Opinions wanted: what was causing the oil-burning?

[tlmsf]

My friend recently restored a 1973.5 2.4L 911T with CIS injection and had the motor professionally rebuilt by a Porsche shop that does mainly racing motors. After 750 miles, the motor was still blowing lots of blue smoke, with no sign it was getting better with mileage. Before rebuild, his motor was relatively low mileage and the pistons and cylinders were well within specs. The shop mainly works on later motors with Nicasil cylinders (that don't get honed before new piston rings), so they did not hone the iron cylinders before reassembly with new rings. A leak-down test showed good values, very even and low.

We thought the rings were failing to properly seat because the cylinders were not honed. After only 750 miles, I helped him disassemble the motor to solve the oil burning. See the photos below. No spark plug or cylinder showed more oiliness than others. Note the even carbon pattern in all the cylinders -- very uniform across all cylinders. Piston tops were still clean.

However, we found one assembly error: one of the oil scraper rings was caught behind the expander ring. But this cylinder (#6) did not show any more carbon or oil on the plug than the others.

Some of the ring gaps were aligned, instead of opposite sides of the piston.

There is noticeable carbon build-up on the valves, especially on the exhausts.

The cylinders walls are shiny but still show some of the deeper cross-hatch scratches from the original honing.

Questions:
1. What does the carbon pattern in the cylinder heads show? Normal for only 750 miles? Does it indicate oil coming from the valve guides, or from the fuel spray as it enters the cylinders?

2. How much did the mis-assembled oil scraper ring contribute to the oil-burning?

3. How much did the un-honed cylinders contribute to the oil consumption?

Let the opinions flow!

Mis-assembled oil scraper ring on #6:


Cylinder #6:


Cylinders 4 - 6:


Cylinders 1 - 3:

[Flat6pac]

One problem is Deves rings.
the rings will line the way they want to run regardless as to how they are aligned on build.
Bruce

[panama911]

Not sure if it's related to your problem - but where did you/they source the piston rings?

I'm asking because the second ring is 1,75mm on the 2.4 CIS, different from all other 84mm engines (1,5mm or 2mm), and no longer available from Porsche as far as I know?

[tlmsf]

The rings were aftermarket Deves rings…

I think my friend has sourced a set of Porsche rings for the rebuild.

I'm surprised at how much the rings rotate when run.

Bruce, are you saying that even that oil scraper ring on #6 could have become caught because of ring rotation when running?

[Lapkritis]

New rings but no fresh hone... certainly will contribute significantly to sealing and oil consumption. A quick ball hone would be perfect.

Do you have ring gap specs from the assembly?

[Flat6pac]

No, but the rings do rotate in a running engine.
Oil control ring is probably from an installation problem that moved when the cylinder was installed and wasnt seen.
Bruce

[mamut]

Also check the valve seals, and valve to guide clearances.