Rear Spring / Differential Bolts

[Dave Rossi]

Hello All,

Is there anyone out there that knows what grade the four large bolts 9/16" 12 tpi x 3 1/4" (1963 7 leaf) that hold the spring plate to the differential should be. I purchased grade 8 from McMaster Carr but in looking at the originals which are slightly bent with ugly threads I can't imagine that they were grade 8 . The original head markings are long gone so I have no way to tell. Any help would be appreciated. Thanks, Dave

[William Othick (36100)]

Re: Rear Spring / Differential Bolts

It is most likely there are Grade 8 but grade 5 maybe possible, Jobbers like Long Island can sell you correct replacments.

[Wayne Midkiff (3437)]

Re: Rear Spring / Differential Bolts

Dave- GM shows these 4 spring mounting bolts as #3833486, 9/16" - 12 x 3_7/32" lg. Gr 5.417 and/or 5.729. Shows as Grade 260-M, which is the lowest of 4 grades. Did you see my pic of a "WB" headmark original on the board a few days ago ? I don't think they have to be a high grade, as in normal operation, all they do is keep the spring aligned normal to the axis of the car. There is no tension involved.

[Joe Lucia (12484)]

Re: Rear Spring / Differential Bolts

Dave------

The original spring plate retainer bolts, GM #3833486 were of GM material grade 280M (equivalent to SAE grade 5). The GM replacement for the aforementioned bolts, GM #9428598, were of material grade 300m (equivalent to SAE grade 8. Both are discontinued now.

[Dick Whittington (8804)]

Re: Rear Spring / Differential Bolts

Dave, there would be no problems using grade 8 bolts. The main issue is to make sure that you do not use a bolt too long. If it bottoms out in the hole the spring will not have the proper torque.

[Dave Rossi]

Re: Rear Spring / Differential Bolts

Hi Guys,

Thanks once again for sharing your knowledge. My main concern was that there may have been a specific reason why GM would use a lower grade bolt for the application of the cast housing. If they used a grade 8 as a service replacement later on, I guess I'll be O.K. Thank You.

Dave

[Terry Fiala (21624)]

Re: Rear Spring / Differential Bolts

If you use too long a bolt it will also possibly crack the housing of the differential. Terry

[Joe Lucia (12484)]

Re: Rear Spring / Differential Bolts

Dave-----

I expect that the specific reason, in this case, that GM originally used a grade 5 bolt rather than a grade 8 is that the grade 5 was competely adequate and cost less than a grade 8. For this sort of application (but not all), a grade 8 bolt is completely fine as a replacement. As you surmise, the fact that GM replaced the original bolt for SERVICE with a grade 8 completely supports that.