Sbc leakdown test reveals leak to ARP studs threads

[Justa1time]

Good Afternoon,

Engine appears to run and drive along fine. No overheating. No misfires.

I have never seen a leakdown test leak pressure from the cylinder to surrounding head studs.The test pressure was conducted at 100 psi with the engine cold. Outside air temperature @80°F. The tank of the compressor was fully drained and filled with fresh air in our arid desert oregon climate so the moisture in the cylinder as far as air injected was minimal. I pulled the plugs the day prior and blew out the holes to ensure they were not saturated with fuel or oil and let it sit overnight. I remove the valve covers and make sure all all rockers are loose prior to testing and the harmonic balancer is marked in 90° increments (also verified near TDC movements with screwdriver) to ensure the test was performed at the same piston position on every cylinder.The Radiator cap was removed and fully topped off with no air bubbles in the slightest during the test of all 8 cylinders. There are egt bungs on the primarys of the exhaust so those plugs were removed to hear any air moving through the exhaust. Carb wide open and air cleaner off to detect air. The results of the test were as follows:

1. 4%
2. 2%
3. 1% leaks past studs threads
4. 12% leaks past studs theads
5. 5%
6. 21% this cylinder when initially charged with air made a slight noise and the gauge fluttered.Then gargling was heard and I look around to see where the noise is coming from past the threads of the ARP stud. I disconnected and completely tested again and cylinder showed 5%. Whoa what?
7. 4% leaks past studs thread
8. 8% leaks past studs thread

** I checked the torque on all the studs and they were correct!**

The block is a GM sportsman 400 block (specific bore size unknown).
AFR heads (AFR cannot identify these heads so I am going to tear it down and measure the valve size, runner size and chamber size to find out.
ARP studs
Shim type gasket (see photograph for reference)

I used a turkey baster and some oil to saturate all the studs during the test as I discovered this initially on cylinder #6. I had to go back and saturate the others with oil and perform perform test again and found many leaked- even as low as 30psi test pressure. I can only imagine what may happen during peak cylinder pressures. This engine has a vacuum pump for for crankcase ventilation anyway so this may be why the anomoly wasn't detected by the previous guy? You would think this would froth the oil and cause all sorts of cavatation problems. I realize the head gasket is failed near cylinder #6. But that doesn't explain why the others with respectable leakdown results leak just the same past the studs threads. I took a three second video of this situation on cylinder #2s stud that is recessed in the head where oil submerged the fastener for anyone who wants to see. I am unable to post a video to speedtalk though. If someone tells me how I will gladly.

Has anyone else had this situation happen? Is this only a metal shim gasket situation? Does this seal up once warm? Perhaps I should run this engine and immediately test again? Would this happen with a head bolt and washer where the machined flat mating surfaces (and obvious bolt head to shank casting) would likely prevent the air from excapaing? Is this a potential surface finish issue? Do some of you machine a snug fit locating Dowell in the head for each stud and also in the block to avoid this situation?

Thanks for your time,

Bryan from Oregon

[dannobee]

Time to pull it apart and check flatness and surface finish for what is recommended for those particular head gaskets.

[BOOT]

Is the block blind holes or thru for the studs? If all the studs leak a little, I would think it's a thru hole bock and the block is maybe cracked. I mean if the leakdown passes the rings on the good cylinders and it's a well sealed engine as you said it has a vac pump.

IDK just my WAG

[rustbucket79]

Leak down at operating temperature is the correct way since everything is at actual size/shape/etc.

What lead you to the test in the first place?

[Justa1time]

Rustbucket: The previous owner ran the engine on 110 octane. I bought this complete car to use parts and the engine in another vehicle I have. I decided to do a cold cranking compression test to try and get an idea of the fuel resilience of the engine to see if I could carefully make it run on 91 from the pump. I removed a valve cover to have a gander and discovered the poor valvetrain geometry in my post earlier in the week. Since then I have been second guessing the others work and carefully combing through the engine checking most everything and forming parts lists for a tear down over the winter. I also have a leak down testing obsession.

Boot: Not to sure on the block design. I have not taken the whole thing apart yet. I am just scratching the surface.

[Momus]

Tear the whole thing down and check everything.

You are pissing around if you don't.

Steel shim gaskets want a very good finish. Yours may be several times reused as well.

[Momus]

Tear the whole thing down and check everything.

You are pissing around if you don't.

Steel shim gaskets want a very good finish. Yours may be several times reused as well.

[nwhp]

With the exception of #6 those are good numbers from a cold engine. I'd test it again warm for comparison before tearing it down.

[rfoll]

Using studs instead of head bolts is the best for a 4.125" or larger bore sbc, especially a factory block. But...they can be a real pain to seal. The threads on the block and stud, or bolts for that matter, need to be clean and free of oil for the thread sealant to work. I learned this the hard way. Fortunately the engine was on a test stand. If you call AFR and give them the info on the various markings on the heads, they should be able to identify them.

[BillK]

Some of the replies are talking about sealing the threads but in this case that has nothing to do with it. You are leaking from the combustion chamber to the studs. Only thing that can do that is the head gasket unless there is a crack.

What I would do is go around the head bolt sequence and one at a time, loosen the nut about 1/2 tun and then retorque it to spec. I bet if you mark a few of them you will find that they turn quite a bit when you retorque them.

Realistically speaking though if its leaking then the gasket is probably compromised and you need to pull the heads and take a look.

[rebelrouser]

I have seen this issue, crack in the block from the head bolt hole. On one mud truck we used water glass to fix it, and it ran for several seasons. The engine ran great no issues, just leaked it down at the end of the season to see if it needed any attention. Had one cylinder with a crack in the block to a head bolt hole.

[bassettmustang86]

Either way the heads need to come off the block needs to be checked. First check the gasket,check block for cracks, and see if the block has blind bolt holes are not.

[Tuner]

I would put a half-teaspoon of this and one ounce of sodium silicate (water glass) in it and watch it stop leaking.

[rebelrouser]

Either way the heads need to come off the block needs to be checked. First check the gasket,check block for cracks, and see if the block has blind bolt holes are not.

One thing I will point out, when using water glass, READ the directions, no antifreeze in the engine when sealing the leak. I have had customers on stock vehicles try and use water glass and did not read the directions, totally destroyed their radiators and heater cores. When using with antifeeze it makes a kind of jelly. Not good. They told me they drained the radiator, and I said what about the block?

[rfoll]

Some of the replies are talking about sealing the threads but in this case that has nothing to do with it. You are leaking from the combustion chamber to the studs. Only thing that can do that is the head gasket unless there is a crack.

What I would do is go around the head bolt sequence and one at a time, loosen the nut about 1/2 tun and then retorque it to spec. I bet if you mark a few of them you will find that they turn quite a bit when you retorque them.

Realistically speaking though if its leaking then the gasket is probably compromised and you need to pull the heads and take a look.

I confess, head stud and leak leads me to water.