Another ARP 425 vs 625 thread

[Brandoncoffey]

So I have been doing a ton of research on this, and from what I can find, most all pros are saying the 424s are good up to 750 800 hp 50 to 60 psi MAX on boost.

With that being said, they are saying after the first initial installation, run two heat cycles, then check and re-torque them, but that goes for both the 425s and 625s as far as having to get back under the head; you can take either one of them back apart.

The only downside to 425s is that they will still stretch a LITTLE overtime for the first year or two after the two heat cycles and re-torque.

The solution to that problem is straightforward, though after they have been run through two heat cycles and retorqued, there suitable for 6 months, then you should (check)- re-torque them and then repeat 6 months later (1year from initial installation) and then just one more time in 6 more months (1year and 6 months from initial installation).

They should be fine from there but most that I have talked to said they checked there’s one last time at the 2 year mark and were still at 125 to 123 FT-lbs.

I would personally check them every 6 months just because I’m OCD about my truck and keeping things right.

In check hope this helps as many of you as possible so you don’t have to spend the hours, well actually days, researching online and calling multiple reputable diesel builders-repair shops for actual numbers on the 425s. Best of luck to you all.

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[BuckMeister]

As long as you are not over 45 psi on boost, you should be okay a few times.

If you over that, it will fail soon as far as stock head bolts.

450-500 hp; you are probably around 40-45 psi.

I believe the driving pressure is what lifts the head.

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Cylinder pressure is what lifts heads.

Your stock head bolts clamp down 2000+ psi from combustion.

45lbs of drive pressure isn't going to bother it.

Just turning over your engine creates a lot more pressure against the head.

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[SKIBBEE4thGEN]

Cylinder pressure is what lifts heads.

Your stock head bolts clamp down 2000+ psi from combustion.

45lbs of drive pressure isn't going to bother it.

Just turning over your engine creates a lot more pressure against the head.

Not at all meaning to sound like an *******, but what is the reason for changing out the head bolts then?

I thought they were garbage, so you change them if you're running higher than stock hp.

I'm honestly asking cause I don't know anything about it.

I know what I read in these forums.

Thanks.


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[Chrisn162001]

Cylinder pressure is what lifts heads.

Your stock head bolts clamp down 2000+ psi from combustion.

45lbs of drive pressure isn't going to bother it.

Just turning over your engine creates a lot more pressure against the head.

Alot of confusion around when studs are needed seems to stem from the drive pressure/boost number recommendations.

Of course drive pressure won’t lift a head the exhaust valves are open when drive pressure occurs.

The recommendations for studs being based on drive/boost psi is you can measure drive pressure and boost pressure.

You can’t measure cylinder pressure very easily.

When you run a certain amount of boost/drive psi over stock on a 6.7 it is generally accepted that you increased injection timing in the tune which raises peak cylinder pressure it also allows more efficient combustion and lower egts along with many other benefits.

I will always recommend studs when you start swapping turbos and adding fuel.




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[BuckMeister]

Not at all meaning to sound like an *******, but what is the reason for changing out the head bolts then?

I thought they were garbage, so you change them if you're running higher than stock hp.

I'm honestly asking cause I don't know anything about it.

I know what I read in these forums.

Thanks.


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Basically what the other poster said, we cant measure cylinder pressure easily so we use proxies like horsepower, boost, timing, etc.

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[revivalroadfabworks]

After initially installing 425's on my truck with an ATS 3000 VFR VGT turbo and an Aurora 7500 twin-turbo system, I still blew my factory head gasket and spent the money on both a 425 set and now a 625 set.

Save some money and get the best.

Follow ARP's instructions unless your head manufacturer requires you to re-torque differently because of fire rings.

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[Schoolbus]

I got lucky and found a set of 625s used on eBay.

Picked them up for less than $500.

I am installing them this weekend.

Then I can crank it up tune wise without worry.

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