My car has had compression test numbers around 130-140 for a few years now. Always figured it was blow-by but the car ran well enough, didn't burn oil, and I never had the means to fix it, so I just lived with it.

I finally got a chance to tear everything down and am looking to reuse the stock 6 bolt pistons and just replace the rings. Also going to replace the rod bearings and the main bearings. I am on a budget, hence the reuse of the stock pistons and borrowing of tools from people, like a bore gauge and micrometer set. Anyway, I have a couple questions about these pistons.

1. The tops of the pistons were all wet with oil, so it definitely was blow-by and therefore worn rings. Is that analysis correct?
2. If it was blow-by, why do all the rings look clean and intact? Is this a case of them not seating correctly or the ring gap being wrong, or something else?
3. How do I verify these pistons can be reused? I checked through my FSM but it only mentions looking for "scratches and seizures", no piston to cylinder wall clearance measurement numbers or anything. I just really don't want to put in new rings, check ring gap, and for this to happen all over again because these pistons are actually the root of the problem.
4. Do these visually look alright to all of you? I've never even held a DSM piston before, so aside from needing a cleaning, they seem fine to me.

Another big reason I took the motor apart was the oil leak that's been making a mess for years. From what I gather, the oil was weeping from some of the front case bolts or around the front case gasket (or maybe both). I think the oil seepage was related to the blow-by/crankcase pressure. The previous owner said the motor was rebuilt, which I believe, cause a lot of the motor's gaskets and seams look aftermarket. So maybe the person that rebuilt this motor didn't do some things right, like skipped checking ring gap

Physically they don't look horrible, but you need to measure the cylinder bores and the pistons themselves to verify if you have proper ptw clearance, bore taper, cylinder ovaling etc.

If you've been running that block for "years" then I'd bet it needs a rebuild and fresh pistons/rings.

While I did reuse the pistons in my 92, I measured everything and it was all still in spec. Nowadays I won't east my time doing that with these motors as they're 20-30 years old and abused.

My advice is check all specs and rebuild if necessary. If you can't properly do this yourself, have a reputable shop, supporting vendor or freelancer so it for you. Don't skimp or you'll do a bunch of work for nothing.

Also, are you sure the wet piston is from oil? It almost looks like fuel soaking...

Theres only a few things to check. You ideally need a dial bore gauge and a micrometer. you can get by with snap gauges as well.

First thing to do is use a razor blade and scrape the carbon off the top of the cylinder where the ridge is. This will allow you to make a really good assesment of wear without any tools. If there is a large ridge you can easily feel, its worn out. If there is no ridge, a ballhone and fresh rings and it will likely seal up again.

Next step is to measure the piston to wall clearance. Measure the OD of the piston with the mic perpendicular to the pin, about a half inch from the bottom of the skirts. Record the dia of each piston. Now use the snap gauge or dial bore gauge and measure the diameter of the bore in many directions and places. Your wanting to see how much taper and out of round there is. Ideally it is none, but up to about .002" is pretty good. Record these measurments. Now you can subtract the piston diameter from the bore diameter and get the piston to wall clearance. Ideally it's 0.0015-0.002"

If that's all good you can move on to inspecting the pistons. There is a spec for the ring side clearance, you'll need to look that up in the FSM, as I don't know it off the top of my head. You'll also want to check that the grooves are clean of carbon, free of signs of wear, and galling. Cleaning the carbon out is a cocksucker, a chunk of broken ring with the end ground square usually works ok, but be careful to not scratch the sides of the grooves. The bottom doesn't matter, just needs to be free of carbon.

Last thing to do is check the oil drains in the piston. These are usually coked up, and it's a 2hr procudure with a pin vice and a 1/16" drill bit to clean them out.

So here's three cases, and what measurments fall into each.

IDEAL: This is a nice low mile engine that you are freshining up and hope for it to last a long time. Should seal up well, and not use oil
PTW: 0.0015" - 0.002"
Ridge: None
Cylinder out of round/taper: 0.000" -0.001"
Cylinder conditions: Nice and smooth, no scratches goughes.
What to do: Ball hone with a 280grit ball hone, clean it well, and re-assemble with new rings/bearings.

GET YOU BY FOR A WHILE: This is one that has some wear, but it'll still run good. Might have a bit of blowby, and use a bit of oil. Should seal up decent and will still make decent power. I went 10's on one like this.
PTW: 0.002" - 0.003"
Ridge: very mild. Pistons should still come out easy.
Cylinder out of round/taper: 0.001" -0.002"
Cylinder conditions: Nice and smooth, might have some small scratches.
What to do: Ball hone with a 280grit ball hone, clean it well, and re-assemble with new rings/bearings.

BETWEEN A ROCK AND A HARD PLACE: This is one that has more wear, but it'll still run good. Might have a bit of blowby, and use a bit of oil. My first engine was worse than this, and I ran the piss out of it for a couple years.
PTW: 0.003"+
Ridge: easily felt. Seems like "you could pick block up by it" Pistons might come out hard.
Cylinder out of round/taper: 0.002"+
Cylinder conditions: Obvious wear patches, maybe some scratches, maybe rust.
What to do: Use a three prong hone, and work the top a bit, and reduce the ridge/smooth it as best as you can without increasting piston to wall clearance. New rings and bearings. Don't expect it to be a perfect engine, but it'll run and should be ok for a while. Might haze a little when cold, compression might be low.

FYI I don't think those are factory pistons.

Your compression numbers are fantastic for our elevation. I make right near 900whp on my engine and I have 125-130psi. I have had them be as low as 90psi and still make boar loads of power here in Denver.

my compression in Denver is
113
119
117
120

on a stock 1990 with a motor that has 145k on it. Runs fine

Wow! What a wealth of information! Thank you for the replies! The previous owner gave me a sheet that outlined a rebuild from the previous PREVIOUS owner. It looks it’s just an estimate though, so I was never SURE it was rebuilt, just assumed it was. But if those aren’t factory stock pistons, it’d point to it being rebuilt in the past. They do have the Mitsubishi symbol on the other side I didn't photograph, so maybe they're OE Mitsubishi piston equivalents.....?

The only reason I don’t think it’s gas on the pistons is because it doesn’t smell and it hasn’t evaporated after sitting for a couple days.

I’m going to spend some time going through the measurements and advice you all gave and report back. One question, if main journal bearings or rod bearings are within spec, can I reuse them if I put them back where they were? Or is that an engine rebuild no-no?

Those 130-140 compression results are adjusted for altitude. The test was done in Pueblo, so a little lower than Denver. I can’t find my little notebook from last year when I did the compression test, but I remember vividly using a correction factor to adjust my results.
The car has run pretty well on a small 16g, but it hiccups and oscillates at idle. It would bounce and constantly correct between 850 and 900 rpms for example. Never a steady idle.

Edit: biglady I think I picked up some DSM parts (including a home made 3” exhaust) from you a while back. You had a 4g63 rat rod truck. Or maybe that was someone else in Commerce City...

That was me and my truck.

My compression tester always said around 118-122 for 1Gs at our altitude and 124-128 for 2Gs. Everyone would always say, "oh, it's rebuild time," even though the engines ran perfectly. This was the cheap kind of tester with a rubber tip that you just hold in place. Same engine would often have a different result by about 5psi with a different tester. At any rate, with the altitude and the fact that you can get different results from tester to tester, I stopped paying attention to the total value and only considered the uniformity. If you have 130-130-130-118 that indicates a problem in my opinion, but I wouldn't bat an eye at 118 across the board.

I had AMS in Fort Collins rebuild my 6 bolt with original crank, rods and pistons, and upgraded to ARP mainstuds/rod bolts. Motors still going strong with 30+ psi on a 35r, not sure if your looking for a shop, but they got my vote

So a long update:

My dad found my notepad with the compression numbers and sent me a photo of it.
The last test I did on a hot motor a bit over a year ago was:
Cylinder #4 - 107
Cylinder #3 - 101
Cylinder #2 - 117
Cylinder #1 - 112

Using .86 to adjust for altitude I got:
Cylinder #4 - 124
Cylinder #3 - 117
Cylinder #2 - 136
Cylinder #1 - 130

Sorry for the misinformation in the first thread. Only two cylinders were in that 130-140 range.

So I spent the week checking the bores with a bore gauge and a micrometer set to 3.3465" (stock bore size according to my manual). Also worked with a digital caliper and telescope gauges to help verify results.

I'm away from home and my notes with the numbers atm, but my bore measurements resulted in all the cylinders being more or less within the 3.3465" range. But two of the cylinders were out of spec for taper and one was out of spec for out-of-round. About .0008 instead of the .0004".
Also looked at crank OD and it's within spec on both the rods and mains.

So from what you all explained and I've been reading about, I think I could get away with using those old pistons (all of which are in spec) with new rings. Of course then I'd probably continue to have blow-by and/or low-side-of-normal compression numbers.

This all started with me just trying to plug some oil leaks around the front case and pan and I ended up just pulling the whole motor. But I get the feeling the oil seepage is related to the blow-by and I don't want it leaking anymore.

So I've decided to bite the bullet and get the motor bored out to the next size (.020"). Being still on a budget, I got replacement OE style pistons -they're NPR brand- and I'll reuse the old rods.
Also got a set of King bearings (the softer OE style, not the XP series).

I was planning to go with Greeley Automotive for the machining. They look to have good reviews on the google.

Also plan to eliminate the balance shafts. I managed to get the two back bearings out without destroying them, so I flipped them 180° and put them back in (left the front BS bearing alone). Getting a stub shaft and freeze plug from JNZ.