[maxmitchell]

Nearly all inline-6 cylinder engines employ 7 main bearings. That is, there is a main bearing between each rod bearing, and one at each end of the crankshaft.

Occasionally an inline-6 engine will be designed with only 4 main bearings. That is, there is no main bearing between cylinders 1&2, nor between 3&4, nor between 5&6.

I think this was common in some of the older American I6 engines, and maybe some aircraft engines too.

On the page for "Straight-six engine", Wikipedia indicates that BMW has also done this at some point:

"Many of the more sporty high-performance engines use the four bearing design because of better torsional stiffness (e.g., BMW small straight 6, Ford's Zephyr 6)."

No further details on the BMW engine(s) is given.

Does anyone know which BMW engine(s) used only 4 main bearings as opposed to the normal 7?

I find the concept of the crankshaft being potentially stiffer in torsion very interesting, and would like to research this further.

Thanks!

[NickyC]

Considering BMWs history with rod bearings, I'd be surprised if they put any in some of their motors.

[GuidoK]

I wouldnt know any bmw crank to have 4 main bearings.
I can only imagine a bmw called a "small straigth 6) if it was a m20 or m50, as back in those days (80's) they also had the m30 which was their i6 block for 2500cc+.
The only thing I know is that the place of the thrustbearing can vary. sometimes in the middle, and sometimes more on the flywheel side.

In theory a 4 bearing crank can be stiffer as it can be shorter. But the lenght of the crank also depends on the width of the cylinders and spacing inbetween. But if you have the space for the bearings, more bearings will always be stronger.

^usually bmw engines have very very strong rodbearings. Look at m54 engines. they've been using the same bearings since the m10 and have no issues whatsoever. a bulletproof design that has lasted 35 years or so. It's only a problem in a number of S engines but thats just a very small portion of the engines they make.

[Chihuahua]

Not sure where wikipedia is getting its info from. M20 has seven crank bearings, M30 has seven crank bearings, that takes you back into the 1960s. I've never in my life seen a BMW I6 with only four crank bearings.

[GuidoK]

^well they do talk about a ford zephyr... thats a 60's? 50's? car?
I'll bet any of the bmw crankshafts since the 70's to be stronger than that one
Anyhow all stuff from way before wikipedia started and most of us were born

[MalibuBimmer]

Originally Posted by GuidoK

^well they do talk about a ford zephyr... thats a 60's? 50's? car?
I'll bet any of the bmw crankshafts since the 70's to be stronger than that one
Anyhow all stuff from way before wikipedia started and most of us were born

Speak for yourself, youngster.

[Dackelone]

BMW's N55(and N54) engines use seven main bearings not four.

http://www.realoem.com/bmw/enUS/show...diagId=11_4446

http://www.realoem.com/bmw/enUS/show...diagId=11_4458

[Maynard]

I think Wiki is way wrong (I know - shocking, right?). I can't think of any performance 6's that skimped this way - probably some that were developed up from passenger/farm models, or some of the older British motors. I think that most of the 4-cylinders used only 3 (at lest the older MG's and Triumphs?). Perhaps way back in the 50's, but I DK my BMW history well enough to know if they had 6's back then.

[01gpr20]

Maynard, you are correct ... thats just it.

[MarcoZandrini]

Checked realoem. The B58 engine has 7 main bearings.