NPR rings on factory pistons and related woes

[riceman]

Hi all - 

 

It's the following issues/concerns that brought me here.

 

first, a little background (in case i'm missing something, which isn't unlikely considering that this is my first venture into a subaru motor): 

 

bought my 1999 forester with 90K miles on it, running pretty decent. by 100K miles, we had a problem. Previous owner had never adjusted the valves by the looks of things. cylinder #4  was dead with next to no compression (50psi vs. 195 in other cylinders). both exhaust valves stuck partially open (not enough to contact the pistons). lots of carbon build up.

 

pulled the heads. pulled the valves. had the two in question cleaned up along with the seats since the carbon build up had caused a bunch of pitting. nothing too severe, but enough that i wanted the heads looked at. i went to the most reputable shop in town that I could find and he said he just lightly touched the valves and seats, not removing much material. a general clean/inspection on the heads and a clean bill of health was given. new gaskets, bolts, etc and back together she went.

 

 - I don't recommend ITM gaskets. both head gaskets blew within 500 miles and they blamed me. basically told me to go pound sand. local parts shop warrantied the gaskets and the bolts though! put felpro stuff in and no issues - 

 

motor ran strong after fixing the valve issue. MASSIVE oil consumption though... 15-20 quarts between oil changes.

 

so now i'm at 115K miles and the motor is out and apart again. best my buddies and I could figure is that while i was driving with a dead cylinder I probably developed deposits on the rings that gummed things up. pulled #4 out since that is my problem child and sure enough... oil ring was crammed completely full of gummed up/burnt oil. though there was a bonus feature this time. valve with a hole in it. one of the valves that had been machined...

 

bought two new valves. cleaned the heads. cleaned the valves in both heads and then lapped all of the valves to make sure everything was going to seal up okay. they are done and set to the side for now.

 

 

 

now, on to my question: 

 

how the heck do i orient the pistons and rings when reassembling the motor?!

 

OEM oil rings just have a tang on the upper ring that goes into the little notch here (the lower ring is free floating):

 

 

NPR rings have a tang on the upper and lower rings and no instructions i can find tell you where to place that tang. Best I can tell, it's supposed to point upwards, towards the oil ring spacer. the problem with that is that i can't put it where it's supposed to go according to this:

 

 

 

because the tang will hit the wire that bridges the gap on the spacer.

 

 

 

i did some additional googling and found these instructions straight from NPR:

 

 

i again can't follow these instructions because the lower tang will his the wire and the gap will be on the lower half of the piston as well as on the skirt a little (i'm told ring gaps on the skirts are a big no-no). But these instructions do match the ones on the box the rings came in at least

 

 

so.... i have two sets of written instructions, neither of which can be followed. I am assuming that the oil ring gaps need to be on the top half of the piston since there's a likelyhood of oil resting in the bottom portion of the cylinder, so i can't just put the lower ring 180° out from the top ring.

the other question i have in regards to the rings is: 

 

the guide says to position for the top two rings 180° out and place them directly over the rist (sp?) pins. yet i've read elsewhere not to place ANY of the gaps over the rist pins or the skirts. so which is it?

Edited July 3, 2015 by riceman

[riceman]

 

In regards to the pistons:

 

when removing them, i made sure to number and place an arrow on each one point towards the front of the motor. i cleaned up the #2 piston a little and found the telltale dot that points towards the front of the motor. i put a little scribe mark on the inside of the piston so that i could make sure that i got them back into the correct cylinder, but figured i could use that dot to line them back up correctly as far as which side points forward. whoops.

 

the first pic shows the #1 and #2 pistons side by side with their dots pointing the same way. the second pic shows the back of the pistons as they will be oriented in the motor if the dots point towards the front (the side towards the bottom of the picture is the bottom of the piston).

 

 

 

in the second pic you can see that, on the number 2 piston the side with the extra "meat" will point up when installed in the motor. the #1 piston will have the extra "meat" pointing down. also, the recess that accommodates the top oil ring is on the upper half of the piston on #2, but on the bottom half on #1 if oriented in this manner. yet i've been told that the oil ring gaps must all be on the top half of the piston...

 

So which way is the correct way?!

Edited July 3, 2015 by riceman

[Fairtax4me]

1 & 2 go on opposite sides, so number 2 will face the opposite way.

 

 

As far as the rings. There should be a gap somewhere in the center wire right? Can the tang on the lower control ring fit into the gap in the wire?

 

FSM recommends the Compression ring gaps be 180° apart

With the control ring expander gap at 90° from either compression ring gap.

 

Look here for a service manual for your car. http://www.ultimatesubaru.org/forum/topic/152944-factory-service-manuals/?do=findComment&comment=1292882

99-05 use the same 2.5 engine.

Edited July 3, 2015 by Fairtax4me

[riceman]

1 & 2 go on opposite sides, so number 2 will face the opposite way.

 

 

As far as the rings. There should be a gap somewhere in the center wire right? Can the tang on the lower control ring fit into the gap in the wire?

 

FSM recommends the Compression ring gaps be 180° apart

With the control ring expander gap at 90° from either compression ring gap.

 

Look here for a service manual for your car. http://www.ultimatesubaru.org/forum/topic/152944-factory-service-manuals/?do=findComment&comment=1292882

99-05 use the same 2.5 engine.

 

if 1 &2 face opposite, the bonus meat that you can see on one side of the piston will be towards the intake on one side and the exhaust on the other. This isn't a problem? 1 & 3 will also have the top oil ring gap on the bottom half of the piston. is that okay? i was under the impression that both oil ring gaps needed to be on the top half of the piston.

 

yes, there is a break in the wire. but the wire encompasses about 180°.

 

Thanks for the link! i'll take a look and see what i can find.

[Fairtax4me]

Cylinder layout is:

 

Bell-housing

3 - 4

1 - 2

Front of engine

 

So if number 2 is placed opposite 1, all dots pointing forward, is the orientation of either of those pistons different than the 3-4 pair?

[riceman]

Cylinder layout is:

 

Bell-housing

3 - 4

1 - 2

Front of engine

 

So if number 2 is placed opposite 1, all dots pointing forward, is the orientation of either of those pistons different than the 3-4 pair?

 

pistons 2 and 4 are oriented the same way. pistons 1 & 3 also the same as eachother, but are mirror of 2 & 4.

 

so 1 & 2 and 3 & 4, when viewed as a set, will be the same as each other. my only reason for questioning the orientation was that i didn't understand what the extra meat on the piston was for. and, whatever it is for, wouldn't cylinder 1 have the same needs as cylinder 2? so shouldn't the extra meat be on the same side of the cylinder? (both to the top or both to the bottom)? in the picture i have above where you can see the back side of the piston, the two pistons are oriented as they would be if installed with the dot on them both facing the front of the motor. you can see that the extra material will be on the exhaust side of the cylinder on piston 1 and the intake side on piston 2

 

 

 

it may be more clear if i make a little video of what i'm getting at. i will do this later and post for clarity.

Edited July 3, 2015 by riceman

[riceman]

in the meantime, i looked up the FSM that was linked. Found the following page in regards to the rings:



so, on piston 2 for instance, the tang on the upper ring will have to go at position C. The spacer gap can go in position D. but the bottom ring cannot go in position E like I assumed it needed to. It can, however, go in position F and clear the wire ring in the spacer. this will put the upper and lower ring gap only 60° apart, but they will physically fit onto the piston. Is 60° of separation sufficient?

Edited July 3, 2015 by riceman

[Fairtax4me]

I see what you're asking about now... The wider area on the inside where the vent holes are for the oil ring.

 

I would guess they cast it that way for balance when the piston is moving.

The force on the piston isn't equal when the piston is on the compression and power strokes.

On the transition from compression to power the piston also rocks from one side of the cylinder to the other.

The extra weight on one side can help to counter that motion.

Of course, those engines piston slap bad anyway, so maybe the extra weight is the problem!

[riceman]

I see what you're asking about now... The wider area on the inside where the vent holes are for the oil ring.

 

yes! that's the spot!

 

at first i figured that the extra material was likely to serve as a bit of a heatsink if it were on the same half of the cylinder as the exhaust valves. but then i realized that on the #2 piston (the only one i'm 100% certain about the orientation of), the extra material was on the intake side of things.

 

so then i thought about balance, but my reservation against that was that the #1 and #3 would have the extra material on the lower half, near the exhaust valves, so they'd be balancing opposite of the driver's side of the motor. i would assume that all 4 pistons would need the extra weight on the same side if it were for balance.

 

unless, of course, the rist pins weren't 100% dead center of the piston and the location of the extra weight wasn't needed in a particular spot in relation to the rest of the motor but was instead balancing each piston individually. if that were the case then orientation of the extra mass wouldn't matter and i could just go by the alignment marks on the pistons. I'll grab my calipers and check that theory right now!

 

 

[riceman]

 

unless, of course, the rist pins weren't 100% dead center of the piston and the location of the extra weight wasn't needed in a particular spot in relation to the rest of the motor but was instead balancing each piston individually. if that were the case then orientation of the extra mass wouldn't matter and i could just go by the alignment marks on the pistons. I'll grab my calipers and check that theory right now!

 

 

 

well, so much for that idea. best i can figure the pin is dead center. i've only for crappy harbor freight calipers to use, but the pin measured within a few thousandths of dead center.

[riceman]

anyone on here have a brand new set of pistons that they could post pics of?

also, any response to my question of whether or not having the gap of the the top and bottom oil ring only 60° apart is okay?

[Fairtax4me]

Step 6 on the instructions. Position lower rail gap in position E or F.

 

At position F they will be 60° apart.

 

 

 

Yes, wrist pins are center on the EJ blocks.

The newer FA/FB blocks use an offset wrist pin IIRC.

 

If the weight is on top of the piston on one side, it should be on the bottom on the other side if its for balance.

[riceman]

Step 6 on the instructions. Position lower rail gap in position E or F.

 

At position F they will be 60° apart.

 

 

 

Yes, wrist pins are center on the EJ blocks.

The newer FA/FB blocks use an offset wrist pin IIRC.

 

If the weight is on top of the piston on one side, it should be on the bottom on the other side if its for balance.

 

I assume position F in step 6 was valid only if position D was used in step 4. the initial instructions i posted only show C for the top ring, D for the spacer and E for the bottom ring. the FSM adds the other positions that would be needed for pistons 1 & 3 because the top ring will HAVE to go in position D due to the relief for the tang and all of the positions needed to be 180° from pistons 2 and 4. or so I assumed. if this is not the case and indeed position F can be used regardless of where the top ring gap is, then my ring issues are solved!

 

as far as the piston being weighted to one side for balance... what are they balancing, exactly? the crank and rods should only be relying on all 4 pistons to weigh the same for balance. if the pistons are balancing themselves only, i don't understand why they should be weighted up on one cylinder and down on another. assuming the rotating assembly to be self-balanced, the pistons should function like individual entities, with the weight and orientation of one not affecting the other. this would mean that all 4 should be oriented the same way. at least that's my non-subaru experience. in most V engines that i've worked on, the same face of the piston goes towards the exhaust end of things, regardless of position in the motor. inline motors, best i can remember, have pistons that are fairly balanced across the rist pin. granted, none of these motors have their pistons oriented horizontally so all of my previous knowledge is potentially useless. I just don't understand what could be going on in the motor that would make it so that it needs one side of the motor to be counterweighted in one direction but the other side to be weighted in the opposite way.

 

not arguing the point. just trying to understand the "why" of it all.