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Discussion Starter · #1 · (Edited)
I have a 355 SBC that I built about 7 years ago with close to 8,000 miles on it. Since day one it had sever blow by which I installed a PCV system to help with thinking the rings needed to seat. Well fast forward to now, this thing burns about 1qt per 250 miles give or take. I tore it apart 4yrs ago thinking it was my intake gaskets, no change. I then pulled it apart, ball honed it with new rings, no change. At this same time I also installed positive lock valve stem seals on the exhaust valves since I thought it was sucking oil past them. These heads were new when I built the engine. I also setup a homemade catch can for the PCV (running OEM valve cover with baffles) and it’s not collecting anything substantial.

While installing new headers last month I could see oil on all exhaust ports, heavy soot build up, and ash deposits on my plugs. Since the plugs were out I did a leak down test with a HF leak down tool. All cylinders were about 25-35% leakage straight past the rings. I am nearly 100% sure my machine shop did not use a torque plate. I don’t see any oil smoke when pulling away from stop lights, and very little at start up but there is the scent of burning oil most times. My right tailpipe is also oily, but the left isn’t. Both sides have significant leak down though so I just assume the right bank is “worse.” Before pulling this thing apart again to have it bored 0.040 over by a reputable shop, I want your opinions.
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When you pulled it apart 4 years ago, how did the cylinder walls look? Did they appear to be evenly worn around the entire circumference? I had an issue with a machine shop not using torque plates and when I pulled it down, you could see 4 very noticeable contact spots. I had to rebore and rehone to correct that. The other issue can be if the cylinders aren't honed to the specifications recommended by the ring supplier. I've had that too, where the shop honed them as they usually do and ignored the recommended honing grit. In the end, I had to pull it down again and have them rehone to spec and that solved the problem. Obviously oil consumption can be from many causes, simple to complex, but sometimes cylinder honing gets overlooked as real cause. Good luck finding the issue. Bill
 

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Discussion Starter · #3 ·
When you pulled it apart 4 years ago, how did the cylinder walls look? Did they appear to be evenly worn around the entire circumference? I had an issue with a machine shop not using torque plates and when I pulled it down, you could see 4 very noticeable contact spots. I had to rebore and rehone to correct that. The other issue can be if the cylinders aren't honed to the specifications recommended by the ring supplier. I've had that too, where the shop honed them as they usually do and ignored the recommended honing grit. In the end, I had to pull it down again and have them rehone to spec and that solved the problem. Obviously oil consumption can be from many causes, simple to complex, but sometimes cylinder honing gets overlooked as real cause. Good luck finding the issue. Bill
I didn’t pay attention when I did that because I wasn’t aware of the torque plate situation. This time I will definitely pay attention and I’m betting I’ll see what you are talking about.

When you had it rebored how much did that have to take off? I’m hoping I can stick with 0.040.
 

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Before you re bore , spend some time with a dial bore gauge . I had similar oil consumption problems , after exhaustive testing & checking with no single cause identified , I tore it down , found 2nd rings upside down , honed it & put in new rings !
 

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I didn’t pay attention when I did that because I wasn’t aware of the torque plate situation. This time I will definitely pay attention and I’m betting I’ll see what you are talking about.

When you had it rebored how much did that have to take off? I’m hoping I can stick with 0.040.
What is the bore now? Mine was a standard bore and had to take it to .030 over to true up the cylinders.
 

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Discussion Starter · #7 ·
Before you re bore , spend some time with a dial bore gauge . I had similar oil consumption problems , after exhaustive testing & checking with no single cause identified , I tore it down , found 2nd rings upside down , honed it & put in new rings !
Will definitely do that. It’s been like this since day one with two sets of rings and in all cylinders.
 

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When it comes to stem seals it’s the intakes that are the biggest problem because there is near constant vacuum in these ports which wants to pull oil around the valve stems. The exhaust side sees mostly pressure that wants to blow oil out from the stem clearance back into the rocker cover.

Use of torque plate is mostly incidental to oil consumption, it is a modern refinement with very small improvement to no torque plate.

Poor machining where the bores are not aligned well to the crank centerline. This forces the piston to operate out of alignment to the wall do to mismatching the retention direction to the imposed forces which prevents ring seal.

Improper hone either as a finish mismatch to the ring type i.e, smooth for moly, rougher for chrome. Or improper finish for the type of stone as silicon carbide doesn’t usually need a plateau follow up hone where a diamond stone does.

Using a dingle ball hone frequently results in uneven metal removal most often at the middle of the bore length, but depending on the operator this can also happen on the bore ends making an uneven or wavy wall in the vertical direction which breaks the ring seal compression and oil control as the piston moves up and down the cylinder.

Probably the most common screw up is rings installed in the wrong location, the wrong up to down to the bore orientation, busted on installation, oil scrapers fouled on the separator. Improper piston size for the bore size or improper ring size for the piston size. Or end gaps improperly sized, usually not enough as too wide really needs to be big before you see a problem but it does happen. When not enough the ends butt which in mild situations distorts the rings in worser situations the rings break in super bad situations they carve up the ring lands or actually bust the piston. They can also do some scrimshaw on the cylinder walls.

The fact that this has had 25% leak down from the git-go says there is some sort of cylinder, piston, or ring problem from day one.

Bogie
 

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Unless you washed a cylinder with a bad tune, you shouldn't need to bore unless it wasn't done correctly on the 1st go round.

Before tearing it down, do a leak down and compression test. Make sure the intake ports aren't oily? And how are the valves, valve guides, valve seals?

If you do decide to have it re-bored, then I'd be interested in how your pistons look. They are probably fine. Perhaps finding another block and having it bored .030 rather than going .040 on this one. My thinking is that if the original bore job was the issue, then it might be difficult to straighten this out with an additional .010.
 

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I missed the 25-35% leakdown , that's like no seal at all . I was upset when I had one cylinder over 4%...
What rings were used , what hone ?
Its very possible what you're seeing on the exhaust valve stem is unburned fuel .
 

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Discussion Starter · #12 ·
I originally thought it was the guides too, but when I did the leak down test the rings were shot. If I disconnect my PCV while running it will smoke like a freight train out the vent 😂. These are forged pistons (not sure of the rings) and I would love to reuse them but I’m finding it very hard to find a 1-piece Block with the provisions for roller lifters. If 0.010 wont clean it up I’m not sure if I can go 0.060 over without issues.
 
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