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Not a big question, ain't no thang.  Just kinda cool.

While splitting this EJ25 block, with each of the block bolts I removed there was a little hiss.  Didn't take long to figure out that wherever it was assembled - probably the factory - must have been rather lower altitude than my roughly 4000 ft.  The US and Japanese plants are both in the 700 ft neighborhood.

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Probably vacuum. I have heard it too when removing bolts that still had oil on the threads.

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No, I'm thinking pressurized air escaping.  It was assembled at low altitude with more air packed in.  Come to higher altitude w/ lower air pressure and it escapes.

 

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Posted (edited)

depending of course on 'if/how quickly' a seal is formed by the threads, there could be pressurized air at the bottom of any 'blind' hole right?

Edited by 1 Lucky Texan

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10 hours ago, jonathan909 said:

No, I'm thinking pressurized air escaping.  It was assembled at low altitude with more air packed in.  Come to higher altitude w/ lower air pressure and it escapes.

If threads were air tight it seems some would get notably hard to tighten in the process of tightening...I’m thinking this isn’t what’s happening.

There are engine assembly bolts (not Subaru) that require sealant when they pass through certain areas as well - because the threads aren’t sealed.

In theory threads aren’t engaged fully - one load bearing face of the bolt thread is seated against the facing load bearing edge of the receiving threads. It is not 3 dimensionally tight between the valleys and peaks. This presumably leaves a permanent spiraling air gap all the way up the shaft.  Not sure what then happens at the bolt head surface but given most metal to metal areas get orings or gaskets I’m assuming it’s not air tight.

Thats “theory” - in practice, an infinite number of interacting variables, some variation of possibilities would present themselves.

So if you’re hearing this, no matter the cause, I guess it is right in the beginning when the head leaves the surface, the “potentially” sealed part and before the volume of the bolt hood shaft increases during removal.

 

 

 

 

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You’re doing it way too slowly and quietly if you’re hearing a fraction of a teaspoon of air escaping! Lol

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11 hours ago, jonathan909 said:

with each of the block bolts I removed there was a little hiss

If you are hearing the hiss when you first loosen the bolt (when the bolt head breaks its seal to the head), I would say your assumption is correct. There is a volume of air between the shank of the bolt and the the bolt hole in the head.

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Posted (edited)

I'm pretty certain it was air at the bottom of the hole and not along the shank; this would be supported by there having been quite a bit of residual sealant in the holes, so I don't think the argument about the spiral gap along the non-contacting thread face holds up - that is, it's the sealant breaking that's allowing the air to escape.  And I don't think assembly would be noticeably affected by the pressurizing air as the bolt is tightened - it's just not tha big a space and air is highly compressible, right?

But I wasn't working that slowly, and there'd be a much larger volume along the shank, so that's possible too.

[edit]

Hmm... no, now that I think about it, you're probably right.  It was when the head initially broke loose that I heard it, so it's probably the air along the shank.  Any escaping the bottom of the hole would come later and not be as loud.

Edited by jonathan909

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Air is a compressible fluid. I never said “assembly would be affected”.  When holding a finger over an orifice of a rigid object containing increasingly pressurized air, the PSIs gets noticeable quickly. I meant that I would expect to notice it more commonly or in other circumstances if threads were air sealed - which I don’t think they are.

Thread engagement is a mechanical concept distinct from any “sealant”. If you found sealant then that changes the system. You never mentioned sealant in the first thread.  

It’s an EJ25....

giving up one last burst of head gasket exhaust gases meant for the combustion chamber....lol

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Posted (edited)

File the sealant part under "goes without saying", as the FSM specifies it, and when the halves get bolted together quite a bit gets squeezed into the holes.

Edited by jonathan909

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47 minutes ago, jonathan909 said:

File the sealant part about "goes without saying", as the FSM specifies it, and when the halves get bolted together quite a bit gets squeezed into the holes.

Indeed!  Good point.   Hence "in theory", I wasn't totally discounting the idea, but am hesitant on causation.  Though, can't be too many other options, so i was considering what physical realities are in play.

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Maybe the sealant out-gasses when it cures?.....anyway, interesting observation.

 

(I thought those bolts got oil on them, not sealant???)

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Nah, the vapour pressure wouldn't be high enough.  This is mos def trapped air.

The threads get oil, but the sealant is on the block mating surfaces surrounding the bolt holes, so it's inevitable that some squishes into the bolt holes.

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Nice observation and theory, but they give off the same burst of nasty smelling gasses here at sea level. So I can say it's not altitude related. 

GD

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Didn't notice a smell, just the hiss.  So what do you figure is going on?

 

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Likely - and essentially what I always assumed was happening. You have 35mm of thread engagement on the head bolts and oil is used to seal the threads so the bolt is acting like a piston that is compressing the air in the blind hole beneath it. In some cases it leaks past the threads, and in some cases it just waits many years till someone loosens it. There may be additional off-gassing of the thread lubricant/sealant that further raises pressure over time as it changes from a liquid to a gas and expands in volume once the engine is heat cycled and it's vaporization temperature has been exceeded. 

GD

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It is interesting - along with reasons why it could happen. Best of luck on the major project, separating the halves of the engine. Just wondering - is it because of a thrown rod?

 

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1 hour ago, GeneralDisorder said:

Likely - and essentially what I always assumed was happening. You have 35mm of thread engagement on the head bolts and oil is used to seal the threads so the bolt is acting like a piston that is compressing the air in the blind hole beneath it. In some cases it leaks past the threads, and in some cases it just waits many years till someone loosens it.

I figure both are true - that the air in the blind hole is being compressed as you describe, but that it's also at higher pressure to start with, so when vented at 4000 ft. it makes more noise than it would otherwise.

1 hour ago, GeneralDisorder said:

There may be additional off-gassing of the thread lubricant/sealant that further raises pressure over time as it changes from a liquid to a gas and expands in volume once the engine is heat cycled and it's vaporization temperature has been exceeded. 

GD

Not with you on that part.  We'd need to know exactly what's being outgassed in order to determine its vapour pressure, but I doubt it's that high.  But on the heating thing:  Phase transitions aren't a one-way street.  When it cools down it's going to condense back out.

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Posted (edited)
2 hours ago, Stelcom66 said:

It is interesting - along with reasons why it could happen. Best of luck on the major project, separating the halves of the engine. Just wondering - is it because of a thrown rod?

Well, the more times you do it, the less major it becomes, right?  High-miles engine spun a couple of rod bearings.  Most annoying part is that the bearings went about 8 months after I did the head gaskets.  Biggest challenge was just finding a good crank - I'm jammin' econo, since it's my daughters' Forester and I'm not real crazy about those things to start with.  Ended up being kinda funny, though.  Buddy doing some acreage yard cleaning popped up on the local online buy+sell (kijiji), hoping to get someone to haul off a pile of motors of unknown condition his kid had left lying around - two EJ25s and an EJ20.  So I gave him a few bucks and took 'em home like scratch'n'win tickets - how many do I have to pull apart to find a good crank?  Two, it turns out, then I gave the EJ20 to the friend I sling these things with.

Edited by jonathan909
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Posted (edited)

Wow - so you've done it more than once. I guess you lucked out 'only' pulling apart two!  I've heard multiple reports of rod bearings going within a year of replacing head gaskets. I respect the work you and others here can do on these engines. All 3 of my sons had Subarus at some point when they were living here, the most I did was oil changes, replaced a belt and a transmission fluid change. I had several Subarus, so I stocked up on PH3593 oil filters. I still have one in the garage, so I should get another Subaru with an EJ25 so it won't go to waste. 

Edited by Stelcom66

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Posted (edited)

To be clear, no props deserved at this end - I've done a few now, but that only makes me a slightly-less-clueless noob.

GD's the guy here to be reckoned with - the number that I've done in my life, that's how many he did this morning before lunch.  Oh - and he actually knows what he's doing.

Edited by jonathan909

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Posted (edited)
4 hours ago, jonathan909 said:

Not with you on that part.  We'd need to know exactly what's being outgassed in order to determine its vapour pressure, but I doubt it's that high.  But on the heating thing:  Phase transitions aren't a one-way street.  When it cools down it's going to condense back out.

Well - we can both agree that the thread lubricant goes in as a liquid. After many years of heat cycle it definitely doesn't come out as a liquid anymore. It dries out. So whatever lubricant is used loses it's ability to condense back out into a liquid. Not being a chemist I myself I'm not sure of the exact property that defines this phase change and inability to go back but it ends up "dried out" and that means that some component of it turned to a gas and whatever was left wasn't able to absorb it again. Leading to the hissing and foul smell that we experience. At least that's my theory. 

Rod bearing failure - was the block main-line honed to restore the clearances? The biggest issue I find is that the main line (especially on #2,3, and 4) gets compressed along the axis of the cylinders and the resulting excessive clearances end in rod bearing failure when the excessive main clearances drop oil pressure to the rods. 

GD

Edited by GeneralDisorder

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Posted (edited)

Understood, but no such this time.  As I said, this is a super low budget rebuild on a car that we got for $500, that my girls like, that wasn't written off a couple of years ago when a moose ran them off the road (after the previous two Foresters were destroyed in deer strikes), and that I don't care much about - beyond its ability to provide me with some more experience.  I've got a couple of hundred bucks worth of bearings/rings/gaskets going into this rebuild that's otherwise just junkbox, so I wasn't about to start multiplying the investment with pricey machine shop time.  Most of all, I just want it the hell out my garage so it's off the list and I can get to all the other work that's queued up!

Oh - on the lubricant... yeah, I'm sure you're right about there being volatiles that don't stick around.  Maybe some of them do remain as vapour, get absorbed into the metal, etc.

Edited by jonathan909

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