Transmission offset between framerails

[eric8]

I was reading Holley's instructions for their LS swap crossmember kit for 73-up C10's and it mentions finding the centerline between the framerails, and then offsetting the transmission output centerline 1/16" to the passenger side of the vehicle for the final mounting location (see last page). http://documents.holley.com/199r10760rev1.pdf

I have a '72, but have been doing some research on this and have seen that on many GM vehicles the engine centerline is offset toward the passenger side. Does anyone know if this is the case with the 67-72's? Just want to be sure everything is squared up once I do my swap and was curious if anyone has bothered to do this offsetting for their swap.

[dayj1]

Originally Posted by eric8

Just want to be sure everything is squared up once I do my swap and was curious if anyone has bothered to do this offsetting for their swap.

I centered the transmission output shaft on my '72 swap and the driveshaft is centered in the hole through the crossmember. I'm amazed that Holley put that in their instructions. 1/16" in engine placement isn't going to make any difference one way or another.

[BR3W CITY]

Originally Posted by dayj1

I centered the transmission output shaft on my '72 swap and the driveshaft is centered in the hole through the crossmember. I'm amazed that Holley put that in their instructions. 1/16" in engine placement isn't going to make any difference one way or another.

Well, there might be room for debate here. Ujoints don't like being head-on, they need a certain amount of flexion to not bind the yolks. Offsetting the motor that tiny bet helps prevent the driveshaft from ever going dead-nuts straight to the output shaft and rear end. Normally, the amount of vertical drop in the driveshaft angle does this just fine, but on bagged trucks with 1 peice ujoints, its possible to get a drive angle so flat that you can actually bind the ujoints up.

I'm guessing this is something thats mostly prevalent on trucks with rear ends which are centered. If you look carefully on late model RWD vehicles, the rear end is actually offset just slightly to one side or the other (in trucks, this is sometimes to ease the angle of departure from 4wd transfer cases).

My $1 on the topic. Holley might have some other crazy reason for doing it, but thats the only logical one I can deduct.

[eric8]

Yes you make good points with the driveshaft angles Brew. I've been reading into this today and seems like many GM passenger cars from this era had a small offset to the passenger side too. Now I'm assuming this offset is built into the motor mount locations as well. So essentially, if this is true, if one centered the transmission output between the frame rails with the front motor mounts having a small offset to the right, the engine would be sitting at an (exaggerated) diagonal in the frame rail. Now imagine putting a laser pointer on the centerline of the engine and facing it rearward on something like a LWB truck. I assume the small 1/16" out of square condition up front would put the laser point waaaaay off out back at the differential, no?

Maybe I'm overthinking it.

[crazy longhorn]

I read the document....my guess is that their eng mount kit offsets just a tick to the pass side, & they have you do the same with the trans mounting? If in doubt, call Holley tech....I would I do agree, that in a "perfect" world the lines should be parallel so as the engine/trans are not cocked in the frame. The engine/trans can be off center....or the rear diff can be off center (and it is)! I agree with you, that if you were to hang a laser on the trans, & one on the pinion....the lines need to be parallel. The offset, the u joints will handle. In the real world, many guys will ask why? Was it that close from the factory? Also a little thought.....after she runs for a while on those rubber or poly mounts, where does it all sit then....just questions to think about If you talk with Holley tech, relay back....I can always learn a new trick. Longhorn

[dayj1]

Originally Posted by BR3W CITY

Well, there might be room for debate here. Ujoints don't like being head-on, they need a certain amount of flexion to not bind the yolks. Offsetting the motor that tiny bet helps prevent the driveshaft from ever going dead-nuts straight to the output shaft and rear end. Normally, the amount of vertical drop in the driveshaft angle does this just fine, but on bagged trucks with 1 peice ujoints, its possible to get a drive angle so flat that you can actually bind the ujoints up.

I'm guessing this is something thats mostly prevalent on trucks with rear ends which are centered. If you look carefully on late model RWD vehicles, the rear end is actually offset just slightly to one side or the other (in trucks, this is sometimes to ease the angle of departure from 4wd transfer cases).

My $1 on the topic. Holley might have some other crazy reason for doing it, but thats the only logical one I can deduct.

You did say there's room for debate

I agree with what you said in theory. However, the minimum u-joint operating angle is 1 degree. The source for that is Dana: http://www2.dana.com/pdf/J3311-1-DSSP.pdf (see rule #1 on page 5)

In a SWB truck application, a driveshaft is going to be in the neighborhood of 60 inches. 1/16 inch offset at 60 inches is 0.0597 degree. That's 1/20 of 1 degree!

I stand by my original statement that 1/16 is insignificant in this case. For the record, with that same 60" driveshaft, you'd have to offset on end by just over 1 inch to get 1 degree of offset.

[crazy longhorn]

Originally Posted by dayj1

You did say there's room for debate

I agree with what you said in theory. However, the minimum u-joint operating angle is 1 degree. The source for that is Dana: http://www2.dana.com/pdf/J3311-1-DSSP.pdf (see rule #1 on page 5)

In a SWB truck application, a driveshaft is going to be in the neighborhood of 60 inches. 1/16 inch offset at 60 inches is 0.0597 degree. That's 1/20 of 1 degree!

I stand by my original statement that 1/16 is insignificant in this case. For the record, with that same 60" driveshaft, you'd have to offset on end by just over 1 inch to get 1 degree of offset.

When you say "offset", I take it that both ends (crank & tail of the trans ), are offset the same amount, so as they are not cocked in the frame? Everything I have studied/done, goes to the route of "parallel" angles(front to rear). Offset is ok, but the lines need to be parallel, from the pinion, to the trans/crank line? Offset ok, but not crooked! I will watch,listen... Longhorn

[eric8]

Yes, you are correct about the pinion yoke's centerline and transmission output's centerline needing to be parallel in both planes (when viewed from the top from front to back as well as the side from front to back). Offset and parallel is good. Offset and not parallel is bad. Maybe you're right though dayj1.....it may be very insignificant. I guess I was more curious of the "why" Holley felt it significant enough to mention that 1/16" offset. Maybe I'll call their tech line and try to pick their brains.

I'm admittedly a perfectionist to a fault. I read and read and dwell on making sure stuff is done right to the point of going crazy. But when you put it all in perspective, these trucks definitely weren't built to close tolerances.

[eric8]

Actually, thinking about the "stack up" error when you consider the frames probably weren't laser square even from the factory, coupled with getting the engine square with sliding engine mounts, coupled with getting the transmission square when mounting/drilling holes for the crossmember, is enough to warrant a Xanax prescription. Then, let's think about the fact I have an adjustable track bar for the rear end, adding even more error for the rearend being square with the driveshaft. So maybe the 1/16" is trivial.

[Willie Makeit]

the frame will flex more than 1/16" from engine to rearend mounting points.

[DLW]

The rear end pinion is about 3/4" offset to the passenger side on our 12 bolts in my 66'. As far as the trans being centered and the driveshaft being cocked to the passenger side for the rear pinion offset, perfectly fine. I to am curious why the 1/16", don't see how it matters much.

[crazy longhorn]

No fight, no issues here....just think guys? If the centerline is here....move it 3/8 ", to one side/ Now measure the complete mess? Double the number....if you measure from 1 side to the other? Centerline is 1/2 of that measure I drop in the ? , because I want you to think about it, as my Dad showed me! Longhorn