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Old 06-15-2009, 10:50 AM   #1 (permalink)
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Default Low compression on fresh engine

I did my first rebuild on an 89 1.6L Sidekick and have compression problems. I bought a rebuilt head for it, and it has new .50mm over pistons and rings.

Compression is 85-89 psi on all cylinders, goes up to 135-140 on wet test. Book says 170-199 is the normal range. Engine started and ran OK, but has very little power on test drive. I tried seating the rings with accelerations and decelerations but dry compression was still the same. Didn't start a day later and I suspected it was due to the low compression. When cranking slowly with a socket and the valve cover off, I easily hear air squeezing past the rings listening down the oil passages to the crank case. I heard nothing listening at the intake and exhaust ports with manifolds off.

I checked the ring gap during assembly and it was .020-.025" (over the limit), but a note in the manual said ring gap is not critical unless .040" or more. I suspected the machine shop messed up the bore and pulled the engine and tore it down again.

I don't have a great micrometer, just a dial caliper supposedly good to .001". It shows about .001" difference between the piston skirt and bore diameters, but hard to tell if it is accurate. Book says clearance should be .0008-.0014 but this is beyond the accuracy of my caliper. One thing that surprised me was the top of the pistons were .020" smaller than the skirts.

Any suggestions? Is it normal to hear ring leakage on a freshly rebuilt engine? Is the ring gap too wide?

John
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Old 10-14-2009, 02:07 PM   #2 (permalink)
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Default

some ring leakage would be normal or how could you get blow by ?

piston heads get hotter so they would need to be smaller.

You said that you replaced the head, did you do a leakdown test to be sure a valve was not leaking ?

add a little oil thru the plug hole, if psi goes up it is rings, if psi stays the same it is valves, same test for checking compression in worn engine, assuming everything else is correct.

just an idea.
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Old 10-14-2009, 05:19 PM   #3 (permalink)
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The machine shop screwed up. The bore was about 3 thousandths over the piston diameter with the service limit being about 1.6 thousandths IIRC. I had to buy another set of new pistons, .75mm over this time, and brought it to a different machine shop since this was the second time the first shop screwed up (they missed a crack in the old block). Compression is much better now. Leakdown test looks fine.
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Old 08-12-2010, 09:02 AM   #4 (permalink)
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Hi,
I did my first rebuild on an 89 1.6L Sidekick and have compression problems. I bought a rebuilt head for it, and it has new .50mm over pistons and rings.

Compression is 85-89 psi on all cylinders, goes up to 135-140 on wet test. Book says 170-199 is the normal range. Engine started and ran OK, but has very little power on test drive. I tried seating the rings with accelerations and decelerations but dry compression was still the same. Didn't start a day later and I suspected it was due to the low compression. When cranking slowly with a socket and the valve cover off, I easily hear air squeezing past the rings listening down the oil passages to the crank case. I heard nothing listening at the intake and exhaust ports with manifolds off.

I checked the ring gap during assembly and it was .020-.025" (over the limit), but a note in the manual said ring gap is not critical unless .040" or more. I suspected the machine shop messed up the bore and pulled the engine and tore it down again.

I don't have a great micrometer, just a dial caliper supposedly good to .001". It shows about .001" difference between the piston skirt and bore diameters, but hard to tell if it is accurate. Book says clearance should be .0008-.0014 but this is beyond the accuracy of my caliper. One thing that surprised me was the top of the pistons were .020" smaller than the skirts.

Any suggestions? Is it normal to hear ring leakage on a freshly rebuilt engine? Is the ring gap too wide?
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Old 08-12-2010, 10:27 AM   #5 (permalink)
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Your description sounds exactly like the things I ran into and the stuff I read. The wide ring gap is indicative of the bore being wider than it is supposed to be. The ring manufacturers normally get it pretty close for the advertised bore size, so if the gap is big, the bore is probably too big. I also found the tolerances on this engine for bore to piston size are much tighter than other engines, which machine shops might not check and thus be more liberal than they should be with the bore size.

It is normal for the top of the piston to be a lot smaller than the skirt. The skirt to bore clearance is what counts.

Since you already tore down the engine again (I did the same thing), bring the block and pistons back to the machine shop and have them measure them which they can do easily and accurately. Have the specs handy so they can see how tight the tolerance is. It is likely they bored the block too much. Good luck.
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