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I use Mahle open bowl 4BT/non-IC 12v pistons in all my builds, but I relieve the aluminum portion of the top ring land.

I do not recommend anyone use those pistons without relieving them. Every single used set I've removed has had distorted/scuffed upper ring lands.

They are a great piston. Open bowl/low ring design makes them efficient and resilient, but I don't think Cummins knew the upper land would expand so much. All subsequent piston designs had a relieved upper land, but they also had the ring pack much higher as well.

There's a common industrial piston with the same bowl design, relieved top land, but the ring pack is high like the rest. I'm sure they would work fine for most, but I prefer the non-IC pistons for my builds. Atleast to me, the 155 degree bowls seem to make more power for a given fuel rate/timing compared to closed bowl. They also work fine with common/cheap 155 degree marine nozzles.
 

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The newer non IC pistons have a steel keystone ring landing that is 14.62mm down from the top of piston & a non issue for 99% of users. Much better than the regular 17.8 : 1 bowl, less distance to ring landing pistons.
What your talking about is the portion between the keystone ring landing & top of piston. Which if you are going to run more than 1250 degs for any length of time, you should machine a tapered relief for expansion of said portion of piston, so piston scarfing doesn't happen at the higher egt's..... but this is only for the guy(s) that are pushing past the safe limit of egt's.
99% of the guys here are not in that category & do not need to do anything but install the pistons per regular install.
 

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Probably removed 20 sets of those pistons from stock/mild engines. All of them had distorted top lands. Every single piston. Like .015" to .030" out of round. That's a mile in a cylinder bore.

They ran, they worked for a long time. It didn't cause many failures. I prefer not to run them without land modification as the manufacturer changing the design on all subsequent revisions suggests they knew something was up.

I think there's not only the component of temperature, but also the carbon that builds at the top of the bore. When I machine those pistons I cut them exactly like the later style OE pistons come. They aren't tapered. They have a straight relief with a radius.
 

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K, I'll correct you. Original non IC pistons in OP's engine are/were 17.5:1
I see...well Quickserve & my hardcopy Cummins books disagree with you about the OP's pistons being 17.5:1 CR. Here are my sources...what is your reference?



 

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I stand corrected then. That is either one odd ball piston or that piston of the OP is not the original, as the large bowl piston design are 43.2cc & 17.6:1 or lower CR...marine being the lowest at 52.3cc & 15.3:1 CR. The larger the bowl is the lower the CR is.
18.5:1 is very high for a Cummins mid range engine & would require a small bowl to raise the CR that high. I'm puzzled because the OP's pic doesn't match a 18.5:1 piston
 

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Discussion Starter #26
I would guess that the motor has been apart before by the rtv on the oil pan. How far? I have no idea. I have rebuilt gas motors over the years, but this is my first Cummins rebuild and don't a have a reference point to know what is stock or not. However, I could not find a part number / manufacture name on those pistons for the life of me, (or CPL or SN) which is why I turned to you guys. I appreciate the posts - Im learning a lot. My intention is to follow the 200hp recipe - nothing crazy. So if I order new pistons and this changes my CR - does that effect what injectors I need?(pop pressure etc, pump timing?)
 

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I would guess that the motor has been apart before by the rtv on the oil pan. How far? I have no idea. I have rebuilt gas motors over the years, but this is my first Cummins rebuild and don't a have a reference point to know what is stock or not. However, I could not find a part number / manufacture name on those pistons for the life of me, (or CPL or SN) which is why I turned to you guys. I appreciate the posts - Im learning a lot. My intention is to follow the 200hp recipe - nothing crazy. So if I order new pistons and this changes my CR - does that effect what injectors I need?(pop pressure etc, pump timing?)
If those big bowl pistons are truly 18.5:1 which I doubt very much & you go to a set of big bowl 17.5:1 pistons, the only thing you will notice, running the same inj's, same bar pressure ( nothing needs to be changed on pump/timing or inj side of things ) etc, is a slight loss of off idle torque ( so lowest bottom end range ) but will gain slightly more power in mid-upper rpm.
 

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Apparently, that is one of the odd pistons in the B engine series. 3903582 is 18.5:1, 3907163, 3919041, and several others are 17.5:1, 3917619 and some others are 17.3:1, 3903655 is 17.2:1, and 3907156 is 16.5:1. Even the marine types varied. Piston 3802160 has the dual keystone rings with plain skirts and is 17.5:1 where the 3800756 which is the same with coated skirts is 15.3:1. I believe that last one came in the very high power P pump marine engines where the higher compression version came in the VE pump marines.
 

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Discussion Starter #30
So I ended up getting the 3907163 pistons and went to put the rings (Part number 3802421) on in them and they are to large - I assume I have the wrong part number for the rings? Also, the new pistons have the cutaway under the oil ring, the old ones have the holes on the inside. Why the change in design? Is one better than the other? Thanks
 
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