Melo928

Hi,

Has any one firsthand experience of significantly worn piston rings in 928? In theory I know what should be the symptoms but I’m interest to hear how worn rings should be and what the first indication is.

My 4,7 S is now being rebuild and it had pretty wide ring gap. Rings were not actually worn and I’m wondering if gap has been out of specs even from factory. Has any Rennlister had similar experience? Engine hasn’t consumed any oil, compression was over 170 psi in all cylinders etc.

Is alusil engine with thigh cylinder and piston tolerances less depending on ring gaps than traditional engines?

and yes I know, I shoud put new rings in..

Lizard928

excessive blow by,
heavy oil smoke on decel those are the two most common ones.

GlenL

Uneven compression test numbers will tell you about the rings. They never fail evenly.

Melo928

According to mechanic all compression ring gaps were above wear limit W/O any sign of actual wear. Compression equal in all cylinders and above 170 psi, no oil consumption at all within 2 000 miles driven last year. Only sign of blow-by so far is filthy oil.

Car has 140 000 km on clock and engine is in original state. Reason why we did RR was a sweeping head gasket.

Anyone who has done engine overhaul to seriously driven 928 – what has been your findings on ring gap vs compression&blow-by?

GregBBRD

Rings are a "standard" wear item. They are the only thing that is actually in contact with the Alusil material.

Do a quick calculation about how many millions of times those pistons have been up and down the cylinders, in 140,000km, and you will quickly understand why there is wear and why they need to be replaced.

Just becasue they are expensive, from Porsche, doesn't mean they don't wear.

GlenL

It sounds to me like you don't need new rings.

Or are they already out? I'm perplexed as to how the mechaic could tell the gap was off otherwise.

karl ruiter

DR had a sale price on there rings a few days ago that was smokin'. In addition to wear, I think they can be damaged by heat. I recently rebuilt a motor that had smoke on decel and the rings were so brittle they fell apart on removal.

GlenL

Broken rings aren't unusual. Knocking will do that.

Melo928

To GlenL – yes engine was almost fully disassembled, only crankshaft/case was let intact.

To Greg – I agree, rings are wear items. But in this case visual wear marks are minimal. Ring face contacting to cylinder is slightly angled and you can see there are area that has not wear at all. Compression of the rings is also strong.

If mechanic wouldn’t be a true 928 geek (Erkka), I would suspect his measuring skills.

Greg (or anyone), do you have an example case of ring gap taken from 100,000 miles driven 928? It seem typical based on Rennlist discussions that gaps are round 0,5 mm in engines that have been overhauled.

It seems that I’m not seeing any normal problems caused by excessive ring wear, except dirty oil. My experience is that blow-by can cause temperature problems when exhaust is leaking to crankcase. This is my primary concern.

FBIII

I thought smoke on deceleration was valve guides and or valve seals.

Vilhuer

Crank chamber in that engine was by far darkest place I have ever seen.

jeff spahn

When I took the rings off my one piston I replaced, they were not brittle. I didn't measure gap before I threw them away. had 125,000 on them.

Lizard928

once you remove the rings, they should be replaced IMO

Ducman82

If they are off, replace them. Why risk it?

Melo928

Budget.

To do things right new rings would mean:

-taking crank out (and later renew bearings)
-lapping process for cylinders
-new rings or even rings&pistons (Wössner)

That would cost me another 2000 € which I don't have. So their back again and we'll do our experiment - shall it consume oil, overheat, how much power its giving out.

Still waiting someone to come up with experience of lot driven engine and its tolerances...