2G 7-Bolt Engine Rebuild, Looking For Opinions/Experience/Knowledge

I’ve got my motor apart, the crank is currently at a machine shop getting spec’d and polished. I figured there’s no point in buying or boring anything until I know the crank is good.


In the mean time, a little bit about the motor.


7-bolt with 200k km (~124k miles), no fuel upgrades (yet), no bigger turbo (yet), balance shafts are getting tossed when I put the engine back together. Basically, it came apart as a factory 7-bolt.


Bearings all looked healthy, even wear on the surfaces. Thrust clearance was at its service limit, so it’s probably a good thing I took it apart when I did.


Why take it apart? Compression. My compression across the board sat below the service limit, with cylinder #4 sitting at a measly 100psi dry, 120psi wet. A leak down test showed no sign of compression leaking past the valves or into the cooling system so I’m pretty sure my rings are dead or piston #4 is bad.


I’m on a tight budget (surprise ) so I need to make sure I do this right. My number one goal here is to get the motor back up and running with good compression. I’ll chase more than 200 horsepower after I know my motor is alive and healthy. After I know it’s all good, I’ve got a small 16g I’ll slap on with the hopes of pushing it towards 300 whp, maybe, we’ll see.


I’m thinking about going with some .020” overbored NPR pistons and rings. I heard they originally manufactured the OEM pistons so I figure it’s a safe bet. Are cast pistons like these safe to take to 300whp?If anyone has experience with these I would love to know how they faired and what power levels they were pushed to.

 

UPDATE! I went with the .20 overbore NPR's and finally with some nice weather up in the frozen wasteland of Canada, I have a fully assembled rotating assembly. Everything is on the looser/wider side of spec which is what I'm looking for. Hopefully, I'll be able to keep working away at it and have the thing running by mid July.

Just for anybody who may be looking at this thread in the future, the rings came pre-gapped on the wider side of spec, but still be sure to check the gap regardless. The pistons look identical to the factory pistons, minus the Mitsu symbol and they're slightly larger (as in .20 overbore). I plan on updating this again after the break-in period to let everyone know the compression numbers or if if blew up. Fingers crossed!

 

ANOTHER UPDATE! It's alive!.. for now. I've only put 100kms on it so far, but all seems good for the time being.

I deleted the balance shafts, kept the oil squirters, and opened up the head oil port to relieve some of the pressure from the balance shafts being deleted. At operating temps my idle oil pressure is 10-12 psi, exactly what I wanted to see. I've read around the forums that you should expect to see +10psi for every 1k rpm, but I'm seeing +20psi for every 1k rpm. At about 5k, it peaks at 90psi and holds there up to 6k. I haven't gone beyond 6k yet so I don't know what redline oil pressure is.

My understanding of the OFH port job is that its supposed to allow for more oil volume through which will result in a lower pressure, but only once the relief spring begins compressing. I may be wrong, (please correct me if I am) but I heard that the 2g relief spring is rated for 100psi. So if my understanding of this is correct, porting the OFH won't really do anything for the +20psi per 1k rpm increase I'm seeing below 5k. I'm not sure I've heard of anything that could drop the pressure increase to the expected +10psi per 1k rpm increase.

What I am thinking of doing is getting a 75 psi rated relief spring (MD015988) just so I'm not seeing 90psi nearing 5k. If the oil pressure creeps up past 75psi with that spring, then I'll port it. Thoughts?

If you guys have any insight or could point me in a direction which you think could provide me with more info, I'd love to hear it.