Advice on scored cylinder

[PrimeMvr]

I'm doing a top end on an 87 930 with 98mm Mahles. As I thought, one piston has broken rings. The piston has no damage, but the cylinder has vertical scores, one that I can clearly feel. Hone, replate, replace? These P/C only have 14,000 miles on them and there are no other signs of pre-ignition, so I'm not sure why both compression rings were shattered. Thanks for any advice.

[Flat6pac]

I had one come off a 930 with scoring like that but worse. It ran in #4 position. Using deck height and size info on the cylinder, I just replaced it.
Bruce

[KTL]

Measure the bore with a bore gauge and send it out to Millennium Technologies or US Chrome (Millennium seems to be the preferred by the pros) to have it replated in Nikasil.

I had a single cylinder that had previously been replated (SC cyl bored from 95mm to 98mm) and there was a considerable chip in the "dead" region of the cyl where the carbon band always forms. Meas'd the existing bore dia. and had it replated "worn" so that I would have similar used piston and cyl clearance as the other 5 cyls. Sent it to US Chrome and they stripped, plated & honed it to exactly the dimensions I asked of them. Worked out fine from what I could gather.

[Steve@Rennsport]

Hi,

Broken rings is caused by detonation (pre-ignition is totally different and comes later) and I would find out why that happened in the first place.

I would also very carefully measure the piston ring lands for wear since detonation really hammers the hell out of them.

We use US Chrome exclusively for cylinder plating services and they should be able to effect a good repair on that damaged cylinder. They may need the piston as well to measure its diameter in several places so the cylinder will be back to original specs.

[PrimeMvr]

Thanks guys. New dilemma, I just found that the oil ring land is tight in one spot, so I guess there was detonation damage after all. Thoughts on mixing in one new P/C set? What's the best source? I've never had to buy these before.

[PrimeMvr]

Steve, I have a feeling it's been like this since I bought it three years ago, because I have always had excessive oil in the air box all the way through the intake tract. I was a rookie and didn't know what a PPI was. I've since put in an adjustable WUR, MSD ignition and AFR logging. This was #6 not the common lean 2/5. The exhaust port and plug have good color now. As part of this tear down I flow tested the injectors. They all matched flow and only #4 had an odd pattern that will be addressed.

Thanks to all of you for sharing your expertise in this forum. I've read many of your posts!

[r-mm]

Is that wall to wall carpeting on your piston dome? I hear it outperforms ceramic coatings by some margin for turbocharged applications...

[PrimeMvr]

To bring closure to this, I had the cylinder replated by Millennium and put in all new True Seal rings. #1 had broken rings also, but zero cylinder damage. The one oil ring land that was tight was ok after cleaning it up and hitting that one small spot with a couple passes of a really fine file. It couldn't have been more than a thou deformed and less than a half inch area.

Shame to see that damage on a 14k mile engine. Mind your timing and AFR....

Thanks again for your guidance!