[77HANDMEDOWN]

I have worked for BMW since 99.

In only one engine I can remember they ran a rod that the cap was offset. Looks like the beam stuck out at 1 o'clock from the journal.

Is that just called offset rods? What is the purpose for these? Is it for smoothness or strength? How would this work at super long strokes where side loading is an issue like our Pontiacs with big cranks. Are they a joke at huge power levels? Seems like a great idea but never heard of it any where else except for some old Austins or Mgs I believe.

It may be a weird question to ask here but I know some of you guys really know your stuff and have been around a few more years and a few more cars than me. So school me or point me a direction if you know a place with more info. Thanks.

[Schurkey]

The ONLY reason I can think of for those goofy rods is that the cylinder bore is too small for the rod to pass through if it's machined "conventionally".

Spinning the rod-cap so it attaches at an angle reduces rod width, and allows it to pass through a bore that would otherwise be too small.

In short, it's poor engineering where one part (the rod) is deliberately screwed-up to accommodate the fault of another part that's deliberately screwed-up (the too-small bore size of the block) I see this in ancient farm-machinery from before the time when ring sealing or piston cooling was understood--lots of stroke (to maintain displacement), and microscopic bores (to promote heat transfer away from the piston).

I suppose hateful government involvement--like taxation based on bore size--artificially promotes long-stroke, small-bore engines. I suppose that would be the reason that "old Austins or MGs" would have that sort of connecting rod--the English government's tax laws made no engineering sense.

[54nomore]

When I hear the term "offset rod", I think it refers to the beam not being centered front to rear over the big end of the rod like when the bore center is not exactly inline with the crank journal. I agree with Schurkey on the purpose of the rod design you are describing and I've seen John Deere rods for example, where the parting line is nearly 45 degrees to the beam because the big end is so beefy it's the only way it would ever fit through the bore.

[gtofreek]

Cummins diesels also have offset rods. So do several other diesel engines. In the case of the MG's and Austins, it may have partially been done to get the rod in the cylinder, since they have a 2.9something inch bore. But, I believe the main reason this is done in diesels is to concentrate the heaviest load on the center of the rod bearing when the cylinder reaches max cylinder pressure, which is somewhere around 14° ATDC. On a conventional rod, the heaviest load is applied off center of the bearing. The shorter the rod/stroke ratio of the engine, the further off center it occurs. Offsetting the rod, fixes this.

[HWYSTR455]

When I hear offset rods I think of the beam being offset from the centerline of the big end. When I see a cap that's offset on the parting line, I think offset parting line. But I'm not a machinist, and have no formal automotive training, so grain of salt.

.

[77HANDMEDOWN]

Originally Posted by gtofreek

Cummins diesels also have offset rods. So do several other diesel engines. In the case of the MG's and Austins, it may have partially been done to get the rod in the cylinder, since they have a 2.9something inch bore. But, I believe the main reason this is done in diesels is to concentrate the heaviest load on the center of the rod bearing when the cylinder reaches max cylinder pressure, which is somewhere around 14° ATDC. On a conventional rod, the heaviest load is applied off center of the bearing. The shorter the rod/stroke ratio of the engine, the further off center it occurs. Offsetting the rod, fixes this.

your post kinda reminds me of what I was told at the school for that engine years ago. So it is more for the combustion stroke and not as much of an fix or perk for when the piston is in the bottom of the bore?

[hurryinhoosier62]

Originally Posted by gtofreek

Cummins diesels also have offset rods. So do several other diesel engines. In the case of the MG's and Austins, it may have partially been done to get the rod in the cylinder, since they have a 2.9something inch bore. But, I believe the main reason this is done in diesels is to concentrate the heaviest load on the center of the rod bearing when the cylinder reaches max cylinder pressure, which is somewhere around 14° ATDC. On a conventional rod, the heaviest load is applied off center of the bearing. The shorter the rod/stroke ratio of the engine, the further off center it occurs. Offsetting the rod, fixes this.

Dittos, Paul. Cummins as well as CAT and some Detroits use offset rods precisely for the reasons you state.