Dave W 1971/240Z rebuild

[davewormald]

I occasionally ground coffee in cylinder 4. Is that bad?

[Patcon]

The marks don't really look like a foreign object in the cylinder. You would expect them to be evenly distributed and they're not. I think Zed might be right. Too much timing advance?

[Patcon]

I found two different forums with similar pictures. Some interesting thoughts in these threads

https://www.bimmerforums.com/forum/showthread.php?2384311-Need-Advice-on-Damaged-Piston

https://motoredbikes.com/threads/detonation.42539/

Detonations seems to appears as finer pock marks almost bubbly looking

[grannyknot]

1 hour ago, Patcon said:

The marks don't really look like a foreign object in the cylinder. You would expect them to be evenly distributed and they're not. I think Zed might be right. Too much timing advance?

@Zed Head, you are absolutely right, good call.  I always thought detonation also need a relatively high compression ratio in addition to a lean condition, lesson learned.

A friend just sent me some pics of his RB26 head and it is pretty convincing. Now I have to remove the peaks and try to smooth it out.

 

[Mark Maras]

 To me the damage looks like it did come from a foreign object. Aside from the obvious damage on two opposite sides near the edge I also see numerous very small pecks? scattered around the piston top. I'm picturing something like a small nut bouncing around in there. In the center of the piston it falls nearly flat causing the tiny marks but if it lands near the cyl. wall it has something to lean up against instead of falling flat. After a short time it escaped past the exhaust valve.

[Captain Obvious]

5 hours ago, grannyknot said:

This is the exhaust valve and seat for cyl #1  with 160psi, the next 2 pics down are valve and seat from cyl #2 with 100psi

On the #2 those crusty bits are hardened carbon that I had to scrape off with dental pick, I'm pretty sure that is the cause of the low compression.

OK, so it wasn't simply a valve lash issue. Not surprising, but too bad. And those valves look a whole lot worse on the stem side than the cylinder side. Definitely enough carbon chunkies to cause the leakdown you saw. Smoking gun there.

So as for cleaning up the valves.... Two words. "Wire. Wheel":

[Captain Obvious]

Oh, and despite the brass color, I think the wheel on my grinder is fine steel bristles. I think they're brass coated, but steel. Use a light touch. The junk came off surprisingly easy for me.

[grannyknot]

8 hours ago, Mark Maras said:

 To me the damage looks like it did come from a foreign object. Aside from the obvious damage on two opposite sides near the edge I also see numerous very small pecks? scattered around the piston top. I'm picturing something like a small nut bouncing around in there. In the center of the piston it falls nearly flat causing the tiny marks but if it lands near the cyl. wall it has something to lean up against instead of falling flat. After a short time it escaped past the exhaust valve.

I hear you, that's why I had such a hard time believing it was detonation but I went back through some of my pics of other engines that had a foreign object bouncing around and there is always a recognizable shape of the object imprinted repeatedly.  Also, in this case the top edge of the craters is raised above the surface like an explosion whereas a nut or screw just makes an indentation with a very little edge.   

7 hours ago, Captain Obvious said:

OK, so it wasn't simply a valve lash issue. Not surprising, but too bad. And those valves look a whole lot worse on the stem side than the cylinder side. Definitely enough carbon chunkies to cause the leakdown you saw. Smoking gun there.

So as for cleaning up the valves.... Two words. "Wire. Wheel":
 

A heavy wire wheel is a great thing but I try to do passive cleaning as much as I can these days, you end up with less crap in the air.  Now I stand the valves up in a bucket with enough industrial cleaner ( Lye - Sodium Hydroxide) to cover the deposits and leave them over night, the next day you can wipe most of it off with an old rag. 

[Captain Obvious]

2 hours ago, grannyknot said:

I try to do passive cleaning as much as I can these days, you end up with less crap in the air.

Gotcha, and makes sense. You can't tell from the pics above, but I took that grinder with the wire wheel mounted outside and I'm doing that cleaning out in the yard and wearing a mask.  No way would I want that cloud of dust inside.    

[grannyknot]

This engine was in desperate need of an Italian tune up.

 

[davewormald]

49 minutes ago, grannyknot said:

This engine was in desperate need of an Italian tune up.

It got plenty of those when I first picked it up in 1994! I remember a time just before I put it in the garage (1999?) for its long sleep when I was heading up Hwy 427 from the Gardiner Expressway. There was a guy right on my tail, so I dropped down a gear and floored it. It didn't impress him with its get up and go, but it caused him to wave his hand in front of his face to clear the smoke. It was soon "replaced" by an Infiniti J30. I had two young kids at the time, and the sedan made a lot more sense, but the engine was still fun. I'm looking forward to the first spirited drive of the car's new life next year. That will be you in the driver's seat, of course - I can do the boring break-in, if you want.

[grannyknot]

All the valves are out and clean, they are in very good shape, this the worst exhaust valve seat and I think I can lap that little bit of roughness out.

I will take the head to my local machinist and have his skim the surface and remove a couple of broken studs for the exhaust manifold, I'm sure you all know which ones I'm taking about, either end of the manifold. Once I get it back I'll drill and tap those holes to M10.

I big box of goodies from Rockauto showed up today.