aaddpp

I measured the gap on my Goetze rings prior to install, and came up with numbers for the top and middle rings that were all outside of the max gap specified in the WSM / TSB (especially the middle rings). Single piece oil rings were OK.

http://www.flickr.com/photos/51967142@N00/7335683560/
Am I just incredibly unlucky to have landed a complete set of rings that are so far outside of spec? I'm not keen on what I see...and I prefer to be within the range specified by the WSM. I'm also worried about what will happen down the line as the Goetze rings wear and the gap get bigger still.

What have others done / experienced?

atb

Nice catch. Do your rings match the tolerance group of your pistons (may be oversized) ?

Fabio421

Are the cylinder bores within spec?

aaddpp

That a good point worth confirming. My pistons / cylinders are tolerance-group 2 (TG-2), so bores are 100.020mm IIRC. I'll need to look at the rings and see if there is anything to indicate TG.

aaddpp

Quote:

Originally Posted by Fabio421

Are the cylinder bores within spec?

At the same time I did the ring gap measurements, I took some readings with a bore gauge. The gauge I had only goes down to 0.0005" accuracy - which I understand is short of the accuracy required for the 928 bores - so I'm not sure if the numbers are valid and didn't post them initially. When setting up the gauge, I zeroed my measurements on tolerance-group 2 numbers of 100.020mm. TG numbers I understand are stamped on the piston head, as well as just above the cylinder number on the block. FWIW, engine has about 70K miles on it.

Here's what I came up with....

http://www.flickr.com/photos/51967142@N00/7336812764/

GregBBRD

If you want Porsche specifications....you probably are going to have to buy the rings directly from Porsche. All of the "Internet mechanics" think that because a company makes a product as OE, every single one of that companies' products are going to be the same as OE.

Not necessarily true.

The other point to consider is if the ring gap is going to make any difference in a 928 engine that virtually never has "ring" issues. Other that the GTS model engines, which are an "engineering" mess, I'm not sure I've ever seen a 928 engine (unless the bore was damaged) that had a "ring" problem.

CraigL

So, are you advocating for reuse of the original rings?
(Presuming they were removed intact)

And, what would you recommend for the cylinders?
Lightly hone to break the glaze?

Thanks in advance ...

Vilhuer

There is no tolerance group for piston rings, only bores and pistons.

vze2jshn

And, what would you recommend for the cylinders?
Lightly hone to break the glaze?

Honing of 928 cylinders is a no no - verbieten.

GregBBRD

I consider rings to be "wear" items and to be "standard replacement" items. I'm not sure I've ever re-used a set of rings.

Never hone the bores on an Allusil engine. It is a very "special process" and requires the aluminum to be microscopically removed, leaving the silicon exposed.

Fastest928

Buy another set of rings and get a better match across all cylinders.

A ring mfg may supply aftermarket and Porsche, the parts they sell to Porsche are highly spec'ed. What you buy aftermarket is not.

aaddpp

Greg, I'm not entirely clear what you mean in the last paragraph. I understand there to be two issues with the rings (i) too small a gap, and possible contact of the ring ends...leading to possible ring failure / possible block damage, or (ii) too large a gap, and consequent loss of compression. I'm pretty sure (i) is out here, so are you saying there's low risk of problems / or performance loss associated with a larger gap like the ones I've found? Thinking ahead, I fear where I might end up, if I sell, and someone wants to run usual engine tests.

I came across a tread some time back that covered the special process used in Allusil...that said, whats your take on the need for the special process when replacing rings. IOW, do you just replace rings w/o the special process, or do you run the block through a machine with the special paste? What influences the decision?

As alway, thanks for taking the time to help out. Alway deeply appreciated!

aaddpp

I've been thinking about that too...maybe another set gets me to uniform .45 on the top ring. Its that middle ring being 50%+ above the max that doesn't sit well.

GregBBRD

In terms of re-ringing an engine, I've never ran an engine through the process of "honing" with the special paste. I simply clean, without scratching, and install a new set of rings.

My logic is simple....I think that most people that do this job have no clue of what they are doing and most will do more damage than good. That has certainly been my experience with seeing different people "bore and hone" these blocks....most are not done properly.

I figure that if the block is bad enough to need to be honed, it probably needs to be bored and honed to the next size....

atb

It's interesting that Porsche would tolerance group the pistons and not match the rings.
Any mathmeticians out there that can calculate what the end gap would expand to using ring with a 0.4 mm end gap in a 100mm diameter bore, then placing that same ring in a 100.020 mm bore?