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Old 04-08-2002, 11:49 PM   #1
Patrick Olsen
NASIOC Supporter
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
1997 Legacy 2.5GT
1996 Impreza coupe

Question Is anyone actually running SOHC gaskets on a DOHC 2.5L?

My car is at the dealer now and will soon be receiving a new set of heads and cams. (Well, not new, a used set that has been ported by Cobb Tuning). The engine has 95k miles on it, and has blown the head gaskets twice - once at just under 60k, and again at around 76k. Needless to say, I'm wary of putting the same type of 3-layer DOHC gaskets (part number 11044AA471) back in there.

In the past, I've read here on the i-Club that using the one-piece SOHC gaskets (part number 11044AA521) on the DOHC engine would bump the compression ratio from the factory 9.7:1 up to around 10.0:1 (the same CR run by the '00+ SOHC engines). However, I'm not sure that's correct anymore based on the numbers that my fellow NESICer, wac, posted on his website http://wac.addr.com/auto/obs/turbo/ejcalcs.html . Using the numbers on Wayne's site I get a CR of 10.7:1 if I put the thinner AA521 gaskets on my DOHC engine - that's a bit too high for my liking.

The second time my head gaskets blew I had talked to Cobb Tuning about what my options were, and I was under the impression that they recommended using the thinner SOHC gasket. When I called them today, though, they said I should stick with the stock DOHC gasket. They also said that the AA521 gaskets would only bump CR to about 10.2:1 which isn't nearly as bad, and that it has been done before. One other thing they pointed out was that intake manifold and exhaust header alignment might be a problem, what with each head being nearly 0.100" closer to the center of the engine - definitely a valid point.

So, all that background is to support a couple of simple questions. Is anyone here on the i-Club running a SOHC gasket (AA521) on a DOHC engine? And if you are, did you have any problems doing so?

Pat Olsen
'97 Legacy 2.5GT sedan

PS - From what I've read here, I'm almost positive the so-called "Phase 1.5" engines (Phase 2 block with Phase 1 DOHC heads) used the 3-piece AA471 gasket, so as far as I know there is no factory precedent for using the SOHC gasket with DOHC heads.
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Old 04-09-2002, 12:58 PM   #2
Scooby Newbie
Member#: 9432
Join Date: Aug 2001
Location: Brampton Ontario Canada
1997 Outback

Unhappy Another part no.

My dealer is just finishing up changing the head gaskets on my 97 Legacy Outback (2.5L DOHC). Over the past 2 weeks, I did a LOT of reading here (and elsewhere) about head gaskets. Patrick, I've seen your name come up a quite a bit.

I've been meaning to post this info in your topic of head gasket part numbers, but just hadn't gotten to it yet.

The part no. going into my car is 11044AA610! Yes, yet another number. They told me that this number supercedes 11044AA471. They didn't know about any other numbers (which perhaps are prior supercedes?).

The "610" gasket is a 4 LAYER affair! It looks similar to the prior 3 layer ones, but has an additional thin metal layer on one side. They also showed me the gasket for the SOHC engine. It looks like just one of the single thin layers of the multilayer gasket. I wasn't brave enough to try it, let alone double stack it. Just a couple of weeks ago, Cobb suggested that I use this gasket. I decided not to. I'm sure the dealer wouldn't have warrantied the job if I had (but I never asked). Their point about the fit of the intake etc is correct. I'd also be concerned as to whether one might run into valves contacting the piston with a thinner gasket, especially if the head was machined.

My dealer said that they've done "a bunch" of head gaskets since the new year. Nothing last year; now they are seeing a rash of it. 2 a week over the past couple of months. And it's NOT just the DOHC either. There was a 99 Forester in just ahead of me with the SOHC engine. I saw it.

From what I was able to find out about copper gaskets, I don't think I'd try them WITHOUT O-ringing.

I wish I could tell you what would be the best gasket to put in. Heck, I wish someone could have told me a week ago. I don't know if there is a "best" answer.

I should add that I'm in Canada, but surely, the part numbering system is the same for all of North America... One would think.


Last edited by Commuter; 04-09-2002 at 01:06 PM.
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Old 04-09-2002, 03:29 PM   #3
Patrick Olsen
NASIOC Supporter
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
1997 Legacy 2.5GT
1996 Impreza coupe


Very interesting, thanks for the info, Commuter. I'm about to head over to the dealer now to drop off some more stuff for them to install. While I'm there I'll ask my parts man about the AA610 part.

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