I vaguely remember thinking about whether to use new rings or not when i had the GTS engine rebuilt.. pros vs cons of the cylinder wall and new rings seating. I used new rings, but i didn't touch the cylinder walls. The engine uses some oil but then we only drilled half the oil return holes.

Andrew,

Those numbers on a cold engine are still way out of whack. Mine with the crappy condition had less than 8% and most were under 5%.

Even cold there should be almost no leakage. Also with that much leakage you will definitely hear where it is leaking to.

Pull the cam covers and pull the cams. Then you know all the valves are seated. Then when you pressure up the system, you can tap the valve to make sure it is seated on the valve seats. Guide seals do not factor into a leak down or compression test. Neither does valve guides. Those are factors in dynamic engine issues (oil consumption and valve float for example).

He has said he is re ringing it. And doing the valves. The leak dwm does not matter.

The cheap HF tester only operates at 10psi. That's not enough to seal the rings. They work by cylinder pressure pushing them out against the bore walls. 10psi is also too low to get accurate readings. A very small error on the gauge results in massive errors in leak down readings. Run the test again at 100psi and you'll get more representative results.

All you need for a leak down tester is a pressure regulator and an inline gauge. Set the line pressure at 100psi. Then connect a hose with an inline gauge to the spark plug port and read the pressure. 90psi is 10% leak down. The only trick is that you have to hold the engine from rotating by having it in gear or by some other means. 100psi is enough to push the piston down unless you have it exactly at TDC and there is no moment pushing on the crank to turn it.

I did but they aren't in my current motor. It is stock everything.

I had them dished to a CR in the low to mid 9's...been so long now I cant recall. Also had the tops ceramic coated. There is a thread on it here somewhere. I think it was over 8yrs ago now? I still have the entire motor disassembled and boxed up in the attic.

"BlownBeast" has the same pistons in his SC'd 928, but I haven't seen him on here in ages? Its really easy to do and I imagine even easier today with more modern machining technology. Mine were turned on a lathe but honeslty I think it would be better to have them CNC machined. Ive got pistons cut in half that have the profile on them, let me know and I can send it to you and you can just copy it.
Pic below...most of the material is removed on the outer part of the dish. The center of the piston was the thinnest area so no material was removed there..it was just "skimmed over" as you can see. In a nutshell, the slope of the stock dish was increased yielding more volume.

No, that is not correct.
Boost alone does not increase blow-by unless something else is wrong.
Most boosted cars with higher than stobk blow-by can be attributed to the tuning, something wrong with the brether system etc...

928 blocks do NOT have a "special coating".
Our blocks are made out of alusil.
They are aluminum blocks with a high content of silicone.

The honing process is to explose some of that silicone which is what the rings "ride" on. If the bores are not damaged or being bored out, no need to hone them, even with new rings.

For grins, here's the Sunnen procedure for honing an Alusil block. Not that I would ever do this at home, unless I was prepping a coffee table...

https://webfiles.uci.edu/redwards/pu...s%208-5-11.pdf

Vaguely reminiscent of the chemical process those instant-clean deoxidizing tarnish removal kits use when cleaning copper, silver, etc. Nobody ever asks what the film of material in the bottom of the chemical bath is...

Andrew,

Good luck with the tear down and rebuild. Did you have any obvious symptoms to feel a full tear down is really needed?

I do not buy those leak down numbers- not from a motor that can turn out 500 bhp.

Regards

fred

See post #34 by ThetaTau87

10psi is good for testing a lawn mower or weed whacker.........

I didn't realize the HF tester only went that high. Do they sell two different units?

Its technically a chemical etching process to remove some of the aluminum but leave most of the silicon crystal structure that resides in the aluminum material.

Also, it doesn't actually remove anything dimensionally speaking - only chemically. So it will not increase the bore size. It simply prepares the bores for new or nearly new ring (the latter is what I will try next on a cheap motor)

Rather than risking your stock pistons by machining them and effing up the coating, you could get some from an '87 (through early '88) with the factory deeper dish, and retain the factory ferrostan coating.

87 S4's are usually 9.3:1 compression. (Tuomo probably has pics of the difference?)

Good for you man. Always nice to see someone getting **** done. Thanks.

It matters if he wants to know what shape the engine is that he is refreshing. Since he did an leakdown, I would guess he wants to know the condition.

When he takes it apart, he will see

1) Bore surface
2) Piston Skirt condition
3) Ring Land condition
4) Valve to guide clearance (probably large)
5) Valve to seat contact

My point simply is that it doesn't matter when replacements as he is suggesting are planned.