One broken ring...Should I rehone?

porschett · 10-31-2005, 09:17 AM

Compression #s:
3 - 115 psi 6 - 119 psi
2 - 125 psi 5 - 119 psi
1 - 108 psi 4 - 115 psi

Found out why #1 is so low...the 2nd compression ring is broken.
Can I simply replace this and go on my way? Do I really need to hone the cylinder? Or just re-clock the rings?

What's the risk of just re-clocking the rings.

87 930
40,000 miles

Things I'm doing as part of my rebuild:

- ARP studs
- ARP rod bolts
- 964 cams
- port intake/manifold/plenum to 36mm
- headers
- custom full bay IC
- full head job (valves/springs/seats/regrind)
- new seals

Eagledriver · 10-31-2005, 09:48 AM

I don't know what re-clocking the rings means. If there are no scratches in the cylinder then you don't need to hone it. There are numerous threads on here about honing and scuffing the cylinders for new rings. You have to be careful to not go through the Nickasil coating. If the other rings are within the wear spec. (available in all engine books including the dreaded haynes manual) then I'd just replace the one ring and make sure the cylinder looks good (maybe a little scotch brite on that cylinder to help the ring seat).

-Andy

porschett · 10-31-2005, 09:57 AM

Thanks Andy. I was hopping that was the answer. All my searches were replacing all new rings and wasn't sure what to do about just replacing one ring.

Re-clocking the rings:

1st compression ring gap set at 3 o'clock (or 9 o'clock)
2nd compression ring gap set at 9 o'clock (or 3 o'clock...just as long as it's 180 degree opposite of 1st ring gap)
3rd oil ring gap set at 12 o'clock

Henry Schmidt · Henry Schmidt's Garage

One thing to remember is that the ring didn't break itself. The most common cause of broken rings in turbo engines is detonation. When the engine detonates hard enough to break a ring there is a good chance the the piston is damaged not to mention the cylinder might be out of round. Check the ring groove for damage and measure the cylinder to ensure roundness. If the heads are off, chuck those lame Dilivar head studs. If you don't replace them they will break and you'll be in there again sooner than you want. I see that your choice is ARP but perhaps you might want to investigate a better stud, the Supertec head stud kit, sold here by our host.
Good luck

One more thought: why did it detonate? Check injectors, waste gate function, ignition function and fuel mixture.

porschett · 11-04-2005, 07:02 AM

It detonated cause of a few factors. I increased the boost to 1 bar, but didn't recheck my timming. I knocked badly for a couple of runs until I realized what was going on. I retimmed it and it was better, but then I decided to put water/alcohol injection to cool down the charge intake.

Stupid me, I inlined the boost sensor for the water pump with the wastegate so the wastegate wasn't functioning properly which caused overboost conditions. Since then I've "T" the sensor.

I inspected both the P&Cs and it looks really good still. I'll measure for roundness.

Thanks Henry.