Connecting rod options?

[Cabriolet]

I have a 1990 325is with a tired engine. I will be refreshing it fully, new 272 cam, rockers, rings, bearings, etc. no boost or anything crazy.

I was given an M50NV so i have extra parts laying around.

Should I run the M50nv or M20b25 connecting rods?

[mtech325]

From what I gather, I think you may want to stick with the stock m20 "i" rods. The non vanos m50 is a bit of an odd ball as it uses 140mm connecting rods, while the rest of those motors in that s/m5x series use 135mm rods. I'm not an engine builder by any means, so I'm sure others with more experience will be able to chime in. It should be noted however that if you poke around on Ireland's or even Metric Mechanic's sites, you'll see that all of their stroker kits use 135mm rods. I've not seen any mention of 140mm or non vanos rods. Hopefully this helps.

--Joey

[Cabriolet]

the TU (vanos) motor used the 140mm, but the nv motor used the 135mm.

[whodwho]

...It should be noted however that if you poke around on Ireland's or even Metric Mechanic's sites, you'll see that all of their stroker kits use 135mm rods. I've not seen any mention of 140mm or non vanos rods. Hopefully this helps.

--Joey

You need to review their builds:

we use four custom rods that all have the “H” beam design and all use the 21mm wrist pin. The lengths are 140.5mm, 138mm, 137mm and 134.5mm long. Of these, the 138mm rod is used in the majority of our engines. You asked if we still sell our 140.5mm “H” rod? The answer is YES! We use it with the 3200 Rally piston and an 84mm crankshaft to make the MM 3000 Rally engine.

[mtech325]

Oops looks like I didn't read hard enough! Thanks for clearing everything up.

[Cabriolet]

also my typo in the title. im asking if the m20 or m50 135mm rods i should use?

[raudonis]

small and big end bearings and length are the same on these rods, but m50b25 NV rods are lighter.

[digger]

if the important dimensions are the same then run the lighter rods. you wont notice anything but certainly it is easier on the parts

[Cabriolet]

thx digger.
im kinda socked that other people havent done this.
has no one else done this?

[digger]

people use s50 rods which are 135mm so using rods from a different engine is not really new

[Cabriolet]

the M50nv are also 135. but idk if there is a differenece between the M50 135 and the S50 135 rods. i also now have a eta crank so i make just get pistons from IE and do a 2.8... "while im there"

[kickinindian]

Why don't u save money and get eta rods and m52b28 crank and not have to pay 1k plus on pistons

[digger]

Why don't u save money and get eta rods and m52b28 crank and not have to pay 1k plus on pistons

he is doing a stock rebuilt bottom end?

[nando]

M50 rods aren't that much lighter than M20 rods. just a bit.

S50 rods are definitely lighter than M50 rods.. and S52 are even lighter. M52B28 are good too.

[Cabriolet]

thx for the input.

I have pantless's old engine and if the crank is good. i could do that crank, m50nv rods and custom pistons and get that compression up.

or is the M52B28 a better choice? i'm not aware of the difference between the m52 and the eta crank.

(weight) would be the factor for me. i almost want the shorter stroke of the b27. but there is a m52b28 crank for sale in the north bay. if that is better i would pick it up.