yes stupid me re the tensile strength.

Paul if I do swap out the heads, would you advise on changing the bolts too (the ones you reused/replaced - can't remember - when you did my build). Or do you reckon they'll be okay. What's the price of a set of genuine OEM Subaru 11mm ones?


Paul, purely out of academic interest, what sort of max relative boost pressure can the Cossie, ARP and OEM ones keep a lid on things?

 

Think the answer will always vary to some degree, but for example we have seen up to 2.4 bar on standard ARPs with out issue which is were we stopped, this was with the old style small top locking nut, they have since been improved. Others have seen less and some more im sure.

Personally I would always replace OE bolts if you’re planning on pushing them further, but for piece of mind ARP's over cosy for cost on most applications

 

I suspect that answer also takes in to account block type used as well.

 

Cheers guys. The gen cons seems to be ARPs for the 'fit for purpose/price' equation.

Ta.