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Compression Test

the_jack

By the_jack,
in Engines & Forced Induction

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the_jack Enthusiast

the_jack

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Posted (edited)

i put my 25/30 together on the weekend with a genuine nissan rb30et head gasket and ARP studs for a rb25det head. i did a cold compression test after i had set the timing yesturday and i only got 30psi. the head is rebuilt and the bottom end came from a running motor. the studs were torqued up to 88nm as per the rb30 service manual.

all i can think of is that the nuts on the arp studs are wound all the way down and that the gasket isnt sealing properly.

or is 30psi normal for a cold test?

cheers

Edited by the_jack
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the_jack Enthusiast

the_jack

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when i put the head studs in i wound them all the way in then backed them off a turn but then when i put the nuts on they went all the way to the bottom and bound up so i wound all the studs all the way in.

SargeRX8 Grand Master

SargeRX8

Posted
Posted (edited)

Are you guessing that the pressure is seaping out across the thread of the stud? I want to keep eye on progress here, I feel I could learn something.

If it was me, I would have suspected timing could be off. You could do a leak down test before you take the head back off and see where you hear the PSSSH from. Even if timing is off one degree, you will get almost no pressure at all.

What I would do is a leak down test on the first cylinder(I'm guessing you are getting 30psi read across the board), listen to the air psssh. If its coming from the exhaust or air filter, your timing could be off or something more. If you take out the plugs from the other cylinders and can hear air coming out of the next piston, you know the gasket isn't sealing. Also what you can do if you do decide a leak down test, get some rubber hose and run it around the head gasket around cylinder one and listen if you can hear the air pssshing from where the studs are. If you can you know the issue.

Also after quick searches etc, the arp stud kit looks only to come with one set of washers...

Edited by SargeRX8
JonnoHR31 Grand Master

JonnoHR31

Posted
Posted

leak down and figure out where its coming from. i'd also get another set of washers anyway, you really dont want the nut on the end of the thread. not that it'd give you low compression but you should use 25 torque figures for the head bolts/studs not 30e(t)

SargeRX8 Grand Master

SargeRX8

Posted
Posted

Don't pull it apart. Pinpoint the source first. You'll ruin the head gasket as its been crimped already. You've also over torqued them a little(I don't think it should impact it that much though...) so you want to be careful the bolt isn't weakened by fiddling with it too many times. Its MUCH MUCH easier to do a leak down then it is to rip the head off.

r33_racer Grand Master

r33_racer

Posted
Posted

Yeh Mafia is right about the torque for those studs. ARP states 80ft/lbs which is about 108nm. You have under torqued them if you did it to 88nm.

If its the head, sometimes when they cut the seats, they dont seal properly until the engine is run for a while and then they come good. Unless ofcourse the valves were lapped in before assembly.

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