m20b27

[jurcha]

Hello everyone!

I have a question concerning m20b27 bottom! `86
I am considering piston ring change due to blue smoke when decelerating and hitting redline but I wanted to know if eta flat top piston rings can take high rpms - I have 731 head, 2.5i intake, nozzles, throttle body and ecu.

Head was recently rebuilt!

[jeffnhiscars]

The pistons don't care how high you rev but the cam does. The 731 head has the 7 cam bearings you need for high RPM but it does not come stock with dual valve springs so you need those as well to survive mechanically.

To use the 2.5 parts you mention (and t oraise your rev limit) you also need to change your harness, AFM and a few other bits and pieces. Lots of searchable info on this

[jurcha]

I just read somewhere on the internet that 325e 86` piston rings where designed to about 5k rpm, after that it is hard for them to seal oil!

Just wanted to know because I have to decide - rebuild block or get m50b25!

[jeffnhiscars]

Ive never heard of anyone changing rings when modifying a 2.7. Doesnt mean there isn't a reason to, just that no one has ever said they did or should have that Im aware of.

[e30sh]

Hello everyone!

I have a question concerning m20b27 bottom! `86
I am considering piston ring change due to blue smoke when decelerating and hitting redline but I wanted to know if eta flat top piston rings can take high rpms - I have 731 head, 2.5i intake, nozzles, throttle body and ecu.

Head was recently rebuilt!

Blue smoke on throttle lift is normally a sign of valve guide sealing issues.

Was it rebuilt by a reputable shop?

[jurcha]

Blue smoke on throttle lift is normally a sign of valve guide sealing issues.

Was it rebuilt by a reputable shop?

Yes! Head was properly rebuilt! I called them and they suggested that rings where going bad!

Btw is there a way to tell that valve guides are not sealing properly, just to double check?

[e30sh]

Yes! Head was properly rebuilt! I called them and they suggested that rings where going bad!

Btw is there a way to tell that valve guides are not sealing properly, just to double check?

Of course they will blame it on the rings, they rebuilt the head lol.

Normally you want the head off to measure guide wear. Most of these heads will need at least exhaust guides by now. I suppose you could pull a vacuum on each cylinder and use a smoke machine with valve cover off.

Good luck.

[jeffnhiscars]

You can just change the seals without changing the guides and in theory, it can be done without removing the head...BUT its a huge PITA and has its risks.

[e30sh]

You can just change the seals without changing the guides .

Since the "Head was recently rebuilt!" you would assume the valve stem seals were replaced. Even if you swap them again, odds are they won't control the oil for long with the valve wobbling in its guide.

[digger]

ask them if they did the guides and seals

did it smoke the same amount before the head was rebuilt?

did you use new intake manifold gaskets?

[jurcha]

ask them if they did the guides and seals

did it smoke the same amount before the head was rebuilt?

did you use new intake manifold gaskets?

yes, they did both!

I think it smoked less, but before seal and guide change it smoked also on startup!

New gaskets all round!

[jeffnhiscars]

Since the "Head was recently rebuilt!" you would assume the valve stem seals were replaced. Even if you swap them again, odds are they won't control the oil for long with the valve wobbling in its guide.

One would hope :)

I was just commenting on the seal swap rather than the problem. Fact is, a fresh head on and old block will often cause blowby since the valves are now sealing better than the rings.

[digger]

Is the valve cover breather dumping a lot of oil into the throttle body and Intake ?

[jurcha]

Is the valve cover breather dumping a lot of oil into the throttle body and Intake ?

No, actually it looks really clean there!

I spotted that around 3rd cylinder spark plug there is some oil visible! Also some oil coming out from 3rd cylinder exhaust side (manifold).

[e30sh]

No, actually it looks really clean there!

.

You can plug the air intake in the throttle body that comes from the valve cover and run the hose from the valve cover to a catch can to keep tabs on blow by.