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In the eyes of the engineers, and especially the bean counters, there are no “unneeded” parts on these vehicles. I’m curious the reasoning behind designing something the way they did.
On many GM trucks, the transmission crossmember is held in place by bolts twice as long as needed, and include spacers to make up the difference. While I have little doubt the spacers cost next to nothing, let’s assume they cost a penny each. Multiple that 4 cents by a few million trucks, it adds up pretty quick.
In my 35 years of building transmissions, I have noticed similar spacers on the transmission mount of TH400s. On some cores I have taken apart, the bolts are much longer than would be needed, with spacers roughly the same size as thimbles of thread for your moms sewing machine.
I have taken enough of the cars/trucks apart to know the spacers aren’t some backyard hack modification, but done from the factory. Just curious the reasoning.
[img alt="This is an example of the spacer on the transmission crossmember. The bolt went up thru the frame, thru the crossmember, the spacer went on, followed by the nut.
"]https://cimg0.ibsrv.net/gimg/classicoldsmobile.com-vbulletin/2000x1504/img_9802_41f6a283a0e059b62e96029e340796827748d3d3. jpeg[/img] This is an example of the spacer on the transmission crossmember. The bolt went up thru the frame, thru the crossmember, the spacer went on, followed by the nut.
Probably the use of spacers saved money. Even though the spacers cost a minimal amount by using them they were able to save money in the long run by not designing an additional transmission mount.
Sometimes I arrive at the conclusion(s) manufacturer's (or those who provide contract work for parts to manufacturers) reach compromises based on milling/machining/forging, etc. - e.g. more expense to create new products when current foundries, mills, forges etc. will save money i.e. no sense creating new 'cold forging' equipment when what is already 'in-place' will work. Just a thought.
It's a mechanical engineering thing. Both crossmember and frame were thin stamped metal. By using a spacer, it gets more length of bolt involved and allows a greater, and more elastic, clamping force than the bolt could do just with the two frame pieces held together as the whole length of the bolt could elastically yield a bit more as it has the length of the spacer to play with. It keeps it from coming loose, in other words.
the spacer is used as an alignment guide. because frames are stamped and welded, and the crossmember is also a stamped.. the driveshaft angle , left to right, can be all over the place.
once the engine trans and drive shaft are in place, a measurement(with a go, no go tool) is taken off the spacer to somewhere on the drive shaft/tans mount/ then adjusted to be within spec…that’s why the crossmember is slotted.
if the driveshaft left to right angle is not within spec, the car can have bad harmonics though the whole car
I have taken transmissions out of a lot of these trucks over the years, the spacers have always been above the crossmember. It’s not an alignment or driveline angle issue, And I find it extremely difficult to believe that GM didn’t have the correct bolt/nut length in its parts inventory to use a shorter bolt without the spacer!!
Im sure there has to be a reason for using the spacer when a bolt half the length would be sufficient. Same thing with the transmission mounts on 400s.
The bolt stretch idea kinda makes sense, at least the most sense out of any of the theories I have heard. I have no doubt that somewhere there is a specified torque for the crossmember bolts, but I would imagine it’s well below what a 12mm bolt can handle.
Just one of the many things in life that makes you wonder why things are done the way they are.
It's a mechanical engineering thing. Both crossmember and frame were thin stamped metal. By using a spacer, it gets more length of bolt involved and allows a greater, and more elastic, clamping force than the bolt could do just with the two frame pieces held together as the whole length of the bolt could elastically yield a bit more as it has the length of the spacer to play with. It keeps it from coming loose, in other words.
Originally Posted by matt69olds
I have taken transmissions out of a lot of these trucks over the years, the spacers have always been above the crossmember. It’s not an alignment or driveline angle issue, And I find it extremely difficult to believe that GM didn’t have the correct bolt/nut length in its parts inventory to use a shorter bolt without the spacer!!
Im sure there has to be a reason for using the spacer when a bolt half the length would be sufficient. Same thing with the transmission mounts on 400s.
The bolt stretch idea kinda makes sense, at least the most sense out of any of the theories I have heard. I have no doubt that somewhere there is a specified torque for the crossmember bolts, but I would imagine it’s well below what a 12mm bolt can handle.
Just one of the many things in life that makes you wonder why things are done the way they are.
I gave you the answer. I'm not sure why people here think I talk out of my ***. Anything I say is backed up by research.
Factory Olds crossmembers do not have slots where the bolts attach to the frame. The bolts to the trans mount go into rubber. The ones at the frame do not. This is not a factory 1968 Cutlass crossmember, despite what the ad may say. This may be a revelation, but not everything you see on the web is correct.
This is not for Oldsmobile specifically, but, if anyone cares, there's three kinds of holes bolting car parts together: the datum, the subdatum, and other holes that are not datums.
The datum is a hole that is just barely wider than the bolt. That locates on the plane of the mounting surface, call that X and Y. The clamping force is part of Z. The only thing left is rotation around the bolt. That is accomplished by the subdatum, which is indeed a slot, but it is not for adjustability. Rather, the slot faces the datum, and allows for manufacturing tolerance stackup between the bolts and the two sets of holes. That takes care of the rotation degree of freedom. The subdatum also clamps, and all the other holes are slightly oversize as they are not datums, and are just there for clamping.
Pardon my bad MS Paint work, but you can see the extra room in the hole pattern is for manufacturing tolerances, not adjustability. Using slots as adjustable features and holding it with clamp force only is bad engineering; and the only way you use that is for a one-off machine that is adjusted, then you drill and pin it to keep it from walking.
This is not for Oldsmobile specifically, but, if anyone cares, there's three kinds of holes bolting car parts together: the datum, the subdatum, and other holes that are not datums.
The datum is a hole that is just barely wider than the bolt. That locates on the plane of the mounting surface, call that X and Y. The clamping force is part of Z. The only thing left is rotation around the bolt. That is accomplished by the subdatum, which is indeed a slot, but it is not for adjustability. Rather, the slot faces the datum, and allows for manufacturing tolerance stackup between the bolts and the two sets of holes. That takes care of the rotation degree of freedom. The subdatum also clamps, and all the other holes are slightly oversize as they are not datums, and are just there for clamping.
Pardon my bad MS Paint work, but you can see the extra room in the hole pattern is for manufacturing tolerances, not adjustability. Using slots as adjustable features and holding it with clamp force only is bad engineering; and the only way you use that is for a one-off machine that is adjusted, then you drill and pin it to keep it from walking.
Good summary. I'll add that often slots are used instead of oval holes to provide location in only one dimension since they are easier to machine. One example of this is the slot adjacent to the no. 3 intake port on factory Olds intake manifolds. Proper location of the intake on the block is complicated due to the fit-up of multiple parts, each with individual machining tolerances. The "V" shape of the intake provides the side-to-side location. This slot is tight tolerance on the bolt for fore/aft location of the intake but doesn't constrain it side-to-side. The other intake bolt holes are oversize and only provide clamping.
Factory Olds crossmembers do not have slots where the bolts attach to the frame. The bolts to the trans mount go into rubber. The ones at the frame do not. This is not a factory 1968 Cutlass crossmember, despite what the ad may say. This may be a revelation, but not everything you see on the web is correct.
you need glasses . They are slotted on the one end. That pic I posted is a convertible cross member, which are wider and have open slots on the end.
here’s my stash. As you can see, the are all slotted and the two convertible ones are wider and open slots.
I was referring to the one in the photo you posted with the open slots. That is not a factory Olds crossmember, despite what the ad may say. If you are going to use photos to back up your statements, try to use correct ones. As for the slot, there is a production tolerance on the frame rail spacing. As Koda correctly pointed out above, the round holes in one end locate the crossmember and the slots in the other end allow for manufacturing tolerances.
This is not for Oldsmobile specifically, but, if anyone cares, there's three kinds of holes bolting car parts together: the datum, the subdatum, and other holes that are not datums.
The datum is a hole that is just barely wider than the bolt. That locates on the plane of the mounting surface, call that X and Y. The clamping force is part of Z. The only thing left is rotation around the bolt. That is accomplished by the subdatum, which is indeed a slot, but it is not for adjustability. Rather, the slot faces the datum, and allows for manufacturing tolerance stackup between the bolts and the two sets of holes. That takes care of the rotation degree of freedom. The subdatum also clamps, and all the other holes are slightly oversize as they are not datums, and are just there for clamping.
Pardon my bad MS Paint work, but you can see the extra room in the hole pattern is for manufacturing tolerances, not adjustability. Using slots as adjustable features and holding it with clamp force only is bad engineering; and the only way you use that is for a one-off machine that is adjusted, then you drill and pin it to keep it from walking.
why did they slot the crossmember in multiple places if it’s bad engineering?
Last edited by CANADIANOLDS; Apr 12, 2025 at 10:13 AM.
I was referring to the one in the photo you posted with the open slots. That is not a factory Olds crossmember, despite what the ad may say. If you are going to use photos to back up your statements, try to use correct ones. As for the slot, there is a production tolerance on the frame rail spacing. As Koda correctly pointed out above, the round holes in one end locate the crossmember and the slots in the other end allow for manufacturing tolerances.
Despite your ignorance on what a convertible cross member looks like for an Olds..that pic is an olds convertible cross member. here’s another one I have . They are different than non convertibles.
you also said they weren’t slotted.. another ignorant statement.
Proper location of the intake on the block is complicated due to the fit-up of multiple parts, each with individual machining tolerances.
Are you sure it doesn't have anything to do with that crazy @$$ intake manifold gasket? It's been 35 years since the last Oldsmobile engine with a turkey tray has been manufactured and I still can't get that darn thing to work!
It's a mechanical engineering thing. Both crossmember and frame were thin stamped metal. By using a spacer, it gets more length of bolt involved and allows a greater, and more elastic, clamping force than the bolt could do just with the two frame pieces held together as the whole length of the bolt could elastically yield a bit more as it has the length of the spacer to play with. It keeps it from coming loose, in other words.
While I understand your premise in that added length in the bolt will provide a proportional percentage of bolt stretch before exceeding the elastic limit of the bolt, I struggle to find this as rationale for the use of a spacer in this application. Loads applied to these bolts are essentially limited to tensile under hard acceleration as the engine and transmission attempt to rotate. Furthermore the rubber in the transmission mount is absorbing a large percentage if not all of this torsional loading.
In my 15 years as a aircraft design engineer I've never seen or heard of a structural design best practice that called for the use of a spacer for the reason you described. That doesn't mean it has never been the case, I am just not aware of one. Especially one as benign as a transmission cross member installation.
tc
Last edited by 4+4+2=10; Apr 17, 2025 at 03:35 PM.
