Freshly rebuilt 6 bolt seems low on compression

[Ill-EagleTSi]

I rebuilt my 6 bolt engine recently with all stock stuff. Everything was within factory spec so I only did a Honing on the cylinders.
I installed NEW 7.8:1 comp pistons with Nippon piston rings, Gaped to stock specs.

I did a compression test and got 135 in all 4 cylinders. I currently have 1,329 miles on the engine. My results seem OK but not exactly what I was expecting.

What do you guys think is causing my low comp? Are my rings not seated yet? I think maybe I didn't HONE enough. I just did it enough to take the shiny glaze off the cylinder walls.

The car runs like a raped APE though....so i guess its NOT a real big deal.

 

[afsdsm]

The lower compression pistons for staters. Also if you didn't leave a cross scratch pattern in the cylinders the rings may not seal properly.

 

[Ill-EagleTSi]

Yeah I left a cross hatch pattern...but I only honed them each for like 10 seconds with 10w30 oil. Just enough to take the shine off the cylinder walls. I thought it was enough.
The hone I used was weird tho. It had 3 legs with little grinder stones on them. I'm used to the ones that look like a big head of broccoli haha.

 

[afsdsm]

What you used were glazing stones and not really going to leave much of a pattern. A bottle brush hone is best and easier for the home user. But if it doesn't smoke i wouldn't wory to much.

 

[lacroixdp]

You most likely didn't use an aggressive enough hone. With chrome rings (which is what I'm assuming you have), you need a coarse hone for break-in. Do another compression test, except use a capfull of oil this time. Also, was the car warm and the throttle at WOT?

 

[ed_dogg23]

the compression on my stock 6 bolt is 125 across the board. I would not worry about it.

 

[jjrock5]

^ I agree on the last post but also did you do any type of valve job? If you didnt, you may be losing comp from there as well.

 

[ed_dogg23]

^ I agree on the last post but also did you do any type of valve job? If you didnt, you may be losing comp from there as well.

Indeed. If this is the case you will see some exsessive crank case pressure. Then it may smoke a lil. Just stay on top of the oil level. I bet its fine though. Good luck.

 

[Sicotic]

i rebuilt my head left bottom end stock and my compression is 175 in all 4 cylinders.

 

[donniekak]

i rebuilt my head left bottom end stock and my compression is 175 in all 4 cylinders.

Not with 7.8:1 pistons.

 

[Sicotic]

bc i didnt mess with bottom end yet.

 

[Ill-EagleTSi]

Yes the head was completely re-done!

Yes the engine was at Op temp and the throttle WAS open.

So do you think over time the rings will eventually seat? or will it stay 135 for ever.

Im not sure if they were chrome rings. They were Nippon stock replacement rings

 

[Team Addiction]

Not with 7.8:1 pistons.

Why not thats what the stockers are

 

[jrohner]

Did you end up putting in a thicker than stock headgasket?

 

[donniekak]

Why not thats what the stockers are

Because the service specs for perfect compression on a 6-bolt are about 165psi.

 

[92_talonGsXt]

:

Because the service specs for perfect compression on a 6-bolt are about 165psi.

Yeah your right on that, but if you use a thinner headgasket or resurface your head more than oem specs, the compression can reach up to 170-180, just got done with my rebuilt head and my compression was 180 across the board

 

[lacroixdp]

Yes the head was completely re-done!

Yes the engine was at Op temp and the throttle WAS open.

So do you think over time the rings will eventually seat? or will it stay 135 for ever.

Im not sure if they were chrome rings. They were Nippon stock replacement rings

They were most likely chrome then. There's no sense worrying yourself to death over it unless you're willing to rebuild it again. You're above minimum specs and it sounds like you're even across the board. I'd forget about it and just enjoy your rebuilt motor.

 

[Black94DSM]

Make sure your compression tester is reading correctly.

 

[Ill-EagleTSi]

I used a stock mitsu composite gasket.

Good Idea about checking the gauge for accuracy. It is a cheapo harbor freight comp tester that is about 5 years old. Could be a little off calibration or something.