Odd Carbon Pattern

[AnnapolisAirhead]

I just put about 700 miles on a fresh set of valves. During the valve job, I cleaned the pistons. Last night I pulled the heads and jugs just to check on things and found this pattern on the pistons.

It's a dual plugged '83 R100 with 65k on it, 700 on it since the valve job. Ring gap measured damned near new. Carbs are super clean, rebuilt, tuned and balanced. It ran great. The light color pattern is bare metal on the piston. I'm running Bosch W6DC top plugs, W6BC bottom plugs with a 0.026 gap. It has factory electronic ignition, plugs and wires are new from last fall, red accel coils and a Dyna booster (all from the PO). Bike is stored indoors.

I would have thought after 700 miles of backroads riding, the pistons would be a uniform carbon color. What do you guys make of this? Ok or something I should be checking?

[crazydrummerdude]

This guy (http://advrider.com/forums/showthread.php?t=410117&page=23) has a similar pattern. No one really yelled about it. :dunno

[47512]

That pattern is because of the fresh fuel charge, older BMW's don't have a squish band like modern engines, so theres not much terbulence in the cylinder. That is also why older BMW's tend to detonate.

Ken G.

[lostboy]

This is caused by poor oil control. Did you renew the rings when you had the top end off? You may may just need more time to seat the rings.

[AnnapolisAirhead]

This is caused by poor oil control. Did you renew the rings when you had the top end off? You may may just need more time to seat the rings.
The ring gap was as if they are new rings when I memasured it. So I didn't put new rings in at the time. I think when the PO had it dual plugged, he probably had new rings put in--he worked at a dealer at the time. But that's just a guess. I'll give it time.

That pattern is because of the fresh fuel charge, older BMW's don't have a squish band like modern engines, so theres not much terbulence in the cylinder. That is also why older BMW's tend to detonate.
Interesting to know. I still would have thought in just a few miles with dual plugs it would have a uniform build-up, like the valves did and they were freshly done too.

[sumran]

This is caused by poor oil control. Did you renew the rings when you had the top end off? You may may just need more time to seat the rings.

If pistons are removed from the cylinders do you go through the same seating process as with new rings? I have heard various opinions on this.

[AnnapolisAirhead]

If pistons are removed from the cylinders do you go through the same seating process as with new rings? I have heard various opinions on this.

I did not. My reasoning is that much of the high rev break-in process is to take the edge off the rings, but since these rings went back on to their respective pistons, in the proper order and orientation as I removed them this was not necessary. The only reason I removed the rings at all was to soak the pistons in the ultrasonic and to make sure I got the grooves clean. While they were off I measured the ring gap.

Since I just pulled the heads and cylinders again, there is no evidence of any glazing on the cylinder walls, so I think I did the right thing...but this is my first adventure into the top end.

What is the consensus on needing a break-in period for re-assembly with the existing rings?

Here is a pic of the head from the same side as the piston picture. Note the uniform carbon build up. Both valves are a tan color, you just can't see the intake valve in the picture.

[vanzen]

... What is the consensus on needing a break-in period for re-assembly with the existing rings?

There will be NO consensus.

I'll start –

First, I am a fan of the "one minute @ 1/2 redline" Gietl method.
However,
IF the rings were seated before dis-assembly, what should be expected to change with any additional "break-in" procedure ?
IF NOT – then your usual method would apply.

Ring gap was measured ...
how's about ring clearance in the groove ?
condition of the cylinders ?
fit of the piston / cylinder ?

Have you done a leak-down ?