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72-CLASSIC_RIDE

Front Coil Springs Specification?

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Hey all.   :)  

 

Been contemplating changing out my front coil springs to big block springs.  When I replaced the 350 CI factory engine with a 402 big block, I never changed out the front springs.

I really liked the look and car handles much better on corners sitting so low.  When I'm alone, I haven't experienced the Headers scraping pavement, but when there are two, going over speed bumps & ramped driveway entrances can cause scraping.

Based on GM Heritage information for a 72 Monte Carlo, these are the front coil springs offered.  I have learned there are different spring specifications & strengths depending on the front end load.

Is there a place to go and see what each spring type shown is made for?

Example:  350 CI, A/C, Auto Transmission vs. a 402 CI, non A/C, 4 speed ect.

 

My fear is that once replaced, the front end stands taller than the rear.  So getting the right combination of spring to load is pretty important to me.  Anyway, here is what GM Heritage shows for specs on a

72 Monte.  Could also be the same for a 70/71 also I suppose.

 

Part Number /  Assy. Code  / Cut-Off Length  / Wire Dia.  / Total Coils  / Deflection Rate Obs^inch)  /  Heights -  Free    /  Working (ln.@Lbs.)

   
3952804                AI                 143.75                  .597              10.82                          250                                      17.73                  11.7 @1500
3952805               AJ                 143.75                  .597              10.82                          250                                      18.01                  11.7 @1570
3952806               AO                153.45                  .610              11.52                          250                                      18.29                  11.7 @1640
3952812              GO                149.85                  .621              11.22                          275                                       17.87                  11.7 @1690 (current)
3952813              GQ                149.85                  .621              11.22                          275                                       18.13                  11.7 @1760
3983300              AK                 170.88                  .645              12.62                          275                                       18.38                  11.7 @ 1830 (proposed)

3983301              At                  170.91                  .645              12.62                          275                                       18.64                  11.7 @ 1900  

 

Had an idea and looked on my build sheet and found the code (GO) for the coils I have on her!  I again am going to make an assumption that my original front coils were rated on a 1690 lb. load.  Now they have lost some of their original properties and the same new springs would likely stand taller once installed as well, but probably not enough and eventually sag.  This is probably wrong in my thinking, but based on what I have and what I think makes the most sense (highlighted in bold print), would be the closest to raising her up but not to the point of to much.

 

Any thoughts?  :coffee2:

 

Doug 

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Doug, 

 

I would be very surprised if you could find/buy new front springs using the original codes or part numbers.  You might be able to use some of the specs to get close to what you want in a current-day replacement, however.

 

My '70 402 was built with the AO spring while my '70 454 got the GQ spring.  The GO spring you currently have is the same wire size (.621) and deflection rate (275) as the GQ  but has a shorter free height at 17.87" (vs. 18.13").  The AK you are considering is a bit taller at 18.38" so if you don't want to sit too high, you should consider both the deflection rate and the free height of the spring you select since both will have an effect on the static front end height.

 

I installed the Moog 6330 on my SS three years ago and Moog 5379 on the rear two years ago.  You can see the resulting stance in the photo below.  

 

The Moog 6330 has a lower free height (17.50") but a higher deflection rate (293) than either the GQ or AK.  Of course, the working height of any spring you select will be 11.7" as dictated by the geometry of a stock suspension so it is the strength of the spring (deflection rate) and the pre-load on the installed spring ((free height - working height) X deflection rate) that will determine the static frontend height.  You can adjust the preload by cutting or shimming the spring, but you can't change the deflection rate.  Good luck - let us know what you find that might work.

 

post-567-0-24702000-1482264862_thumb.jpg

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Thanx Dennis for that explanation.  Admittedly I am looking at what I have and using the specifications in the schematic, ball parking what I believe I need.

That is interesting on your 70 402 car.  I would have thought that to be a small block car spring (again was just an assumption) with the only big difference

would be mine with A/C and Automatic.

I have considered looking at other Build Sheets for the front spring info and matching that information as closely as I can with mine to see what I should be using now.

I also should get some info from the rear springs as well, but the GM Heritage PDF makes no references to rear spring geometry.  It's almost as if the rear springs were common?

I am expecting to gain at minimum an addition 1" chassis height that would probably be sufficient to minimize ground scraping.  I tried the hard rubber spacers between the coils but they end up falling out.  

I could buy larger diameter tires but I like the 60 series Goodrich T/A's.

I agree about finding an exact copy of the original, that could be tough!  When all else fails, use the geometry, find the expert and let (them) advise.

The front end is still pretty tight as I replaced the Center Link right after purchasing and driving her home.  I was all over the highway and that repair seems to have mostly solved front end wandering.  Those wider tires also add to handling when driving on the rutted highway's.

 

If I can find relevant information, I will try and remember to reply back to this thread if/when work performed.

In the meantime, will continue narrowing it down.

 

All have a GREAT day.....................................   :)

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Doug,

 

Take a look at the following thread.  Scroll down until you find a post from Sam that includes thumbnails of two tables, one for the front spring options and another for the rear.  If you click on the thumbnails, you can see and read the larger tables.  Note that there are 10 front spring options but only 3 rear spring options (and yes, they were mostly common).  All three rear springs have the same wire size and deflection rate but different free heights.  So, when there is load on the car, it takes more weight to compress the working height to 9 inches on a BM or BP than a BL spring.  Both of my '70s were built with the BM spring.  Your can find your Monte's rear spring code in Box 14 of its Build Sheet.

 

Compared to the BM spring, the Moog 5379 rear spring has a taller free height (14.75" vs 13.08") but a bit lower deflection rate (122 vs. 130).  This is verified by the photos I took below.  Again, a taller spring provides more load capacity at the same working height because it has more spring to pre-load before reaching that height.  I suspect that the Moog 5379 will raise the rear of your Monte over any of the three factory options - it certainly did on my  SS (without changing the air pressure in the load leveling shocks).

 

Regarding the AO springs selected for my '70 402, the note on the rear spring table (above) says that GM selects the springs based on the total vehicle weight considering the options that are ordered on that vehicle.  My 402/4-speed was a low option vehicle - no air or power accessories - not even power steering!  So that may be why it was built with AO front springs.  

 

As Sam has advised us many times, the springs in our cars are not "small block" or "big block" springs (even though there is an obvious weight difference in those engines).  A heavily optioned small block car may have a heavier front spring than a low optioned big block car - it depends on total vehicle weight, not just engine selection.

 

post-567-0-98565900-1482274197_thumb.jpgpost-567-0-59906500-1482274215_thumb.jpgpost-567-0-24453800-1482274236_thumb.jpg

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in 1970, they used 10 different springs, in the aftermarket world, it's going to be tough finding what you want, my only recommendation is to go to coil-overs

 

med_gallery_1341_4_1413157620_4876.jpg

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Doug,

 

Take a look at the following thread.  Scroll down until you find a post from Sam that includes thumbnails of two tables, one for the front spring options and another for the rear.  If you click on the thumbnails, you can see and read the larger tables.  Note that there are 10 front spring options but only 3 rear spring options (and yes, they were mostly common).  All three rear springs have the same wire size and deflection rate but different free heights.  So, when there is load on the car, it takes more weight to compress the working height to 9 inches on a BM or BP than a BL spring.  Both of my '70s were built with the BM spring.  Your can find your Monte's rear spring code in Box 14 of its Build Sheet.

 

Compared to the BM spring, the Moog 5379 rear spring has a taller free height (14.75" vs 13.08") but a bit lower deflection rate (122 vs. 130).  This is verified by the photos I took below.  Again, a taller spring provides more load capacity at the same working height because it has more spring to pre-load before reaching that height.  I suspect that the Moog 5379 will raise the rear of your Monte over any of the three factory options - it certainly did on my  SS (without changing the air pressure in the load leveling shocks).

 

Regarding the AO springs selected for my '70 402, the note on the rear spring table (above) says that GM selects the springs based on the total vehicle weight considering the options that are ordered on that vehicle.  My 402/4-speed was a low option vehicle - no air or power accessories - not even power steering!  So that may be why it was built with AO front springs.  

 

As Sam has advised us many times, the springs in our cars are not "small block" or "big block" springs (even though there is an obvious weight difference in those engines).  A heavily optioned small block car may have a heavier front spring than a low optioned big block car - it depends on total vehicle weight, not just engine selection.

 

attachicon.gifIMG_3494.JPGattachicon.gifIMG_3495.JPGattachicon.gifIMG_3496.JPG

That explanation confirmed what I suspected, that my logic was flawed and thanks for confirming.  All your information has helped immensely Dennis.  My Build Sheet says my rear springs are (BP) whatever that means.

I will see where this takes me and advise back once/if I decide to get it done.

 

Doug 

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Okay, something I just noticed.  Sam's Spec Sheet has additional Part Numbers and conflicts with mine.

I got those directly from GM Heritage info for 1972 Monte Carlo.  Could Sam's cover a wider variety of Models?

 

Doug

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no, that's for 1970, in 1971 I think it was down to 6 different springs, not sure about 1972

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Easiest solution, as Sam mentioned above, is a set of coil over shocks. I put a set of double adjustable Vikings on my big block Chevelle (550# rate I believe) and now the height, rebound and compression is all completely adjustable.

If you want a low ride height, that doesn't bottom on dips, loosen the coils and tighten the compression. Want the front higher, tighten the coils. You can really dial in the stance and ride.

They are mostly out of sight, so if you're concerned with original appearance, you need to crawl under the car to see them at all.

I highly recommend the Vikings as they are high quality, reasonably priced & made in Dennis's (MCFan) home state of Minnesota

And the best part is you only install them once. No in & out, cutting coils only to find you cut too much.

Scott

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Easiest solution, as Sam mentioned above, is a set of coil over shocks. I put a set of double adjustable Vikings on my big block Chevelle (550# rate I believe) and now the height, rebound and compression is all completely adjustable.

If you want a low ride height, that doesn't bottom on dips, loosen the coils and tighten the compression. Want the front higher, tighten the coils. You can really dial in the stance and ride.

They are mostly out of sight, so if you're concerned with original appearance, you need to crawl under the car to see them at all.

I highly recommend the Vikings as they are high quality, reasonably priced & made in Dennis's (MCFan) home state of Minnesota

And the best part is you only install them once. No in & out, cutting coils only to find you cut too much.

Scott

That is also a possibility Scott.  I had intended to replace original coils with similar coils but after reading your experience, it does look to uncomplicate with adjustability. 

Appreciate your insight. ;) 

 

Doug 

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I have been thinking of having my front big block springs changed to new, Eaton springs has a spring that can be ordered with 1\2 inch increments neg. or poss. My concern is to much hieght or not enough ,does anyone know what spring I would need for a stock replacement ,in a moog ,My car is a 72 with auto, air, 454 ,The eaton springs I put in the back ,raised the back, up about 1 3\4 inch ,and am very happy with that stance, the front springs were replaced in 1980, and 25000 miles ago. thanks,

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Roger, can you identify the spring that was installed in 1980 (i.e. service records, parts receipt, stamped part number, etc.)?  Unless you know the specs on the spring you now have installed, it won't be easy to determine very closely what spring specs you would need to either raise or lower the front end from its current height.  

 

If you have a Build Sheet for your '72 , you could read the original spring code in Box 13 and at least find the specs of the original front spring (see the table Doug included in the very first post of this thread).  You are not likely to find any aftermarket spring (i.e. Eaton, Moog, etc.) whose specs exactly match any of the original springs, but you might come close.  The most critical specs are the deflection rate and the free (uncompressed) height.  

 

If you want to raise the front end, either a spring with the same or greater deflection rate and a taller free height than your current spring will be needed.  Likewise, if you want to lower the front end, you probably want the same or slightly less deflection rate and bit shorter free height. You say Eaton has a spring that can be ordered in 1/2" increments (I assume that is total coil length not free height which will also increase or decrease by a smaller amount) but the deflection rate should remain the same.

 

If you know you are going to replace the front springs, you could always pull one out and measure its free height and the diameter of the "wire" (actually steel rod) used to make the coil.  Since a coil spring is simply a long torsion bar wound into a coil shape, the larger the diameter of the coil wire, the "stiffer" or higher the deflection rate will generally be.  For example, just about any aftermarket spring whose "wire" diameter spec is larger than the diameter of the spring you currently have installed, will have an equal or higher deflection rate.  

 

A higher deflection rate will give you more "preload" capacity for the same free height but will also produce less deflection or a "stiffer" front end suspension under all driving conditions.  If what you want is a higher (or lower) front end stance without affecting the ride characteristics, try to match your current deflection rate (or wire diameter) and experiment with front end height by increasing or decreasing the free height of the replacement spring.

 

As you probably know, changing the front coil springs on our Montes is a lot of work and involves some risk of injury so experimenting without knowing the specs of what you have installed could turn out to be a big project.  Good luck.

 

 

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Dennis ,I do not have a build sheet for my 72 ,or receipt ,or part numbers from when the front springs were installed ,but the original owner who i bought the car from installed the springs, iI see him out around sometimes I guess I could ask him, the eaton 1\2 increments are probable overall hieghth. I would need to call them, I would have these installed by a shop so I am trying to get the correct spring , thank you, Roger 

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