View Full Version : question about 2.5 liter engine head gaskets

10-29-2005, 12:28 PM
When I bought my car this summer with 196,000 km on it, I didn't notice that the previous owner had kept a record of all maintenance and repairs in the back of the manual.

It look like he bought the car with 89,500 km on it, and I don't have any records from before that. Both head gaskets were replaced at 126k, and then again at 154k! That is ridiculous.

My question is, is the problem with the design of the head gasket, or is it inherent to the design of the engine? I just hope the damn things don't go AGAIN.

10-30-2005, 12:03 AM
from the sound, the first replacement wasn't the updated version, or wasn't done right. though done poorly, they lasted a while. if replaced with the new version, my understanding is they'll be good indefinately.

correct me if i'm wrong...

10-30-2005, 09:57 AM
+1 for ooberdoob

or maybe the dude was paranoid. can you get in contact with him and ask him?

gator gt
10-30-2005, 01:51 PM
Depends on the date of the change, not the mileage. So if his records are that good, see the corresponding date with the gasket change.

And yes, (while not indefinitely), the new gasket design should sustain your motor for a markedly longer period of time.


10-31-2005, 08:23 AM
I have no clue who the previous owner was, the car was traded in to a VW dealer an hour away and then bought by a local used car/equipment dealer, and I bought the car from him.

Looks like the head gaskets were last replaced in 2002 or 2003.

10-31-2005, 04:28 PM
The Phase II design for headgaskets came out in late 99 or early 2000, IIRC, so you should have the new headgasket design, and you'll be ok :D

10-31-2005, 05:43 PM
Course if you decide to go with new ones, you can always get some nice Cometic ones. :)

10-31-2005, 05:44 PM

10-31-2005, 05:46 PM
Yup, they make very high quality gaskets...

www.cometic.com (http://www.cometic.com)

A local Subaru guy works there - he also used to work at the dealership nearby.

With the right gasket (ultrathin), you can increase the effective compression without changing pistons. :D

10-31-2005, 05:48 PM
With the right gasket (ultrathin), you can increase the effective compression without changing pistons. :D

I thought that depended on the valve openings? I guess there's more than one way to up compression. Better compression=better low end torque :)

11-29-2005, 10:47 PM
With the right gasket (ultrathin), you can increase the effective compression without changing pistons. :D

I thought that depended on the valve openings? I guess there's more than one way to up compression. Better compression=better low end torque :)

Wait i'm confused, sorry as the tag says i'm a noob...
How does using an ultrathin headgasket change the compression ratio of the engine?

11-29-2005, 10:56 PM
well, ultrathin gaskets would change the airflow a little bit, possibly upping compression, but I don't really understand it myself.

We need Phil, I think...

11-29-2005, 11:08 PM
It changes the volume of the cylinder by bringing the head closer to the block.

Mind you, it changes it very, very little.

11-29-2005, 11:09 PM
Just decreases the distance from the bottom end to the heads, by a fraction of a mm or whatever. Its a simple equation, we all remember geometry!

volume = pi * radius(squared) * height

Less height, less volume, more compression

11-29-2005, 11:17 PM
hmmm, i think I'd like to go with some ultra-thin gaskets then :)

11-30-2005, 12:58 AM
It reduces the volume of the cylinder.

Displacement is calculated as volume of the cylinder in the block, the volume of the head, and the volume of the head gasket.

In real terms, displacement = Piston Displacement + Clearence Volume.

Clearence Volume is the head volume + gasket volume + cylinder volume from deck to piston TDC (plus piston depressions and such).

Compression ratio is (piston displacement + clearence volume)/clearence volume

Reduce clearence volume and compression ratio increases. Thin headgaskets reduce clearence volume.

11-30-2005, 02:15 AM
I'll give you guys some fairly real numbers to play with.....

Phase 1 engine. Phase 1 heads. Stock compression ratio is 9.7:1.

Head gasket thickness is .152cm, bore is 9.96cm. Bore is 9.96cm. Stroke is 7.9cm. Head Volume is 46cc. Quench volume is 11.6cc (area between top of piston and deck of head, excluding head gasket).

So it all adds up.

3.14(4.98^2)x.152 = 11.84273868cc (.152cm head gasket volume)
3.14(4.98^2)x7.9 = 615.5107601cc (piston displacement)
Head volume = 46cc
Piston Volume = 14cc (the amount that fills any cavities in a piston, valve indents or any other clearences)

So plug it all in.

Piston displacement + clearence volume (w/ .152cm head gasket) = 687.3534988cc

Divide that, by the clearence volume (11.84273868 + 46 + 14)

You get 9.567473504:1. Pretty close to stock published numbers. This can be effected by measurement faults or simply manufacturing error. The factory would round up, and call this a 9.6:1 Compression Ratio.

Now replace this with the Phase2 SOHC head gasket, which is .058cm. Just plug the number in.

The new head gasket volume is 4.518939758cc. You'll have to remember, this is COMPRESSED (to proper torque) gasket thickness. That makes a BIG difference. Usually, the fire ring around the gasket is the compressed thickness.

Plug in the new gasket volume, and you get 680.0296999 divided by 64.51893976

You get your new compression ratio of 10.54000116:1. Round this like factory down to 10.5:1.

What will this do for power? According to a webiste I found by using Google (www.bgsoflex.com (http://www.bgsoflex.com)), a jump from 9.5:1 to 10.5:1 on an engine making 165bhp will net you a 3% power increase, making 169bhp.

Of course, that is just an estimate. The gains may or may not be seen.

Premium fuel should be used if you are running 10.5:1. Traditionally, higher compression engines usually yield greater fuel economy.

Hope that helped someone!

11-30-2005, 05:00 AM
wow. I am going to go out on a limb and say you know a LOT more about this stuff than I could ever hope to. Hat's off to you BAC! That made my stupid "volume of a cylinder" thing look like I'm a total bonehead :lol: Thanks for the info and the compression lesson!

11-30-2005, 12:30 PM
BAC - just a quickie, the SOHC EJ25 in the 05 RS runs 10.2:1, and they are fine for running on 87 octane.

11-30-2005, 07:15 PM
where can I find ultrathin headgaskets? :D

11-30-2005, 09:05 PM
www.cometic.com (http://www.cometic.com)

Located in Mentor, OH. Just up the road from me.
One of the guys that works there is also into Subbies - Kevyn Kistner.

12-01-2005, 08:17 PM
BAC - just a quickie, the SOHC EJ25 in the 05 RS runs 10.2:1, and they are fine for running on 87 octane.

They also have much more sophisticated engine management than the Phase1 EJ25 that I used for my scenario.

Going to a higher compression ratio slightly changes the volumetric efficiency of the engine. The stock ECU may be able to adapt to this or you may need some form of fuel control. I've never done it, so I couldn't tell you in truth.

Also, the jump in compression ratio should also yield a slight increase in fuel economy as the engine is now volumetricly efficient.

But because of this higher compression ratio, you are more prone to knocking and pinging. It's a double edged sword. An ECU designed around the idea that volumetric efficiency will be within X range, and suddenly (I say suddenly because when the ECU last ran it was everything OK, and when it starts up tihs time, it is different). It is entirely possible for the new volumetric efficiency to exceed the stock tables in some situations. That will throw a CEL and probably retard timing.

ECU's have a very easy job to make a CEL turn on. If something is outside a parameter, it sets off the light, and kicks into safe mode. Safe mode is running the best possible configuration for the least amount of damage (or no damage) should one of the malfunctions not be remedied. Newer cars are more sophisticated, and can determine best possible running without going immediately to max retard. I've heard of some ECU's setting a "trouble code rev limit" where if the CEL is on for certain things, it will set a rev limit at some RPM (around 3500 or so). I could only imagine that the cars with those types of things have drive-by-wire setups.

OK, this has gone OT. Damn my ADD!

12-01-2005, 08:37 PM
Damn my ADD!

Hey, ADD shouldnt be ma-



12-02-2005, 04:59 AM
This isn't OT, I swear. ;)

SOHC 2.5l with 9.7:1... Which engine code would it have on the VIN plate? EJ251/252/253? Or does the EJ25x number not relate to phase of the engine?

12-02-2005, 10:59 AM
BAC - damn good post. I'm really enjoying reading what you write, because it's the way I think, just with deeper info.


PS. I found out that the 05 RS SOHC EJ25 is actually 10:1, not 10.2:1

12-03-2005, 12:38 AM
Yes, I'm really surprised Phil (BAC) is taking the time to type all this stuff out for his.

You're the man Phil :smt023

12-09-2005, 12:14 AM
blown Head Gasket FTW!

so how much should this cost me?

12-09-2005, 12:48 AM
If you remove the heads yourself, and are just installing (say) Cometic headgaskets...

gaskets = $50
install time = depends on how good you are

12-09-2005, 01:13 AM
how much should i expect to pay at a garage?

12-09-2005, 04:27 AM
how much should i expect to pay at a garage?

When the HeadGaskets on my 1995 Legacy Wagon went out at 100k, it cost $300 for headgaskets and a new clutch. SHouldn't be more than $100-$150.

12-09-2005, 09:33 AM
headgasket replacement time = $80 an hour at Subaru dealership.

12-09-2005, 10:39 AM
Nicely put bac.

Thought i'd throw a few details in.

The newer ecu is at an advantage because its clock speed is faster. When running high compression the air/fuel ratio has to be more closely monitored to prevent detonation. The engine is walking a finer line.

octane ratings refer to the gasses compressibility. Go figure.

CC shape, port flow, timing, etc all raise the engines effective compression ratio. You don't have any more volume to fill, but by filling it better you are changing the ratio. Those things also have some influence on how much compression can be run before knock; fill the cc better and knock is resisted.

Increased CR should provide a solid increase in power all the way across the board, unless the ecu is pulling timing, heh.

At least in my car, the head gaskets can be done w/o pulling the engine. take off the valve covers, then pull the heads, the bolts wil take some finagling.

All second hand info.

12-09-2005, 05:41 PM
Scott - All good, mostly correct information.

Effective compression ratio is also known as "dynamic" compression ratio. If you read information that mentions this, it's the same as effective CR. Like Scott said, everything from valve size to valve timing, to piston dome/dish, all effect the dynamic CR.

When you replace a headgasket, you really should take the heads to a machine shop to be perfectly sure they are flat. If the car overheated to badly, they may have warped. Plus, the gasket indentations made from the headgasket could potentially cause a spot for a failure.

12-09-2005, 05:42 PM
Scott - All good, mostly correct information.

What isn't right?

12-09-2005, 05:46 PM
Oh, I dunno, I wasn't thinking.

I wonder if you could over-clock and ECU....

01-01-2006, 05:04 PM
so as i said before i blew a headgasket...

got it back from the garage the other day.. $1077.99 later i have new headgaskets, the heads were plained at a machine shop along with the flywheel. i got a new timing belt tensioner and a new waterpump. new clutch and pressure plate installed and everything seems great except the NOISE....

i should make a sound clip on the noise this thing makes. especially when its cold it is god awful loud from the engine compartment. like a wierd ticking sound that speeds up when the revs rise. im going to take it back but i dont know what to make of it.. could the internals of the engine have been damaged during the incident?


01-01-2006, 07:46 PM
I sincerely doubt your engine is damaged because of blown headgaskets.

Did they change your oil? Check you oil levels...

gator gt
01-01-2006, 10:30 PM
You're not alone. Right after my HG's were done (at the dealership) my motor started to make a noise under heavy accleration. Tech (whom I've trusted to this point) said that us car guys (he's a Ford guy when he's off the clock) tend to pick up on sounds more so than others, especially after major repair is done.

So I accepted it and decided I liked the extra 'growl'.

Yes, I accepted it at the time...but I'm now thinking it was half right. The noise is getting worse. The motor misfires when its been sitting cold for sometime. 30 seconds after start up, it misfires for 30 seconds until the ECU raises the RPMs enough...then its fine. Its only out of one cylinder. Not sure which one.

No CELs from it and thus, no codes.

But, the sound is there, more prominently in the cold, but still there when warmed.

Used to be heard only under load, and not during free reving.

Now, its louder under increasing load (accel), can be heard under constant load (cruise) and can be heard during free reving.

1. timing belt tensioner
2. piston slap
3. spun conrod bearing

#1. is nixxed as I ruled that out in some investigation.

#2. is the likely suspect.

#3. is unlikely cause I've driven the car for close to 3k miles since the HGs were done. Car still pulls strong. Motor would have stopped (or more catastrophic happenings would have occured) if this was the reason.

Besides, along with HG's, the phase 1 EJ25 is known for piston slap and resulting in fatiguing the block (ie: cracking). Or so says my parts guy and several postings on NASIOC. Piston slapping occurs in these engines due to Subaru's negligence in designing the pistons without skirts. Thus less stability.

Incompetence angers me....but incompetence that is known and gets ignored by the responsible parties angers me beyond no end.

Fix? $3000 short block and labor all said and done. Had this happened before the HG's, I'd have told them to do the 2.5 block and be done with it. I'm just going to ride it out as long as I can. If I can avoid dropping another large bill, I will. The block alone is $1800.

Stupid engineering mistakes.


01-02-2006, 12:37 PM
Guys - Techworks Engineering offer cylinder sleeves to help quell the piston slap. They also offer piston sets in various compressions to quieten the piston noise.

Expensive yes, but they are re-engineering a "buggy" Subaru design.

If you picked up a used block you could have them build it and then just have the heads refitted.

01-02-2006, 09:00 PM
yeah, you could have TWE build your car up with their stuff if you really wanted...it'd be a lot more reliable and very nice, TWE is good stuff.

01-03-2006, 10:09 PM
there is a misfire/ cel..... the thing sputtered out after i started it this morning... went back in my house came out 5 minutes later and it was stalled...

01-04-2006, 11:54 PM
JPaul... How much of that 1000 dollars was the headgasket and install? I almost got 70k on my 97 and I think I might want the headgasket replaced before any problems arise. Also, I freak out about my car blowing up cause of this HG problem.

Hey, my computer is playing Journey (Dont stop believin')! <insert family guy comment here>