2.5 vs. 4.0 piston rings

hi all

I bought a NOS 2001 4.0 short block which someone had cannabalized a piston from. I purchased the right sized piston (Mopar # 04798331AC) which did not come with rings (but it did come with a rod.)

Both Mopar and Sealed Power only make rings available for the 4.0 in complete sets but they do make rings for the 2.5 available by the cylinder. There is only a $26 difference in price between the Mopar single 2.5 set and the complete 4.0 set ($44.50 vs. $70.75) so I guess it's not the end of the world if I buy the whole set. I can buy the single 2.5 Sealed Power set for $22 (#WE924K.)

My question started out being: Isn't the 2.5 piston ring the same as the 4.0? I thought they were. If they are, then why does Mopar only list this for the 2.5? Both rings are 3.875 bore, top and 2nd ring width = .0586, 3rd ring width = .1577.

I really don't know why but I don't want a different brand of rings in one hole. Can anyone comment on that? Am I being pathetic? I do plan on having this engine balanced since I am putting a 4.2 crank and rods in it. Are the rings in one cylinder going to mess up the balance job?

I find that Mopar is usually quite pricey (the piston was $109) however the ring set is only half (at discount) of what Sealed Power gets for their set of 4.0 rings (kit #E925K ... over $100.) Can someone comment on that? Are they that much higher quality? Then why oh why is the single cyclinder set cheaper?:confused1

Thanks for any feedback you can give me.

Doug Dornbos

Today I spent some time in the Mopar Performance Jeep Engines book I forgot how much info is in there) and there ARE minor differences in the ring fit for the two motors:

2.5: Piston Ring Gap Clearance Oil Control Steel Rails: .015-.055
4.0: Piston Ring Gap Clearance Oil Control Steel Rails: .010-.025

2.5: Piston Ring Side Clearance Oil Control: .001-.0085
4.0: Piston Ring Side Clearance Oil Control: .001-.0095

2.5: Piston Ring Groove Diameter #1 and #2: 3.456-3.461
4.0: Piston Ring Groove Diameter #1 and #2: 3.476-3.486

2.5: Piston Ring Groove Diameter Oil Control: 3.445-3.455
4.0: Piston Ring Groove Diameter Oil Control: 3.557-3.566

The oversize pistons mentioned in the book however are quoted as the same part for both motors.

For the reason that the specs are just a little bit different, I believe I'll stay with the 4.0 rings.

Doug