AMX Crossmember
Printed From: TheAMCForum.com
Category: The Garage
Forum Name: Body/Interior
Forum Description: Paint, glass, interior, rust, sheetmetal goes here.
URL: https://theamcforum.com/forum/forum_posts.asp?TID=107468
Printed Date: Apr/24/2024 at 4:22am
Software Version: Web Wiz Forums 12.03 - http://www.webwizforums.com
Topic: AMX Crossmember
Posted By: mbwicz
Subject: AMX Crossmember
Date Posted: Sep/10/2020 at 9:52am
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My 70 AMX had engine swaps in its history, and the motor mount area of the crossmember was torched out to fit the other engine(s). I acquired another crossmember from someone I trust. I believe that the one that was damaged was original to the car, the bolts holding it to the chassis were a bear to remove. 4 bolts with tapered seats and fine threads, like a European car's lug bolts. Yesterday I got out the new crossmember to sandblast it, and notice that the 'replacement' piece appears to have had only one tapered seat bolt per side, and there is a witness that a flat washer was used on the second hole. My gut feel is that the tapered bolt was used to positively locate the crossmember to the chassis. Is there any reason to not trust one tapered bolt and one 'standard' bolt per side on this replacement crossmember? In the pic, the original one is on the top, and the pen is pointing to the lack of a tapered seat on the replacement. Thanks for any opinions. Mike  ------------- 1970 AMX, one step forward, one step back. Both steps cost time and money. |
Replies:
Posted By: 1948kaiser
Date Posted: Sep/10/2020 at 12:03pm
| my 72 gremlin with the factory v8 uses one bolt per slide and the one stud and a wheel nut per side. the stud gets put in first using anti-seize. takes two to hold the cross member up. then start the lug nut onto the stud and hand tighten to hold it in place. then the bolts using anti-seize should start by hand NO WRENCH and go in easily. that way they will not be cross threaded. the lug nuts line up the cross member into the correct position. |
Posted By: Heavy 488
Date Posted: Sep/10/2020 at 12:10pm
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If there was a problem with 1 taper per side i believe it would have been stumbled across many moons ago. |
Posted By: AMXFSTBK390
Date Posted: Sep/10/2020 at 1:45pm
I recently rebuilt the 390 in my 68 AMX. The engine cross member is mounted exactly as you described. ------------- Questions are powerful tools...what's in your toolbox? |
1948kaiser wrote:Posted By: mark09
Date Posted: Sep/10/2020 at 5:30pm
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The pen points to where a weld-nut is missing/knocked off.
Should have 4 tapered "blocks" to locate. 2 get stud-bolts 2 get 1/2" Lug nuts. 1 nut and 1stud-bolt per side
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Posted By: Mopar_guy
Date Posted: Sep/10/2020 at 6:11pm
That's the way every 71-74 Javelin I've worked on is. I think they took 2 of the blocks out as a running change to save money because it's not necessary. -------------  " http://theamcforum.com/forum/hemilina_topic95889.html" rel="nofollow - Hemilina " My 1973, 5.7 Hemi swapped Javelin |
Posted By: mmaher94087
Date Posted: Sep/10/2020 at 7:20pm
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Mopar_guy is correct. I had NOS cross members and they all were the later type made for 71 - 74s. Edit to include: If you are concerned about concours correct then knock two tapered nuts off your old crossmember with a cold chisel and use two old tapered bolts from the old assembly when installing the new. ------------- Mike |
Posted By: 401MATCOUPE
Date Posted: Sep/11/2020 at 5:26am
Mike M is 100% correct it was a running change to the crossmember. The old P/N supersedes to the one with single taper washer. It was a mid year 1970 change from what I have seen from parting out dozens of 70 Javelin's and AMX's. ------------- Ross K. Peterson 68X,GoPac,343,AT,52A(1stCar) 68X,GoPac,390,4sp,52A 69X,GoPac,390,4sp,64A 70X,GoPac,390,4sp,87A,8 70X,GoPac,390,4sp,BBO,8 70 Jav SST,390,AT,BSO 74MatX,401,AT,Prototype 74MatX,401,AT |
Posted By: mbwicz
Date Posted: Sep/11/2020 at 5:53am
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Thanks for all the replies. I figured it was a running change along the way. Amazing all of these little details that show up when you start looking closely. Mike ------------- 1970 AMX, one step forward, one step back. Both steps cost time and money. |
Posted By: Steve_P
Date Posted: Sep/14/2020 at 8:02am
| Running change as Ross said. The second design is better. Two tapered locators like used in the first design will "fight" each other when it comes to front/rear location. |
Posted By: Trader
Date Posted: Sep/14/2020 at 12:28pm
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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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Posted By: Steve_P
Date Posted: Sep/16/2020 at 9:15am
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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. |
Posted By: Trader
Date Posted: Sep/16/2020 at 10:53pm
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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. |
Posted By: mbwicz
Date Posted: Sep/17/2020 at 4:59am
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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, one step forward, one step back. Both steps cost time and money. |
Posted By: Trader
Date Posted: Sep/17/2020 at 7:06am
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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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Posted By: Steve_P
Date Posted: Sep/22/2020 at 12:46pm
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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. |
Posted By: Trader
Date Posted: Sep/23/2020 at 9:43pm
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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. |