fixing broken piston rings

mickster8888 · 12-21-2009, 02:31 PM

forgive me for my ignorance but what is entailed to fix a broken piston ring? can you just fix the cylinder with the broken ring? does the case have to be split? ballpark on cost to fix? i'm getting alot of crankcase pressure in my oil tank and did a search which indicates this may be the problem. car runs fine and just got it back from mechanic who said this was a normal situation but the car smokes on startup and after hard accelarotion on boost. car is a 1983 930 with maybe 6,000 miles on rebuild. any thoughts would be greatly appreciated. mick

Superman · 12-21-2009, 02:40 PM

Was the rebuild done at Motormeister?

mickster8888 · 12-21-2009, 02:49 PM

no, a reputable company.

mpetry · mpetry's Garage

Well a compression test first will tell you which cylinders are questionable.

If you decide to replace the rings, then the engine must come out, take everything apart, and the heads come off. Then you can replace the rings, after sliding the jugs off the pistons.

Maybe it's just me but if I had suspect rings after 6000 miles, I'd be tempted to replace them all.

Then re-assemble everything, get it reinstalled and running right. It's essentially a top end rebuild, minus the valve job, assuming you don't find anything else broken...

good luck with it

billybek · billybek's Garage

Is it possible that the restrictor in the sump vent line is plugged or the line installed improperly?
Just wondering if your crankcase is pressured up for another reason.
+1 to a compression test and leakdown.
Could be that the rings never seated...

James Brown · James Brown's Garage

WOOOOH there Mickster, lets not jump off the cliff quite yet. There are several things that can cause that condition that must be eliminated first. leak down test being first, pull the exhaust off and see if there is any oil coming from the exhaust valve guides, check the lower valve cover gaskets looking for leaks, oil return tubes......See where i'm going? many things mimic rings but aren't that hard to fix or as costly.

Nice looking engine there, Mark (like your cylinder holders, cheapskate!!!)

mickster8888 · 12-21-2009, 03:48 PM

thanks for all the responses guys. the reason i thought it might be the rings is that it turned up in a search i did concerning crankcase pressure. i'll definately leakdown/compression test done to eliminate the rings. mark, thanks, that was the explanation i was after. the thing that's weird is that it runs fine. hopefully it isn't the rings. thanks again.

john walker's workshop · 12-21-2009, 05:55 PM

overboost/detonation can certainly break rings into little pieces. what's your boost limit? there's a ball bearing in the fitting that the turbo oil feed line attaches to on the case. without it, you can get some smoking.

mickster8888 · 12-22-2009, 02:46 AM

boost limit is .8bar. is that line easy to get to and check? thanks john.

john walker's workshop · 12-22-2009, 05:29 AM

you can do it with the engine in the car. intercooler, A/C pump and air filter housing off, then find the red oil pressure light switch over the bellhousing. remove the switch and then the big fitting under it that goes through the banjo end of the oil line. then dip a skinny magnet into the big fitting below that and see if you come up with a ball bearing.

smokintr6 · 12-22-2009, 05:30 AM

Quote:

crankcase pressure in my oil tank

How have you measured pressure in your oil tank? And, are you sure all the breather hoses are hooked up properly?

Quote:

car smokes on startup and after hard accelarotion on boost

It is actually pretty normal for these engines to smoke on startup... Even a relatively new one. Smoke during and after hard acceleration can be the result of several things, including rich fuel mixture. Have you verified any oil consumption? Good luck, and check back with us after your leak and compression results. It might be prudent to have a different shop (who did not build it) check the engine's condition.

jwasbury · 08:13 AM

I had lots of crankcase pressure on my 930, it would actually puff like exhaust out of the oil filler. Car ran great, no visible smoking under boost.

I found 44% leakdown on #6, and busted rings on 3 cylinders:

Overboosting can happen due to faulty wastegate, and overboost switch disconnected (a "mod" that is too often done by dumb POs). Also carefully check ignition timing at full advance and make sure vacuum/boost connections to the dizzy are properly set up. You can use a mity vac to test. If any of this stuff is not right, you end up with the 48 piece piston ring.

midnight911 · midnight911's Garage

my 84 3.2 lost compression, losing the ring in cyl 6. My fix was to get a 3.6 motor...

Superman · 12-22-2009, 11:28 AM

Quote:

Originally Posted by James Brown

Nice looking engine there, Mark (like your cylinder holders, cheapskate!!!)

I used bits of garden hose.

mickster8888 · 12-22-2009, 01:40 PM

gonna do a leak down test as soon as some of this snow disapates. hoping not to find what jacob had.

mickster8888 · 12-22-2009, 02:27 PM

any others things that could tell me if my rings are broken? it's kinda keepin me awake at nighy. might be some time before i can get it to a mechanic.

Walt Fricke · 12-22-2009, 06:26 PM

Mick

The leakdown test will tell you a lot. It will tell you if the seal of one cylinder differs significantly from that of the others. Then it will tell you where the air being fed into that cylinder is getting out. You just listen - intake valve leaks come out the intake, exhausts you hear loudest at the end of the tailpipe. Ring leaks show up as air escaping into the crank case. You want to listen to the crank case (run a small plastic tube from your ear into the oil filler tube with the cap off) for all six of your checks, because there is always some air leaking into the crank, and you are listening for a louder sound from one cylinder than the others. If they all sound the same, they probably are all OK.

In normally aspirated engines, one cause of broken rings is worn ring lands - where the rings fit in the pistons. If these get too wide, the rings can wobble around, etc. You might check to see if the rebuild included new pistons, or a careful measuring of the ring grooves before deciding to install new rings.

Listen to these other guys on turbo specific issues.

You don't sound like you are a home brew mechanic, so the question put to you about how it is you know what your crankcase pressure is is a good one. Did you create a tool to measure it with a gauge? I think I could figure out how to do it, but don't think there is a spec for this. Crank pressure vents into the air cleaner/intake system, so it can't ever be very large (even though a little pressure can move a lot of aereated oil mist).

You just won't know until you do some measuring and the accompanying listening.

Walt Fricke

mickster8888 · 12-25-2009, 11:01 AM

walt, thanks for the info. i just have to wait to get some time to bring it to my mechanic. we just had alot of snow so i'm waiting for that to melt to. anyway, i'll post the verdict as soon as i get the results. thanks again, happy holidays.

Porsche84 · Porsche84's Garage

Mick

My 84 (non turbo) had alot of pressure in the crancase two times. Both times I found broken ring (actually shattered into small pieces). The second time was with only after 1000 miles with new piston and cyl (Mahle euro). My problem was detonation due to no vent in the gas tank (creating too low fuel pressure at high RPM). That was pretty much my most expensive lesson, thus far, on the 911. Pretty sure you'll find shattered top piston rings.....sorry