ARP headstuds...

Right chaps,
I can never remember the difference between regular ARP headstuds and undercut versions.
What's the difference?,i've got both types side by side and i can't tell the difference,both sets have
different part numbers though...

"ARP undercut short bolts can help eliminate head gasket failures through providing more "stretch" to compensate for the additional compression of gaskets."

"You will also note that ARP offers specially undercut studs for several engines. This procedure (done only to the shorter studs) more equalizes the “stretch” of both studs, which makes for a more consistent clamping force—and one that compensates for head gasket compression when the cylinder heads are installed. This helps prevent blown head gaskets, and assures optimum engine sealing!"

gotta love Google.

Does that mean the undercut ones can only be used once, like regular head bolts? Would seem pointless if that is the case?

^^^^^ yes pretty much the undercut ones are like your conventional OEM one, so in theory you should replace them after use.

Headbolts are funny things as they have to undergo extreme changes in clamping force throughout each heat cycle of starting an engine, racing about, sitting in traffic and shutting her down for the day/night.

So when you look at a conventional stretch headbolt it is designed to offer good clamping force throughout the heat cycles as it stretches upon head expansion as the engine warms up and contracts again as the whole lot cools down maintaining even clamping force throughout.

Now here come's the best bit if you instal ARP head bolts you must retorque them to spec after a 1000 miles otherwise you risk headgasket failure.

Also if you speak to ARP's technical department they'll say that VAG headbolts are the best in the industry for standard bolts. Likewise I've seen OEM headbolts and headgaskets from VAG survive 600+hp/tq.

So as a rule of thumb if someone is building a FI engine for under 500ponies I always specify Standard VAG headbolts and headgasket. Why you ask?

Well look upon it this way most FI engines that are going 300+hp/tq are turbo cars. Turbo's unlike chargers are susceptible to boost spikes. Imagine you are on a full throttle pass as you boost spike and you have standard headbolts and headgasket. The result is the extra combustion pressure may potentially stretch your heabolts lifting the head causing headgasket failure. So you take the head of, skim the head, fit new gasket and bolts and torque down.

Alternativelly you are running ARP studs they do not stretch but the pressure has got to go somewhere. Take a guess what happens next.......


...... yep somethings got to give which tends to be the next weakest link, so either you bend the rods or chuck one out the side of the block. I know which one I'd rather have

What's the conrod bolts got to do with headgasket failures??.
I still can't tell the difference between the two,HEADSTUDS not rod bolts..

JNLRacing wrote:Now here come's the best bit if you instal ARP rod bolts you must retorque them to spec after a 1000 miles otherwise you risk headgasket failure.

How would a rod bolt failing cause head gasket failure, or do you mean head bolts?

I even wen't back to sort some spelling/grammar errors. God knows how I missed that one! Can only put that embarrasing faux pas to two things

1) I install lots of ARP rod bolts and never headbolts so force of habit when typing about ARP's

2) Absolutelly shattered from too many late nights making sure someone can pick something shiny up tomorrow

That would be me then! I've always thought standard bolts had a lot more going for them than ARP's, when correctly installed with a metal head gasket.

I've run ARP head and rod bolts for over 2 years, daily (over 30K miles), and after 1000 miles, 2 of head nuts needed nipping up slightly and have been fine since.

The ARP rod bolts have not needed retorquing at all. The tighten, loosen, tighten (3 times) upon installation gets round that.

In my humble one, nearly all turbo related problems come from high boost. Up to 15psi in the majority of well screwed together and setup up engines shouldn't cause any problems.

Mind you, I would add to that I've had cracked manifolds most of those 2 years so probably haven't had the full force of the boost