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Doing a head gasket/timing job on an Subaru EJ251. As I type this I have it down to heads on a bench and a short block on an engine stand... but now I'm wondering if I shouldn't just take it all the way?

I'm trying to talk myself out of doing the job, honestly. The blocks on these are a HUGE PITA to split, and there is a lot that can go wrong, but that would be required to go farther. That being said... there is nothing visibly wrong with any of this.

  • The cylinders still show good hone marks.
  • it had plenty of compression.
  • Pistons are solid as can be.
  • No rod knock that I can feel.

In theory, journal bearings operate as hydrodynamic bearings, so experience zero wear so long as lubrication has been 100% maintained to spec. Similarly, the same could be said about rings and cylinders. So long as the cylinders are maintaining a nice film of oil 100% of the time, all of that could last indefinitely too.

The critical thinking here is, "theory rarely holds up to practice." It would be a shame to put this thing together all clean and proper, just to have it die in < 10k miles because of the few things I didn't overhaul.

If anything is certain that could/should be fixed, it could use new rings. They work, there is just no way they still have all the meat on the bone.

Then again, if it ain't broke, don't fix it...

So... Does it make sense to replace the main bearings and rings? Is the cost and time put in going to tangibly improve the engine?

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If you take the tact of "if it ain't broke don't fix it" then why would you rebuild it in the first place?

If the bottom end is tight and you have minimal to no blow by, just put the head gaskets on it and call it a day. There is no good reason to split the block unless you are having issues.

Horizontally opposed engines are a PITB to split (as you stated) and rebuild. If there's no good reason for doing this, then just leave it together and call it a day.

If you are going for longevity's sake, then break it down and put in the bearings and rings. I don't know what is expected out of the EJ25 as far as mileage, but 250k is a little bit of wear, I'm sure. If you want it to last another 250k, it might make sense to tear it apart since you have it on the bench.

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  • Answer to first question: It is getting rebuilt because the Chinese/Gates water pump ate it's bearings at < 15k miles, went wobbly, snapped the timing belt, then some pistons passionately kissed some exhaust valves. . . It still had the stock "graphite" head gaskets, so, since I'm changing the previously mentioned parts, may as well change the known garbage gaskets as well.
    – Charlie
    Mar 19, 2023 at 2:47
  • To the second point.... There MAY be concerning blow by..... I'm not sure. There is enough oil/tar in the intake downstream of the PCV to suggest that there is more blow by than factory. Still well within the margins of a "good running engine" .... I wouldn't put a turbo on it though. That being said, I'm with you, there is no GOOD reason to split the block that I can see.
    – Charlie
    Mar 19, 2023 at 2:54
  • As for the last point. I would say in addition to the longevity, there is also the performance aspect. With worn rings, there is no way I could expect it to last if I ever put a turbo on it. Though, to be fair, it is hardly a WRX/STI either. Still, that door is 100% closed if the rings are not sealing well enough. My friend nuked a Nissan engine because he ignored signs of blow by. But, also, yes, if I'm not building it for performance, why not then build it for longevity? (other than it's going to be a b!7#h.)
    – Charlie
    Mar 19, 2023 at 3:00

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