jon928se

+928

Where you can still see the lathe marks in full but just obscured it's build up. If it was pitting the lathe marks wouldn't be there.

PS Your pistons shouldn't be purple!

milkman383

Yeah, I've never seen purple pistons. :-)

And agreed with Greg, that actually looks like build up since you can so clearly see the machining marks. I think you might be all good. Do the plugs have any aluminum flaking on them?? That's the best sign you're running dangerous lean and causing issues.

Cosmo Kramer

I think the boroscope is making the dark areas look like pits when they are just carbon buildup. The images aren't clear enough to distinguish buildup from pitting.

GregBBRD

I've never taken a 928 engine apart that had piston damage from low octane or too much timing.....and certainly not a 2 valve engine. I have seen one late GTS that had pitting on the pistons, right below the spark plug, but that engine pinged and knocked so badly that it could hardly be driven. I didn't take that engine apart, I simply fixed the problem and left it alone, since there was no visible damage on the edges of the pistons.

When engines get combustion chamber temperatures high enough to begin melting pistons, the first things that are going to get damaged are sharp edges, like the edges of the valve reliefs. Therefore, a good clue that there is nothing wrong, in this picture, is the very "sharp" edges of the valve reliefs.

Abby Normal

Amazing what an eye of wisdom can see!

danglerb

Its like where's Waldo, once you know where he is, he is easy to see.

Tim968

'92 GTS, 70,000 km, AT, ex-Japan, bone stock, & air pump delete. Recent plugs WR7DC. Recent O2 sensor, New knock sensors, all functioning properly. Recent X-pipe and 100cpi Cats. Life is good. Improved power and a lusty exhaust note. Fuel economy who cares? 13-15 liters/100km. Problem - spark knock when pressed hard beyond 1/2 throttle, can modulate the effect with throttle pressure. Oil consumption is very reasonable, increases when driven hard. M&H Provent200 and PCV set-up installed ~ 1500km ago, works well. Timing at idle verified at 10° BTDC specification. Have previously logged knocks per 10,000 cycles at various "around town" throttle levels 25, 15, 40 with Theo's tools. Can easily get to GregBBRD's danger threshold of 75 knocks per 10,000 cycles (rod bearing damage zone) if pushing hard on engine, but this is not necessary to drive it daily, which is what I really enjoy doing. Too bad it experienced a hard, slow, coked-up and hot weather life without drilled oil-ring holes for its first 60,000 km in Japan....

Ran hard today in an attempt to burn off some coke, about 1.5 hours highway time, 40-50 liters of fuel at 3k rpm, with a few 15-30 second pulls up to 4500 Could hear spark knock when feeding it too hard, always prevalent above 3000 rpm. Ran fine and car feels happy. Came home and pulled plugs 1-4. Good color diff on intake/exhaust side, perhaps a shade lean, but the deadly tiny pinprick flecks of shiny aluminum are there on each plug (photos are poor, but the flecks can be seen with pocket microscope. Photos embedded.)

Tops of pistons 1-4 look like shiny hard black rough-finish coal. No machining grooves on piston top are visible, surface is relatively coarse. (My borescope needs a smarter operator).

So - how to decarbonise in place, or get ready to be very intimate with cylinder heads this winter. Can I feed it 2 liters of water while running 2500rpm in top down the road at 100km/h? Ideas? B&G products? Techron? Other?

Imo000

Combustion chamber cleaner time! you can do the old mechanics trick and use water through one of the vacuume lines. That shouls steam clean it too.

Hilton

I used an equivalent to Subaru/GM Top Engine cleaner made by 3-bond - worked very well, but did take more than one treatment.

Water apparently works just fine, or old brake fluid.

If you're not sure baout sucking stuff through an intake vaccum line, and want to go professional, then the BG product has plenty of good reports here. This thread has a lot of input:

https://rennlist.com/forums/928-foru...er-thread.html

dr bob

I'm a very regular bore-scope user, or was for quite a while, inspecting gas turbine engine sections. Probably the hardest thing to get your head around is the way there's typically no available sense of scale/size/proportion in the borescope images, especially the inexpensve ones. I started using a Sony 'scope since it has comparator gradients on one of the lenses to help you judge what you are looking at. Mine has a 15mm objective lens and is way too big to pass through a spark plug opening on the car. It was also several tens of thousands of dollars when new in the early 1990's, and except for being long enough (four meters) to pass through turbine sections and flexible/steerable remotely, offers no serious benefit over the inexpensive scopes available today.

Sometimes it's handy to pass a small printed paper scale with the 'scope, and place it in the field of vision. This is especially important when attempting to document damage or wear that's not severe enough to deserve a full teardown now but will in the future.


The images in the OP's photos show the machine marks on the piston tops, marks that -might- be 1mm apart. The small shallow-focus wide-angle lens, image projected to a larger screen, makes the minor deposits look like mountains on a moonscape. They are very normal. Detonation damage shows up as little pock marks in the metal. Like the piston top has been hit repeatedly with a small sharp-end hammer, or blasted with buckshot.

I worked years ago with a 2-stroke race engine tuner who used a little fiber-optic type device with a lens on one end and an eyepiece on the other. He was amazing in what he could diagnose from the deposits and damage patterns on a piston crown. Worn or flattened rod bearings, twisted connecting rod, fuel mixture issues, poor flame dispersal, detonation damage, stuck rings, burned piston lands, all of them leave telltale signs on top of the pistons.