any takers

 

Very strange, but not much way to get pressure from the combustion chamber to the crankcase/valve cover except by the rings. There would be other paths around valves/guides, but not in that volume and not without going out an intake or exhaust port. There has to be some sort of "perfect storm" of defective parts for it to only do it at/around TDC. The compression numbers are decent, but the problem with a static compression test is that the tester has a check valve. You could be building pressure as the piston travels up the bore and then losing it when it hits the trouble spot at the top. The compression tester misses this. This is where a leakdown test will show up a problem (which yours has). My best guess would be a gouge of some significant size at the top of the bore, a broken ring possibly combined with a broken ring land. I described my dad's 300 that had this problem. I can see where it would be possible that it could move "just right" as it is changing direction of travel allowing a bleed-off of cylinder pressure. This is about the only scenario I can think of where what you are describing would happen. Also, try running it, pull out the breather(s) and the PCV valve so there is no vacuum sucking on the crankcase and see how much blow-by/oil smoke you get coming out of the valve cover. If it's much at all, I'm afraid your motor is screwed. Check it out and let us know. Good luck! Oh, BTW, try pulling the plug wire from that cyl and see if you see much change in RPM, smoothness, missing. If not, then that hole isn't really doing much anyway. Hope this helps!

 

Your posts are excellent. I did not intend to belittle your experience or skills with my post.

 

another update. i brought home a borascope from work today and stuck it down in the cyls. found in just about each one there is a chip in the piston on the edge. looked at the old pistons from a 300 i previously tore down, and they had chips as well... but on the pistons i had in my hand those chips looked as if they were designed to be there? sounds crazy but if you could see this chip off the old motors piston its so perfect as if someone scored it and chipped it off it makes me wonder? there was no factory thing done to the pistons right, there should be no chips anywhere?

the chip is right whre the piston touchs the cyl wall.

 

One of the few weak parts of the 300 are the stock cast pistons...they tend to break. A number of folks have found bits and pieces of them in the crankcase when doing a rebuild. Hypereutectic pistons are the usual upgrade when doing a rebuild and they maybe cost only a little more than stock slugs.

 

Oh, No. No offense taken. I should have offered a bit more info in the first place. The little story I offered at the end there was an absolutely true experience though. While extreme, I have seen a lot of similar things in my day. Strips of beer cans cut and placed behind bearing inserts as "shims" for one. As a matter of fact, the very first engine I ever built was a 350 Chevy. I bought the car not running. The engine was supposed to have been recently installed and had been purchased from a local well known engine "remanufacturer." Every region has one or two of these big outfits that offer rebuilt short and long blocks for about anything on exchange basis and employ several "professional rebuilders", do machine work in house, etc. Around here it's Grooms or Jasper, etc. Anyway, the guy I got it from had cracked a head, bought a pair and replaced them, never could get it running. Well, the dizzy appeared to have just been dropped in at random, like 90 degrees out, the fuel pump suction line not hooked up, etc. Got that straight and got it to hit a little, but sounded all wrong. Decided to pull heads and check it out cause the behavior didn't make sense. Pulling the heads revealed a large chamber on one bank and a small chamber on the other. Would have had about 3/4 point more compression on one side than other. Cam out of time too. Decided I couldn't trust anything in it (mind you I was only 17, but knew somebody was a moron). Tearing down the bottom end revealed that although competent machine work HAD recently been done, the assembly rendered it moot. Some rods had standard nuts, some castle nuts, etc. Easy first build because it was essentially just rings, bearings, gaskets, re-con rods and a cam just for fun. (Oh yeah, a set of decent heads!)
I could tell you a bunch more such stories, because ANYBODY with a set of tools can assemble an engine, but many shouldn't. You are obviously a very competent mechanic yourself, so you know the folks I am referring to. So, not belittled at all, just wanted to make sure I wasn't being wrongly assigned to the short buss with those fellas!
It's all good here. Any FTE guys, but ESPECIALLY the 300 I6 ones are family in my opinion. All that said, this engine sounds a bit strange. I mean it's obvious that the rings/bores etc. are done for, but the leakdown test results seem a bit odd to me as well. I'd love the chance to tear this one down and see exactly what's in there. Also, just read the post about the "chips" in the pistons and wondering if these are missing hunks from the slugs starting to come unglued, or just the alignment notches.
How 'bout it 8525? Any way to put up a pic? There's not just one per piston is there? Do they go just over the edge of the surface, and do they go to the top ring groove? LMK when you can. Thanks?

 

i'll see what i can do on the pics. do pistons have alignment notches?? that may be it

 

Yeah, usually something to get them in the bore right. Different on different things, but my EFI 300 has a machined notch about 1/8" wide that goes to the front. Look at the pistons you've got out. Is the thing you are talking about right above the end of the pin on all of them (or in the same spot on all of them?). If so, it's just that. If you are talking about something in various different spots, more than one per piston, or extending down the side towards or all the way to the ring groove then it is destruction. The fact that you said they looked like they were made that way made me think this was what you were seeing, in which case, they WERE made that way. LMK when you can. I hate to keep saying it, because I know you are stressed out. I too have been without my truck for a bit now because of the cracked head, but I gotta say, there's not much doubt that your engine is history (at least until it can be rebuilt). I just re-read some of the previous posts. I was thinking that it was only 1 cyl that had this symptom but it is actually more than that? Did I also get it that you changed/replaced head when it started? Did you eyeball the cylinder walls while in there? Any significant lip or ridge at the top? obvious scoring etc? BTW, what part of the world are you in?

 

Yep, rebuilding an engine can be like an archeological dig. I found the same mis-matched nuts on the rod bolts of Datsun roadster I rebuilt many years back. A common trick of 'mechanics' is putting ball bearings or bbs in vacuum lines when they can't figure out what is wrong or how the system works.

I also helped dismantle a bbc racer that had been assembled by 'good ol' boys'. That engine looked like a gernade had gone off in it.

 

I knew some "good old boys" who assembled a Mopar 383 while consuming adult beverages. Sounded funny when cranking over first time. Should have, what with that piston & rod left out. Seriously.

 

picture of a piston from the old motor. this is hard to see but this is not the notch your talkin about is it, i think its a chip and this is what the pistons look like in the motor in the truck right now

[/IMG]

 

That looks like a placement mark to me.

 

That is exactly what it is. OBCB

 

Yep, spent considerable time looking at the same mark on mine just last week while assembling the motor. I can nearly guarantee you that you'll find something ugly if you tear into it. Seriously, if you have no other transportation and need this thing running, you could get a cheap head gasket set, and being a carbed motor instead of EFI, you could have the head off in probably an hour. If there's no warpage of either block or head, clean it up with a Roloc disc on a die grinder or scrape, being careful not to gouge metal Sand with a peice of wet-or-dry paper until both are perfectly clean. Re-assemble with new gasket and torque down. With a little luck you can have it back together in a couple of hours. You won't be able to tell a LOT without tearing into shortblock, but agood look at the bores, watching for scrapes, gouges crack etc. might give you the answer you're looking for (or the one you are not). Look for a significant ridge or lip at the top of cylinders, bring one of the problematic pistons up to TDC and see if you can rock them around, side to side. You might even introduce a little pressurized air into the crankcase (NOT A LOT OF PRESSURE!) and see how much is hissing out around them. It's arelatively small investment of time or money and there's a strong chance that you'll finally KNOW what's up with it.

 

IF THATS A PLACEMENT MARK THEN WHY IS THAT MARK ONLY ON 3 PISTONS, THE OTHER ONES HAVE NO MARK AT ALL. i NEVER HEARD OF THAT BEFORE PLEASE EXPLAIN WHAT THE PURPOSE OF THIS PLACEMENT MARK IS