Cylinder scoring...when to be worried?

[Flynn]

I was told this motor was recently rebuilt by the PO, the intake/exhaust manifold look almost new and the head looks like it was freshened up but when I got the head off I found this in cylinder #6 (see pic). I can feel one of those score marks with my finger, the far right deeper one, the others I can't feel. My local machine shop said to try hitting it with some fine emery cloth but that didn't take it out.

There may be a slight knock from the rear of the motor that disappears completely when warm and is tough to hear when cold. I had some pinging from the back two cylinders over the summer that I was able to get rid of with timing/valve adjustments and the #6 exhaust port was leaking pretty bad (not sure if these would affect the cylinder wall). I did a compression test 1,500-2,000 miles ago: #1 = 155; #2 = 155, #3 = 150; #4 = 145; #5 = 140; #6 = 160. Is it weird that the cylinder with the worst looking wall had the highest compression? Also, I don't have a good micrometer but I got 94.1-94.2 mm on all cylinders with my crappy HF one...I wouldn't trust it too much.

I'm not sure if I'm at a point where it would be better to pull the block or put it back together and keep driving it until it doesn't work. I was getting good power and 16 mpg. How quickly do these things tend to deteriorate? Gaskets get expensive and I'd hate to waste the cylinder and exhaust gaskets I just bought. Do I have any other options?

Thanks!

 

[vtgbeemer]

You didn't mention smoke or oil consumption. With compression readings like that your reasons for pulling the head were?

 

[Flynn]

There's a bit of oil consumption, I think I have to put in a quart or two between changes. Not a lot of burning that I can notice.

I had to pull the head b/c the PO stripped out the last exhaust manifold bolt.

Edit: not much smoke that I can notice, runs pretty clean when warm, if a little rich.

 

[ColonelMustardFJ40]

I have seen this happen before with a freshly rebuilt engine. The problem, in my case, was that the machinist did not allow adequate piston to cylinder wall clearance. The person described an episode where he was driving up a grade when the engine lost power and began knocking. As the piston expanded from the heat load, they scored cylinder walls in two cylinders. The engine also had a knock it it when cold. When the engine was disassembled. all of the bearings were good. The engine was re-honed and new pistons installed, never a problem since.

 

[vtgbeemer]

Sounds like some freshening up would go a long way at this time. In observing this site and others I see too many well intended overhauls result in more pain than gain.

 

[Flynn]

Well...I don't think I have the option of going for a new piston as this is a 78 motor with the dome-top pistons, all but unobtanium now from what I understand, even for one.

There's a local guy with a block that has been bored 0.30 over and has all the parts to go back together from an 83 motor with flat tops and the closed chamber head - should I go that route? It's cheaper than if I had to buy new flat pistons and a late head (or mill the one I have) to get my current block away from the domed pistons.


Could I reassemble the scored motor and drive for a while while finishing the rebuild on the later 2f? I don't know if I would be risking toasting a still recoverable core motor that I could sell at a later point.

 

[Okie joe]

I think I would pull the oil pan. Take the offending piston out and check the ring end gaps and see if they are to tight. If they are fine then put it back together and use it. If they are tight which may be why you have higher compression on that cylinder, then grind the rings to get the right end gap and put it back together. some folks just assume all the cylinders are the same and don't check end gaps.

 

[Flynn]

OK, I think my plan is to pull the piston, as Okie Joe suggested, and check both the piston skirt and rings. At least that way I can take that to my machinist and see what he thinks. Will I have to replace the connecting rod bearing when I do that or can I reassemble if it all looks OK? Should I just replace it anyway? The goal is to get it running again while I figure something else out (if I can't fix the scoring). I was planning on carefully honing the cylinder too, to take off any burrs or ridges.

My thoughts were that since it's a dome top piston, I can't rebuild it as is (b/c of piston availability), so, assuming worst case scenario, I'll do my best to alleviate any problems and reassemble it. Then I'll pick up a spare 2f block and work on getting all the parts together for a 3fe or TBI build, depending on what I can source out.

 

[srgould41]

I think you are on the right track with pulling the piston and honing the bore. I suspect the hone followed by ring gap checking/setting it will solve the issue. I don't see why you would need to replace the rod bearing if it looks good.

 

[cuencanolenny]

Cast pistons run less clearance than do forged pistons. The cast pistons are less likely to swell when hot, that is why forged pistons require more clearance. When hot the skirt of the piston can swell and "grab" the cylinder wall and leave a score like that. Also a broken ring can leave a scratch like that. But a person wouldn't know what the condition is until the piston is removed and examined along with the bore. Use of a bore sight gauge is required to determine the size and concentricity of the bore. A simple micrometer and internal gauge pin is only good for determining diameter, it won't tell you how round or straight the hole is. One problem machine shops often overlook is the use of a torque plate. This thick steel plate dummies the cylinder head and is installed and torqued to spec prior to boring and honing. This assures a more accurate operation. Also, many repair shops use a Bux drill for boring and honing. This rudimentary piece of equipment is a poor choice as opposed to a Sunnen machine which will align bore the cylinders perpendicular to the crankshaft centerline and correctly spaced. Doing your homework to understand what kinds of machines can perform what kind of work and what the machine shop has is valuable. Just relying on someone to do the job right can be an expensive mistake.

 

[DougAustinTx]

I have yet to find a machine shop that had a torque plate for a 2F engine. (Texas)

 

[cuencanolenny]

I guess that would be one big chunk of steel to have laying around for relatively little use. I suspect they do so many small block Chevy's and Ford's that the cost and storage are worthwhile but not so much for the LC engines. Good point.