No ring gap for oil control rings- Prince 1.6 petrol engine

[gwest]

I bought a 2nd hand engine to replace the cooked one in the 2013 BMW F20 18i that I bought as a non-runner. Now I know we are talking about French cars on this forum but given that the N13B16A engine is a derivative of the PSA EP6 series I beg your indulgence. The original engine was leaking coolant and had a softened (~70 Brinell) head that was warped 0.013”. It also had corrosion in the connection for the coolant level sensor and a brittle O-ring in the coolant sensor that is invisible at the back of the motor, that I think explained its demise.


My replacement engine had an uncertain past although it was claimed to have covered only 65,000 km. I removed the head and fortunately it was straight and hard (>100 Brinell) but it showed signs of battering possibly from a broken spark plug, and scratches indicated that the head had been off before. And it was unlikely to have been a low mileage car. I then removed one piston to check the ring gap and found that the 2 compression rings were well outside the upper limit (~30-35 cf 8-20 thou). I don't normally bother to check the oil rings but this had a one-piece oil ring with a coil spring sitting in its groove, so it was easy to check. This was where things became a bit weird- there was no gap when I placed it in the bore. In fact it was distorted indicating that it was bigger than the bore.


I then dug out the old pistons I had from fixing a Peugeot 207 VTi and it had the same type of oil rings. And they were oversize! The Haynes manual gives a gap of 12-20 thou.


What am I missing?

 

[Greenpeace]

I bought a 2nd hand engine to replace the cooked one in the 2013 BMW F20 18i that I bought as a non-runner. Now I know we are talking about French cars on this forum but given that the N13B16A engine is a derivative of the PSA EP6 series I beg your indulgence. The original engine was leaking coolant and had a softened (~70 Brinell) head that was warped 0.013”. It also had corrosion in the connection for the coolant level sensor and a brittle O-ring in the coolant sensor that is invisible at the back of the motor, that I think explained its demise.


My replacement engine had an uncertain past although it was claimed to have covered only 65,000 km. I removed the head and fortunately it was straight and hard (>100 Brinell) but it showed signs of battering possibly from a broken spark plug, and scratches indicated that the head had been off before. And it was unlikely to have been a low mileage car. I then removed one piston to check the ring gap and found that the 2 compression rings were well outside the upper limit (~30-35 cf 8-20 thou). I don't normally bother to check the oil rings but this had a one-piece oil ring with a coil spring sitting in its groove, so it was easy to check. This was where things became a bit weird- there was no gap when I placed it in the bore. In fact it was distorted indicating that it was bigger than the bore.


I then dug out the old pistons I had from fixing a Peugeot 207 VTi and it had the same type of oil rings. And they were oversize! The Haynes manual gives a gap of 12-20 thou.


What am I missing?

Expander oil rings should have a gap as well. Around 0.004" gap per inch of bore diameter is pretty standard.
Maybe it's been rebuilt with OS rings and they only filed the compression rings.

 

[dmccurtayne]

I have found this and found the bore ovality way out of spec maybe 1 out of every 7 heads I took off had this lucky at the time I was in a dealer and this could be solved with a new goodwill engine because the bore coating .

 

[PeterT]

I don't think you've missed anything. Well done identifying the issues. Just make sure it goes back together with appropriate ring gaps as discussed.

 

[gwest]

Well I checked the bore on no.1 cyl on my N13B16A BMW 116i replacement engine- I could just insert a 3 thou feeler gauge down beside the piston skirt near the top on the thrust and non-thrust axis. I then used a dial gauge and the thrust axis was about 0.4 thou greater than the non-thrust axis. So no obvious ovality problem in this case. I then measured the outer diameter of the rings with the gap closed between my fingers. I did it at two places at right angles and got the average. The results were: top ring 76.7mm, 2nd ring 76.7, oil ring 77.35. The bores are a nominal 77mm. I then checked the oil ring from my 2007 VTi and its diameter was 77.25.

The local BMW people very kindly gave me the specs for this engine and the gap for the oil ring is 4 to 16 thou, with a maximum of 32thou.

 

[gwest]

There seems to be little information available on the 1-piece (+coil spring) oil control ring but the replacement set of rings were the conventional 3-piece oil control rings with appropriate end gaps so it's a bit academic. When I got around to pulling all the pistons out I discovered that the top ring in cyl. 2 was stuck in its groove. I freed it up using an ultrasonic bath and kero. So it was just as well that I decided to check the rings. A month or so later I was dismantling my original ruined engine that had covered 99,000 km and discovered that it too had the top ring seized in no. 2 cylinder. I am thinking that this is more than just a coincidence.

 

[GRAHAM WALLIS]

Distortion in the top ring land due to pinging?

 

[Greenpeace]

Distortion in the top ring land due to pinging?

I was going to same the same thing.
I had a small block Chrysler V8 with a 150 shot of nitrous on it. When I rebuilt it the new top rings wouldn't fit until I filed the grooves a little.