<VV> Corvair block studs

RUSSELL DAVIS bluechipmach at verizon.net
Fri Dec 19 11:18:31 EST 2008


Good mornig Clark, Russell Davis here in Wakefield,Va. I finally had an opportunity to measure the pitch diameter of the 3/8" -16 end of the block stud that I had e-mailed you about back in August. I used thread mics and measured 10 studs that were in good shape and free of alum. The P.D. measures .340 to .3405 on all the studs. With a major O.D. of .371 According to the Machinists handbook that would indicate a class 5 interference fit thread. The class 5 pitch diameter for a stud being used in a non ferrous application is .3382-.3414 with a major diameter of .3646 -.371 Also I noticed that the blank size was too small for a 3/8" rolled thread (.280-.285). So the blank was bigger on each end when it was rolled. The blank size needed for a  1/4" to 1/2" threaded stud would be approx. .002 to .0025 smaller than the required  pitch diameter. The 3/8" pitch dia. of .340 would require a blank size of some where around   .3375 to.338 Thew size
 of tyhe blank inbetween the threaded ends in approx.280 to .285.This reduced blank size in the middle of the stud could have been used for extra clearence for the barrels and heads, I assume. I spoke to LMTFETTE Tools co. in N.C. that specializes in thread rolling and asked  them about the noticeable reduced size on the first few threads. They educated me on how to identify the undersized start of the thread. It is reffered to as a 2K lead which gives the blank it starting cappabilities in non ferrous materials. Basically the first 4 threads are imperfect and not rolled up to the required major O.D.This gives the stud its "start" in the material.Also the required length of engagement( for alum) is .9375"  
This info along  with the class 3B internal fit for the tapped holes that you provided tells us that there was a large amount of driving force used to install these studs.I wonder how GM approaced that? The handbook makes no reference to any lubricant used for installation other than oil or no lubricant at all.  Thanks for all your help
Russell Davis
Blue Chip Machine 
Wakefield, Va.   


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