rb25 cylinder 5 bad compression
 

I just decided to do a compression test and I saw that all cylinders were about 150 except number 5 , it was about 75psi. I dropped oil there and it went up. BUT before I call it the rings let me also say that its smoking whitidh blue quite a bit through exhaust but there is no mixing of oil and coolant in the engine or radiator?? There is oil coming from the back of the motor around the cylinder 5&6 area. I can't tell if its coming from directly between head and block but it does kinda looks like it. Could it still be a bad hg? Could putting oil in cylinder and making the compression go up be a sign of a bad hg? Or is that only for rings?

If the compression jumped after dropping oil in there, its the rings. If you dropped oil and it didnt jump, then its something else like the HG or the valves.

[QUOTE=fliprayzin240sx;4746123]If the compression jumped after dropping oil in there, its the rings. If you dropped oil and it didnt jump, then its something else like the HG or the valves.[/QUO

The rings could also be the reason for the smoke? Oil getting past rings?

Yup yup...

Damn was hoping it was the hg or valve lol. So can I just put some new rings on the pistons and put them back in with new main and rod bearings? No need to overbore unless the walls are fucked right?

Correcto...most of the time, if the cross hatching is still there, you dont have to re bore it. Just make sure theres no nick in the cylinder. But 75psi on 1 cylinder when everything else is at 150psi, it might even be a cracked ring land.

Well the thing is that it still runs good and pulls pretty hard so I'm gonna guess its just the rings and not the Piston or walls (hopefully). If the walls need reboring, that means I can't use the OEM pistons right? I would have to buy after market ones to match the bore size correct? Please correct me if I'm wrong man lol

???? Ummm ok

OK guys this is weird and is really puzzling me. I just let it run for awhile to burn out the oil I poured into it and did another compression test, and now cyl 5 is between 150-175 like all the others? Hows that possible?

the oil has coated the top of the piston and the cylinder walls, new engine oil is very thick, not like depleted oil like in the engine and it has sealed the leaking area. give it a few drive cycles and it will go back down. best to take it easy on it until the problem is fixed. without the cylinder combusting with correct cylinder pressure can be harmful.

Nissan do pistons in 86.5mm

What do u mean

You did a bad compression test the 1st time around. Retest again to make sure it's ok now. Lucky you

Or you did it cold the first time.

Original pistons are 86mm. .5 overbore allows you to use OEM 86.5mm pistons.

Oh really they make OEM 86.5mm pistons? Do u have a link?

Ni it warmed up. But even if I did, one shouldn't be that much lower than the other

Pistons.

Nissan A2010-21U72 OEM Engine Piston RB25DET 86.5mm Oversized R33 | eBay

Rings.

Nissan 12036-21U00 OEM Piston Rings RB25DET 86.5mm RB25 ECR33 R33 RB | eBay

I considered chucking a set in my 25 but my pistons and bores are in awesome shape so I just got new stock rings instead.

Thanks man! So you just threw some new rings on the original pistons and the compression was fine?

Can I reuse my main and rod bearings guys? As long as the clearance are good? I'll be checking them with plastigauge

Just replace em, ACL race series are good cheap insurance.

I prefer OEM but my question is, CAN I reuse them if they're good

You are already in there. Replace them.

How do I know which ones to get?

Look up fsm crank specs, rent a mircometer/caliper only takes maybe. 20-30 minutes to go over it. Chances are its all original still and you need standard size.

ok so i can just buy all original main and rod bearings? im kinda stuck on how to check the clearance? and why do i need to if im going with all oem standard parts?

So you know if it has been rebuilt before....

the rod bearings are .001 clearence so can i just order some standard acl ones and throw them on?

How did you measure your clearances?

You need to remove all oil from the bearing surfaces, grab a piece of plastigauge stick it on the bearing surface, tighten the bolts to correct tension(46-52nm for Crank bolts & 14-16nm plus an additional 60-65 degrees for rod bolts) DO NOT TURN THE MOTOR. Then unbolt and check the width of your plastiguage with the supplied card.

You won't have clearances of .001(Thou or MM I'm not sure which you mean but either is a way smaller clearance than you need)

Clearances are.

Rods

Standard 0.2-0.3mm
Limit 0.4mm

Mains.

Standard 0.05-0.18mm
Limit 0.3mm

Yes you can just chuck ACL bearings in but during assembly you WILL need to measure again to ensure you have correct clearances. The standard bearings are all graded with the grade of each bearing in the engine stamped on the front most counterweight on the crank(Mains on the top, rods on the bottom, left to right = front to back) If you want to go stock bearings you'll need to order the right ones or go up in sizes depending on wear on the crankshaft itself.

Hmm that's exactly what I did and the plastigauge was squished out as big as the biggest GREEN little,mark on the paper. Can't remember what it was measured in though I gotta go look in the morning