I think you are onto something here. I have a similar scenario in my head too. Believe me, we will take the GT engine apart, as I really want to get to the bottom of this. All I am hoping is that when we take the heads off I will not see massive scratches down the bores of 5 and 8...

Either way I will be splitting the block and then fitting 86.5 pistons in. Pelican are still doing the 968 valves for $15 a piece. My machinist in Bulgaria is still good for $750 for a complete head job - valves, guides, seals, porting and polishing. I do wonder whether I can see 400bhp crank out of the good old GT lump. In a way every clowd has a silver lining... Mapping it should be a doddle now that I have a ST2 at home.

 

What piston cc you will have? I have one S3 piston set currently under work for 968 valve cuts. It seems result will be around 12 cc. This means between 10.5 and 10.8:1 with S4 crank. GTS crank would require too much skirt removed to work at BDC for my liking. Thus only 5L engine.

 

Although this will not fix broken oil control rings, it may be somewhat relevant to the rebuild effort.

Here's how we are combating ring flutter. My guru engine builder recommended gapless second rings for the build. Initially I didn't understand why the gapless second (not first/top) ring would help with flutter, the conventional wisdom beeing that the second ring gap is used to vent pressure out from under the top ring. But it turns out this does make a lot of sense specifically on the 928 engine, where there's seems to be a lot of oil on the cylinder walls all the time. The oil control ring can't always deal with all the excess oil, so the second compression ring has to do some oil control duty as well. The gapless second ring is really good at it, and reduces the likelihood of the top ring losing seal because of excess oil on the cylinder wall.

Another way to beat ring flutter is to boost the engine. This is also surprising at first, but improving the VE of the engine actually increases the usable rpm range. The ultimate VE improver is the turbocharger. The ring flutter appears when the piston ring deceleration in the later parts fo the compression stroke overcomes the cylinder pressure and unseats the ring from the bottom of the ring groove. Suppose that an unboosted engine that has a flat natural VE curve from 5000-6500 rpm starts fluttering rings at 5000 rpm. Suppose then that one adds ten pounds of boost. By my simple (oversimplified/incorrect?) math, the critical ring flutter rpm is moved up to 6500 rpm sqrt((14.7+10)/14.7)).

Again, caveat lector. This "knowledge" is from reading books and surfing the web. I haven't tried any of this at home. You shouldn't either, at least not based on this kind of "internet engineer" posting.

 

I had it calculated at 11cc, but 1cc per piston is irrelevant difference. The plan is to skim the heads too, as I was hoping for 11:1 with the GT crank.

GTS cranks are a rare beast and don't come around often enough. If something happens to mine, I think I will simply go for a stoker kit....

 

Not really. Pelican parts had them listed for $6.75 each around week ago. Friend bought 6 of them IIRR. They also had GTS pistons for $4.25 and $42.95 each depending on tolerance group. I bought several sets. Quess was this error in their database and did we get any parts.

 

That would be a hell of an error and I suspect you didn't get any parts! Unless I am missing something in my translation from Finnish...

 

Just minor pricing issue they had. $6.75 vs $7900.00 for GTS crank IIRR. In any case we didn't get anything other than email asking if I want to get pistons for $790.00 each.

 

Split the GT lump today and there is no damage to the bores. Wohooo! The rings are not broken either, but it seems that the oil control rings are worn beyond recognition. Moreover, the mechanic has never seen oil control rings like these before. Have you?

I also enclose the pic of con rod bearing N2 after 45k miles, 10k of them with a dry sump. Looks like it came out of the Glyco pac yesterday... So the dry sump does work

 

Oil rings look familiar. Need better picture to be sure but they are probably same what are used in 951 etc.

 

I just looked at my set of 951 rings. And the oil control ring looks a little different.

It is similar in design, but it has 2 ridges around the edge, instead of a smooth shiny edge,

The compression ring of the 951 is chromoly edge, but the scraper is like the 944 ring, just without the slightly smaller step on it.

 

rod bearings look GREAT

 

Alex,

what does he contest the oil control ring wear to?

 

those oil rings look funny. did that engine ever get split and rebuilt before?

should be the same as S4 rings, right.

any mods when you put her back together? bores look good?

bearings sure do!

mk

 

Those don't look like my oil control rings...

This is what mine look like in my GT.

 

Yikes, that oil ring looks like it's out of diesel. Those kind of rings are used in very hi compression low tension engines. There should be a wavy spring or something behind that oil ring. I'm wondering if the inner spring collapsed, or failed and allowed the oil ring to float away from the bore.