[bygddy]

Hey guys, in a nutshell, 350, mild rebuild done app 15000m ago, 268 cam, supposed to be forged pistons, stock crank and rods, i bought it a month or so ago, the previous owner had warned me that it uses a bit of oil and he was certain it was valve seals.....i get it home and drive it app 1000m over a month and its down at least 3ltrs of oil!!! On start up there is zero smoke, driving around town, zero smoke, but at high rpm, 4500-6000rpm it has some minor smoke, i swaped intakes and carbs, in that process i left the pcv out and just had breathers installed, that then caused the dipstick to push out and spray oil all over my headers....alot!!! so back went the pcv valve, and no more exsessive crank case pressure...but still using a significant amout of oil....i thought intake gaskets, but then realized it was doing the same thing before i did the swap, it runs fantastic, has 35psi oil pressure at idle hot, and 50+ cruising....i had plans to go with good heads, more cam, etc etc, but now have those plans on hold thinking the rings are pooched....any ideas or input??? Thanks
Dave

[bygddy]

Originally Posted by bygddy

Hey guys, in a nutshell, 350, mild rebuild done app 15000m ago, 268 cam, supposed to be forged pistons, stock crank and rods, i bought it a month or so ago, the previous owner had warned me that it uses a bit of oil and he was certain it was valve seals.....i get it home and drive it app 1000m over a month and its down at least 3ltrs of oil!!! On start up there is zero smoke, driving around town, zero smoke, but at high rpm, 4500-6000rpm it has some minor smoke, i swaped intakes and carbs, in that process i left the pcv out and just had breathers installed, that then caused the dipstick to push out and spray oil all over my headers....alot!!! so back went the pcv valve, and no more exsessive crank case pressure...but still using a significant amout of oil....i thought intake gaskets, but then realized it was doing the same thing before i did the swap, it runs fantastic, has 35psi oil pressure at idle hot, and 50+ cruising....i had plans to go with good heads, more cam, etc etc, but now have those plans on hold thinking the rings are pooched....any ideas or input??? Thanks
Dave

P.S

On a side note the resaon the previous owner thought valve seals is the motor had sat, alot, the rebuild was done in the mid 90's and between then and now its seen maybe 15000 miles...i do trust the old owner, he is a hardcore gearhead and seems pretty upfront and honest.....but still seems wrong...?

[FmrStrtracer]

Time for more diagnosis.
Compression Test
Leak-down Test
Read your spark plugs
Pull the valve covers and inspect the valve seals.
Have you climbed under the vehicle to look for any signs of leaks, if you have a pressure leak, it may only lose oil at higher speeds.

[bygddy]

Originally Posted by FmrStrtracer

Time for more diagnosis.
Compression Test
Leak-down Test
Read your spark plugs
Pull the valve covers and inspect the valve seals.
Have you climbed under the vehicle to look for any signs of leaks, if you have a pressure leak, it may only lose oil at higher speeds.

Plugs look slightly fouled, havent comp or leak down tested yet, not sure what im looking for with valve seals, although i do have new springs and seals sitting on the bench and may install just cause i have them, yah, been under it lots, very minor leak at rear main, but no signs of any major oil, and the amount im going through should be odvious??? I was looking at rings based on the exsessive cc pressure before the pcv was put back on, which im really really hoping it isnt...!

[bygddy]

Any chance worn out weak valve springs could be related? Espicially since the cam was swaped in a couple years ago and the springs were never replaced, not since the 90's anyways, and it sees 6g's fairly regularly....not sure if in any way a weak valve spring could cause this tho.....but thats the only time it will smoke, is 4500+, maybe even 5 +?

[oldbogie]

Originally Posted by bygddy

Hey guys, in a nutshell, 350, mild rebuild done app 15000m ago, 268 cam, supposed to be forged pistons, stock crank and rods, i bought it a month or so ago, the previous owner had warned me that it uses a bit of oil and he was certain it was valve seals.....i get it home and drive it app 1000m over a month and its down at least 3ltrs of oil!!! On start up there is zero smoke, driving around town, zero smoke, but at high rpm, 4500-6000rpm it has some minor smoke, i swaped intakes and carbs, in that process i left the pcv out and just had breathers installed, that then caused the dipstick to push out and spray oil all over my headers....alot!!! so back went the pcv valve, and no more exsessive crank case pressure...but still using a significant amout of oil....i thought intake gaskets, but then realized it was doing the same thing before i did the swap, it runs fantastic, has 35psi oil pressure at idle hot, and 50+ cruising....i had plans to go with good heads, more cam, etc etc, but now have those plans on hold thinking the rings are pooched....any ideas or input??? Thanks
Dave

Actually an engine can burn a lot of oil and not smoke much if at all.

The fact that it blows the dipstick out tells you the rings are not holding seal.

Forged pistons can be a real PIA depending on alloy. The less silicon the piston has such as 2618 generally the stronger the piston but the greater the thermal expansion issues so they have to be run with pretty loose clearances so the rings have a harder time sealing, which is why racers run a vacuum powered vent systems to help seal the rings and to get the blow-by out. The 4032 high silicon alloy is more like hyper-eutectic material in cast pistons in that it's more thermally stable but is more brittle than 2618, therefore easier to fail. But for a hot street engine I think makes a better choice as it fits daily driving habits better. The 4032 alloy results in closer running clearances which do a lot to help the rings seal as they aren't being tipped as much as the 2618 piston relative to the bore. The 2618 piston needs to carefully warmed to operating temps before getting on the motor, otherwise the angled rings begin to beat out their lands resulting in a loss of seal that can't be repaired only replaced. The 4032 alloy should be warmed up but will react much more like a production piece in regard to riding pretty tight and straight to the bore wall thus holding ring seal.

Of course what the specific problem is remains quite open. Events such as honing the wall for a 2618 piston, then running a 4032 nets the same problems, or worse, than the 2618 because the eutectic alloy will not expand enough to gap the clearance so it continues to rock when hot just like the cold 2618. Its complex, the machinist/builder really needs to know the properties of the alloy to be used and the type of ring among several other things when preparing the bore walls. This is a place where communication between owner'builder and the machine shop operater can easily become a series of missed or assumed information.

A compression and/or leak-down test may not show a problem as these type ring seal issues are often not identifiable when the engine isn't running.

Bogie

[bygddy]

Originally Posted by oldbogie

Actually an engine can burn a lot of oil and not smoke much if at all.

The fact that it blows the dipstick out tells you the rings are not holding seal.

Forged pistons can be a real PIA depending on alloy. The less silicon the piston has such as 2618 generally the stronger the piston but the greater the thermal expansion issues so they have to be run with pretty loose clearances so the rings have a harder time sealing, which is why racers run a vacuum powered vent systems to help seal the rings and to get the blow-by out. The 4032 high silicon alloy is more like hyper-eutectic material in cast pistons in that it's more thermally stable but is more brittle than 2618, therefore easier to fail. But for a hot street engine I think makes a better choice as it fits daily driving habits better. The 4032 alloy results in closer running clearances which do a lot to help the rings seal as they aren't being tipped as much as the 2618 piston relative to the bore. The 2618 piston needs to carefully warmed to operating temps before getting on the motor, otherwise the angled rings begin to beat out their lands resulting in a loss of seal that can't be repaired only replaced. The 4032 alloy should be warmed up but will react much more like a production piece in regard to riding pretty tight and straight to the bore wall thus holding ring seal.

Of course what the specific problem is remains quite open. Events such as honing the wall for a 2618 piston, then running a 4032 nets the same problems, or worse, than the 2618 because the eutectic alloy will not expand enough to gap the clearance so it continues to rock when hot just like the cold 2618. Its complex, the machinist/builder really needs to know the properties of the alloy to be used and the type of ring among several other things when preparing the bore walls. This is a place where communication between owner'builder and the machine shop operater can easily become a series of missed or assumed information.

A compression and/or leak-down test may not show a problem as these type ring seal issues are often not identifiable when the engine isn't running.

Bogie

Wow....thats a ton of info, and i gotta be honest.....im a fairly smart guy, but feel fairly inadequate after reading that lol......

[boatbob2]

Hi,if you say you have a SLIGHT rear main seal leak,look at the drive shaft,bottom of car,and rear end for oil staining,you could be loosing oil only at high revs.

[bygddy]

Originally Posted by boatbob2

Hi,if you say you have a SLIGHT rear main seal leak,look at the drive shaft,bottom of car,and rear end for oil staining,you could be loosing oil only at high revs.

Yah, checked that, and everything else is dry, just at teh rear main and a touch at the bellhousing.....thats about it, and for the amount im going through it would be wet everywhere....

[68NovaSS]

Do a backyard mechanic thing, drive it down the road in low gear to about 40, then let off the throttle, see if smoke rolls out. We did that in the old days, when I was a kid, thinking it was the way to test for wear.

[T-bucket23]

Originally Posted by bygddy

Wow....thats a ton of info, and i gotta be honest.....im a fairly smart guy, but feel fairly inadequate after reading that lol......

If you boil it all down, you need to do a compression and leak down test. If you are getting excessive pressure in the crank case it is due to pressure leaking. Once you validate there is leakage you can try to determine why.
valve seals generally show up at idle after the engine sits for a while, everyone trys to blame something cheap when they are selling it.

[bygddy]

Originally Posted by 68NovaSS

Do a backyard mechanic thing, drive it down the road in low gear to about 40, then let off the throttle, see if smoke rolls out. We did that in the old days, when I was a kid, thinking it was the way to test for wear.

Done that, and nothing, i had a friend follow me last night, zero smoke right up till about 4500rpm, and even then its fairly minor, enough that most people wouldnt notice.....checked my oil level again, down 1/2 a litre in about 500km, and if im simply driving it gently and just dont shift till 5k then no smoke, if im on it then thats when you see a bit...im lost lol....part of me wants to say fk it and just buy my heads and more cam etc etc, but if a mild motor has an issue adding a bunch of compression and cam i suspect is just going to make thinsg worse before better.....

[bygddy]

Originally Posted by T-bucket23

If you boil it all down, you need to do a compression and leak down test. If you are getting excessive pressure in the crank case it is due to pressure leaking. Once you validate there is leakage you can try to determine why.
valve seals generally show up at idle after the engine sits for a while, everyone trys to blame something cheap when they are selling it.

The thing is the guy was upfront and above board, i called on a sat at likr 5pm and said i wanted it, now, when i told him i was driving 2hrs to come get ithe was stunned, wante dme to know it wasnt perfect, had some crappy bodywork done to it, and yes he said it uses a bit of oil, and needed an axle seal etc etc, i drove there, and went around the block, and made him an offer, he accepted...but even then, this car was his baby, he has owned it since 92 or something like that and he loved it, it had never even turned a tire before i got my greasy hands on it, i figure he would likely punch me in teh mouth if he knew what i had done to it or how i drive it lol.....

[68NovaSS]

Originally Posted by bygddy

Done that, and nothing, i had a friend follow me last night, zero smoke right up till about 4500rpm

Yeah, the smoke would roll out when it decelerates in low gear, but he would have seen it.