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I have a friend of mine who I'm helping rebuild his Mitsubishi engine, 2.6 4 cylinder. We've got a new set of rings, same set of pistons (didn't need boring) and a ready block.

I cleaned the piston grooves with a broken piece of ring & installed the rings. They're installed correctly (in this case, bevel up). I get out the spring compressor and compress them, then start installing the piston into the cylinder (arrow facing correctly). I get the piston all the way in (all 3 sets of rings are into the cylinder), and then no more. It literally takes heavy hammering to even slightly move the piston. I get the piston back out, and none of the rings have popped or are bent; they're all in the proper grooves, undamaged. There's no scoring of the cylinder wall that might indicate a ring dragging.

I inserted the rings into the cylinder without a piston, thinking maybe there wasn't any gap once they compressed, but they still have a gap all the way down the bore. The rings themselves seem to compress and expand very easily within the piston grooves. The pistons slip easily into the bores with no rings on them.

The machine shop, which cleaned up the block & honed, made sure there wasn't any ridge on the block (there really wasn't any to begin with). I've tried soaking these pistons in oil, oiling the cylinder wall heavy, etc. Still, it takes an inordinate amount of pressure to even budge these things. I compared the new rings to the old ones and they're the same size. I've got the pistons going back into the same bores they came out of. The rods aren't hanging up on the crankshaft or anything like that.

I'm at wit's end here...I can't, for the life of me, figure out what could be making these pistons so hard to install. Does anyone have any ideas of things I might not have covered here???

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did you check ring end gap by sliding a ring into the bore without a piston in it?

John

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Or did I put it in there? Anyway, I put the rings into the cylinder without the piston to check the gap and there's gap all the way down.

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I know this sounds simple, but....did you check the piston to sidewall clearance? Mic out the piston and take a measurement of your bore at top, middle and bottom, check your manuals for clearance. Let us know what you find
 

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What grit did the machine shop hone to? If the hone is too course on these little import engines, they will do exactly this. These fine little oil scrapers love to hang on course hone marks. BTW, have you checked the end clearence of the oil rings?

Do yourself a favor and don't hammer on the pistons very hard, (you knew that........*G*), even with the back of a hammer handle. Use somthing soft like a shop rag folded up and a block of wood and tap on the block of wood when you get back to that point.

When they are right, they will go in, even if they are a little sticky.

Try going over the bores with a red scotchbrite pad and some WD-40 to break the edges of their hone marks. Also, have you tried other cylinders to see if the problem is the same?

Think about it this way.......... If you were trying to make the piston stick you would likely put course hone marks and then use the finest little rings you could find.......*G*. The machine shop likely ran a "bottle brush" hone down the cylinders and called it good. That is too course. You may need to re-hone with a fine stone hone and try again. First try the scotchbrite trick.

Hal
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Are all three oil rings installed correctly? Is the crank at BDC for that cylinder? Rod and rod bolts not hitting anything?

67 coupe 289 auto
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Are the connecting rod bolts hitting the crankshaft? Put some vacuum hose on them to protect the crankshaft as they slide past.

Could the piston be 90 degrees out of position meaning the rod would hit the crankshaft?

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Something is obviously not right. Are they all like that or just one cylinder?

Make double sure you got the correct rings. If the block is truly a std bore you should have STD rings and STD pistons. Because of the forgiving nature of street motor ring gaps you could possibly get a wrong size ring to slide in the bore but it would bind up once it got on the piston because of the inside diameter of the ring against the ring grooves in the piston(too tight).

Good luck and let us know what you find out.











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Nope, the rods are clearing the crankshaft fine. I've got fuel hose on the bolts protecting the crank. I've also got the pistons faced correctly within the bore...

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I didn't consider the roughness of hone possibility. The bores LOOK as though they're honed pretty finely, since they almost looked glazed, but I'll try the scotchbrite trick anyway. The problem is the same in all of the other cylinders. The end clearance on the rings is good...and I have no idea what grit they honed to, since my friend took the block in and got it back on his own.

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If you have checked that when the rings are in the cyl bore they have the proper end gap throughout their full travel, I would suspect that the ring grooves in the pistons are the culprit. Get hold of a ring groove cleaning tool(like a K-D tool #1722 less than $20.00). This has a cutter that will cleanout the grooves so that the rings can sit properly in the piston and not cause them to bind in the bore

Greg B
 

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Yo Joe,

Is it possible that the rings are bottoming out in the ring grooves of the pistons? I know you checked them and they look similar but did you actually mic the lateral thickness?

I'll bet you're using a spring steel compressor and not a tapered sleeve compressor, yes?

Run your fingernail up and down the hatch marks in the cylinder wall....does it catch at all?

Are the hatch marks at roughly a 45 degree angle to each other?

FWIW, I've done a few re-ring jobs with a 400 grit Flexhone in a drill, some plasma moly rings and lots of hope...and most of the time they'd work just fine, at least until we could get the engine back on the CK10....it's amazing the things one will do with some prize money dangling in front of them....

Let us know what you find out...

Pat
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I've run into this problem before on these engines.I can't remember exactly what year, but there was split year in which they made a few changes to the engines configuration,and I think the ring package was one of them. You may have been sold a set of rings for an earlier engine. The radial thickness of the earlier rings is a hair thicker than the later ones which could be causing them to bottom out in the grooves. I believe that the later engines(88 and up I think) used the narrower rings and the thicker ones on the early engines. I'll have to double check my data to be absolutely sure of when the change was made. Let me know what year,make,model and I'll try to get more info .

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You remember correctly.It was in mid year 1983 they changed the compression rings from 2.0mm to 1.5mm

Greg B
 

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Thanks for correcting me:) I reallized I was 5 years off after I got to the shop and checked my catalog. Unfortunately my catalog doesn't list the radial thickness to know exactly how much of a difference there is. Scott

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