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AMX Crossmember

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Sep/14/2020 at 12:28pm
If the original taper bolts on the cross member are "fighting each other" when doing any reassembly, there is a problem with the sub frame alignment. IF the sub frames are within factory tolerances, the taper bolts will go in with no issues.
Personally, only cars that have been in accidents or really twisted form drag racing have had issues with the bolts not fitting. Then it's getting out the port-a-pack and measuring equipment to straighten the car.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_P Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2020 at 9:15am
No. You're missing the point. You control location with one countersink in one direction (front to rear in this case). Two is over-constrained. One across from each other (right and left side) is correct as used in the second gen cross member.

Think about it. This is why you use one dowel pin with a round hole and one with a slot. The one with the round hole has accomplished what you need to do- locates the item. The slotted hole controls orientation (rotation of the located item) and not location. This is basic machine design.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2020 at 10:53pm
Steve, don't take offence but please explain to me why then an AMC head has 2x dowel locators and an AMC timing cover has 2x dowel locators.
I can list hundreds of pieces of mechanical equipment that use 2x dowels (tapered or straight) to  locate/align parts.
A slotted hole is designed to allow for poor manufacturing locating or to allow movement for alignment by other means.
On my 69 there is a single taper bolt for the cross member left and right sub frame and a slotted hole for the stud left and right sub frame.
If you put the bolts in either side the opposite side has to align or the sub frame is not in it's correct location. All 68's and 69's are like this and all that do not align, when measuring as per the TSM, the sub frames or whole cars are not straight. Alignment and thrust line are all off until you square the car as per the measurements in the TSM. This is front to back and side to side as per the TSM. You use the holes on the sub frames that the factory used when they mounted the cars on the assembly line fixtures for making these measurements.
I've had cars upside down, with levels and pins measuring and straightening. The engine cross member will bolt right in when all measurements are correct.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mbwicz Quote  Post ReplyReply Direct Link To This Post Posted: Sep/17/2020 at 4:59am
Trader,
Generally, if the locators can be put in during one operation, on one machine, then they will accurate enough to use round holes, like in the timing cover or head dowels. That is assuming that the machine (and process) has enough accuracy to do this operation. At least that is how I think about it.
The crossmember that I removed from my 70 had the 4 tapered seats in it, and these parts were spot welded on to the cross member. Similarly, the 'frame' rails had 2 spot welded nuts to mate to the cross member. It is asking a lot to have these parts be located accurately enough, with spot welded components to not have the tapers fighting themselves.
After starting this post, and seeing replies, I looked again at the hardware that was used to secure the crossmember. There was one 'lug bolt' on each side. One side had a 1/2" stud with lug nut on it, and the other side had a 7/16" stud with lug nut. Since my car is at the body shop, I haven't confirmed why the smaller stud was used.

Mike
1970 AMX. Holy Crap its easy to spend money on this car!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Sep/17/2020 at 7:06am
In all likely hood a smaller weld bolt/stud was use because the car is 50 years old and the original weld bolt inside the sub frame was damaged or corroded beyond use. They went with the easy option of drilling and tapping a smaller thread that would leave enough material on the bolt head in the sub frame. 
Putting in a new bolt can be done but is a lot more "challenging".
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_P Quote  Post ReplyReply Direct Link To This Post Posted: Sep/22/2020 at 12:46pm
All of the hardware to fasten the crossmember should be 1/2"

Once you tighten the "lug" nut on each side, the crossmember is located front to rear. That's it, it's located and locked in place from front to rear. The only purpose a second countersink fastener serves is to try and move the crossmember from the location established by the first fasteners. This is why it was eliminated.

And I got off topic, but "standard" dowel pin location and mating hole tolerance is +/-.0005 when using two holes. This is very tight. What is fairly typical in industry is to use a diamond shaped pin in the second hole as this controls orientation but allows for some location error without using a slotted hole.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Sep/23/2020 at 9:43pm
Now I'm off topic but;
Now we know the differences of opinions on such topics, diamond pins were patented in 1985 and not in wide spread use until a decade later.
I am old enough to never have worked on a piece of equipment with a diamond pin. Have heard of them, but never even seen one.
So moving back a few decades, there were dowel pins, round, solid or hollow, straight or tapered, threaded or not, head or not and not much else. Tolerances were likely looser.
Wheel nuts or bolts of the day are similar locating dowel. If the taper of a wheel nut does not match a rim, you have the wrong bolt circle, wrong center clearance or a damaged rim. You either get a new rim, fix the old one, but never just bolt it on.
Engine location off a degree is not that critical. But the old taper bolts are a good indication of how straight the sub frame is.
  
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