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Old 12-29-2012, 01:49 AM   #1
Scooby Newbie
Member#: 318734
Join Date: May 2012
Location: Central, TX
Default 2.5L Knocking - Post Shortblock Rebuild / Head Work

Shortblock rebuild with swapped crank/rods/pistons by machine shop
Heads decked & blued and new stem seals by machine shop
2.5L SOHC swap on 1.8 ECU (was running prior to rebuild)
Engine making knocking noise
Not a cracked flywheel (rode clutch while in first and rev'ing, noise doesn't disappear)
Stethescoped: Noise appears loudest on passenger side, not in shortblock. Sounds almost even throughout engine/intake except on underside
Reset valve lash two times on passenger side and once on driver's side valvetrain
Found two bolts are missing on rearmost portion of passenger camshaft carrier. Filled with material, top bolt hole is half stripped. Possible the bolts were never present, or I was given back the wrong head by the shop
Removed head and carrier, cleared material from the missing bolt positions in the head
Pistons don't match bore markings. Notice pistons are not from a 2.5L SOHC (more dish, ****loads of dish, helladish if you will)

Action items:
1) Install competely different head in hopes cam carrier or rogue dead/collapsed spring isn't the culprit (costs head work and new gasket)?
2) Tear down shortblock because it's totally rod knock and the shop gave me a bad shortblock rebuild?
3) Do the teardown, but after I install a 2.2L block to 2.5L SOHC Heads and add a turbo later down the road?

For the details and process, and hopefully answer any of your initial questions in your effort to help:


This car was run at RallyX nationals with a 2.5L swap out of an '01 Forester. It went into a 1.8L (still using this ECU) Base FWD Impreza. Heads from one engine and the shortblock from another. Ran like a top for two events and street use prior. Story goes the co-driver failed to shift out of first (never drive stick) and the last run of the first day the engine died on him at the finish. Lots of banging around and rough idling DNF'd us and we went home. Two weeks ago I pulled the engine and tore down the long block. Lo and behold a disintegrated bearing

I'm offered funds to rebuild the block and I decide I'm going to save myself some time and effort and drop the heads and shortblock off at a local machine shop.

The Story:

The machine shop calls me to let me know my shortblock and heads are ready, but the crank and rods were used out of a different engine. (You could say there are some (read: one) subaru enthusiasts in the area and the local shop has many parts blocks laying around) Let's pretend, for the sake of this thread, the crank and rods are good though I never saw them.

I assemble the long block, spend the cash on a gasket set, get the lady installed and fire it up. Immediately I have a very lovely banging noise. Quite similar to my rod knock, but a higher pitched bang and very pronounced. I'm a bit surprised but vary the RPM to get a start-up 'break-in' and overfill an additional quart. As the engine gets to temperature the knock disappears at idle but becomes pronounced when I bring the RPM passed 1200. This video is a later documentation and attempt to diagnose the noise just before the 'at temperature' banging goes away; it is obviously still audible at idle here:

Ignore the suction. I have unnecessary sensors plugged in and looped to avoid CELs in ODBI cars. The knocking noise you hear appears to be coming from the top end, but more the passenger side of the longblock. With a stethescope it seems impossible to locate the source. It isn't any more or less pronounced anywhere on the engine. Not the valve covers, front/rear of the heads, or from the shortblock and intake on either the left/right/front/back of the engine. Definitively we cannot say, but decide it is on the passenger side of the engine. The only noticeable place the banging is not as pronounced is underneath the engine (though I'm realizing now I did not utilize the stethescope on the bottom of the engine). I have a friend and one of the guys from the machine shop there and we all agree it doesn't sound to be coming from the shortblock.

Guessing it could be a cracked flywheel, we ride the clutch in first while rev'ing the car hoping to make the noise change up. Unfortunately, it remains consistent throughout the endeavour. OK, not the flywheel.

We agree it seems to be valvetrain related and pop the passenger side valve cover, set the lash to .008" on the intake and .010" on the exhaust as per spec. The noise is still there on the next run. I drop the machinist off and proceed to question the valvetrain work and spend some time readjusting the valve lash on both sides (repeating the passenger side the machinist did but to .007" intake and .010" exhaust) and re-torqueing the rocker arm shafts. The noise is still present after this. Lucky me, I sheared the threads on my motor mounts; we'll deal with this later.

After breakfast the next day my friend and I spend the day getting an oil pressure gauge (two, actually) and find the oil pressure is getting up to at least 45psi at the front galley (stopped the test early, 45+ is good enough for me). I find I don't have an adapter large enough for the rear galley, but there is plenty of clean oil in the rear passage. OK, not an oiling problem.

I end up later popping the sparkplugs out and a compression test gets over 129psi in each cylinder. Ok, it's not a valve magically touching the piston but not bending. Unfortunately I do not have a compressor or leakdown tester.

I'm left with re-torqueing the camshaft carrier (like I should've the first round in the valvetrain) and proceed to pop the passenger cover first. Reviewing the removal and re-installation procedure (torque specs and sequence) I discover no one ever happened to notice two bolts missing from the rear of the head:

OK, maybe we're getting somewhere? I pull two bolts from a head I have laying around and find the bolts won't go in. Unfortunately there is little space and light to see wtf is going on with the bolt holes. Sheared or stripped? Nope, bolts won't even go in that far. I hit it with a mini-screwdriver, some WD-40, and PB-Blaster. Something is coming out, but not enough to get the bolts to catch thread. OK, **** it. I pull the carrier off after some effort finding the right tools. Still not enough room to really get into the bolt holes. Frustration is getting the better of me and I proceed to remove the head from the block (yes, while it's in the car, and yes, without having jacked the motor up off the subframe). Upon doing so I find out a few things.

Cam Carrier removed:

The head either never had those bolts in (and did not exhibit this noise problem ever), isn't my head, or the machine shop wash tossed dirt/shavings into the bolt holes. Seeing the cam carrier had never come off (old gasket and the camshaft rear plug wasn't replaced) we can assume the bolts were never in. An A&E mechanic who works on Subarus informs me the lack of these two bolts isn't likely to cause the kind of noise we're hearing. Thoughts?

Moving on, we see the pistons aren't from a 2.5L:

Further, I find the pistons aren't installed per the block markings. Not knowing if the driver side is wrong too, the mismatched piston/bore combo makes sense since the parts came from a different motor. None the less, does an A piston in a B bore at 99.52-99.53mm likely to cause the kind of knocking we hear?

Piston bore marking and measuring:

If the piston/bore match isn't enough to cause piston slap, and we're assuming the driver side head is not the source of my problem, is the head I dropped off at the shop worth installing with a new gasket; hoping I've 'fixed' the problem without identifying it exactly? If the noise goes away, I could only then assume the problem was a dead/collapsed spring on the head and I'm not able to detect it's 'deadness' by compressing manually.


This car is intended for RallyX and eventually a cage and stage rally [converted to AWD]. I would like to eventually build a 2.2L shortblock + 2.5L SOHC heads and turbo the found 8:1 compression ratio for my street car, but would consider this as a test bed. I do not have enough contigencies to compete in a non naturally-aspirated class and therefore am at an empass. I could submit and possibly get approval for a provisional open-class FWD only license.

That aside, I'm very much spending what I'm starting to think is too much time proving the shortblock is not the problem and avoid having to go after the machine shop. I can either:

1) Install this second head and eat the cost of a gasket and possibly solve my problem. Possibly recoupe the cost of one head gasket and the headwork.
2) Tear down the shortblock, document it well, find the problem there, and re-coupe the cost of the parts/gasket kit and shortblock payment (maybe).
3) Build my intended motor until I get the turbo parts, and do #2 anyway. (There is a supply of 2.2L longblocks I can purchase from a friend)
4) Do something I haven't thought of suggested by you all
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michaeloconnor909 is offline   Reply With Quote
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Old 12-29-2012, 02:19 PM   #2
Scooby Specialist
Member#: 80465
Join Date: Jan 2005
Chapter/Region: RMIC
Location: Bailey, CO
2001 2.5RS
Black Diamond Pearl


B bores are smaller. If anything, there might be less noise.

The engineers at FHI put those bolts in that place for a reason, but I don't know if it's the source of your noise. If you've been running the heads for a while and they've been silent, I say to look elsewhere for the noise.


are you sure you want to use the SOHC heads for a turbo build? will you be using a different cam? the overlap in NA cams isn't good for turbocharging.

You can't accurately measure a bore with calipers, you need to use a micrometer.

Bore sizes for 2.5L 2001 FSM:
A: 99.505 — 99.515 mm
B: 99.495 — 99.505 mm

99.53 is way too large. re-measure.
yarrgh is offline   Reply With Quote

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