Question about uneven carbon build-up on the piston top
10-23-2014, 05:12 PM
#17
Rennlist Member
Thread Starter
Mark,
Short story is I blew the front crank seal, found copper flakes in the oil, pulled the engine (which was already on my winter to-do list), checked the rod bearings (nothing), pulled the girdle to look at the main bearings.
I pulled the heads because I was curious and now was the time to do it. Sort of glad I did.
Short story is I blew the front crank seal, found copper flakes in the oil, pulled the engine (which was already on my winter to-do list), checked the rod bearings (nothing), pulled the girdle to look at the main bearings.
I pulled the heads because I was curious and now was the time to do it. Sort of glad I did.
10-23-2014, 05:16 PM
#18
Rennlist Member
Thread Starter
Greg,
In case you weren't being rhetorical I figure the car has 70k ish miles on it. However I'm thinking that this damage was either done by my lean condition or by something the PO's mechanic did or didn't do when he repaired the TBF.
In case you weren't being rhetorical I figure the car has 70k ish miles on it. However I'm thinking that this damage was either done by my lean condition or by something the PO's mechanic did or didn't do when he repaired the TBF.
10-23-2014, 06:24 PM
#19
Rennlist
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Mark,
Short story is I blew the front crank seal, found copper flakes in the oil, pulled the engine (which was already on my winter to-do list), checked the rod bearings (nothing), pulled the girdle to look at the main bearings.
I pulled the heads because I was curious and now was the time to do it. Sort of glad I did.
Short story is I blew the front crank seal, found copper flakes in the oil, pulled the engine (which was already on my winter to-do list), checked the rod bearings (nothing), pulled the girdle to look at the main bearings.
I pulled the heads because I was curious and now was the time to do it. Sort of glad I did.
For the time investment, from where you are at, there is little downside.
I think 4.7 rings are a bit hard to find and pricey....but these were not $5,000 cars when they were made, it's really absurd that they have their current values, although this looks like it is quickly being corrected.
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greg brown
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Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
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Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
10-23-2014, 06:53 PM
#20
Nordschleife Master
I know the 4.5 rings are pricey and hard to find, but I thought that the 4.7 were reasonably priced.
I could be off on that.
Was there a leakdown test done prior to pulling the engine?
I could be off on that.
Was there a leakdown test done prior to pulling the engine?
10-24-2014, 01:28 PM
#21
Rennlist Member
Thread Starter
Colin,
No a leak down test wasn't done prior to tear down as I wasn't thinking "rings". I pulled the heads only to check the head gaskets and see if there was any problem with PO's mechanics valve job.
No a leak down test wasn't done prior to tear down as I wasn't thinking "rings". I pulled the heads only to check the head gaskets and see if there was any problem with PO's mechanics valve job.
10-24-2014, 06:22 PM
#22
Rennlist Member
How many miles are on it?
Generally, lean running gets things hot and generally helps to seat rings. Rich running is hell on the ring seal.
That being said, it is possible to run these higher compression engines so lean that the pistons/rings get damaged.
From my visual inspection of your picture (tough to diagnosis things from a picture of one piston and a couple of combustion chambers), I ruled out a "water problem". Water generally "steam cleans" things....instantly....and leaves only "hard carbon...not soft carbon. I do not think you could have that wet, soft carbon shown on the intake and exhaust valve with the presence of water.
Generally, lean running gets things hot and generally helps to seat rings. Rich running is hell on the ring seal.
That being said, it is possible to run these higher compression engines so lean that the pistons/rings get damaged.
From my visual inspection of your picture (tough to diagnosis things from a picture of one piston and a couple of combustion chambers), I ruled out a "water problem". Water generally "steam cleans" things....instantly....and leaves only "hard carbon...not soft carbon. I do not think you could have that wet, soft carbon shown on the intake and exhaust valve with the presence of water.
lean does not 'generally"mean anything gets any hotter than rich. in fact, the temp curve is pretty bell shapped, with it going COOLER on the lean side. stoich is the highest temps, and that's where knocks occur.
as far as the evidence goes...I think I tend to agree.. Ill have to pull out the pics of scos motor with the head gasket leak (water leaking issue), and see what it looked like. its tough to have a water problem, as it has to really be leaking to leak in the cylinder on the intake, and not pressurize the cooling jacket on the compression stroke.
10-24-2014, 06:24 PM
#23
Rennlist Member
I thought that as well.... I think I have a set of 4.7 euro pistons with new rings on them... ill have to check.
10-25-2014, 06:38 PM
#28
Rennlist Member
Thread Starter
Pulled the crank and pistons today. I didn't get a chance to inspect the rings carefully yet but it looks like there's some excessive wear on the piston coating considering the miles.
10-27-2014, 04:49 PM
#30
Archive Gatekeeper
Rennlist Member
Rennlist Member
I am just an amateur who has seen only 4 or 5 sets of pistons coming out of a motor but to my eyes those don't look bad (or at least excessively worn) for 70K miles. Happy to be overruled by those with more experience.
EDIT: Overruled, see post #37.
EDIT: Overruled, see post #37.
Last edited by Rob Edwards; 10-29-2014 at 02:34 PM.