ARP Nuts

U

ursus

Well-known member
Following the suggestion of others, I decided to use ARP nuts in fitting up a set of Model B rods. These have a close-fitting threads which would obviate the need for cotter pins. The standard castle nuts available these days all seem to fit rather loosely and are of unknown grade of metallurgy.

ARP nuts have a wider base, like a flange nut and in fitting them up I noticed that the flange base only makes full contact with a raised half circle on the inboard side of the rod cap. The raised area was left in the machining process as the machined area was made to accommodate the standard 7/16 castle nuts. As a result, the nut flange only makes contact with half of the rod cap surface.

Has anyone noticed this when using ARP nuts and are some folks running engines with such a situation? I suppose one could machine away the raised ledge in order to achieve full contact but I don't have that ability.
 
Try Marsden nuts. Far cheaper and more than adequate for OEM or race applications.
Just get ones made in USA though.
 
Its hard to picture what you are describing, but obviously the nut face should make decent contact with the cap spotface. Not sure if your rods have been abused or the caps distorted or stretched, but I don't think its a common problem. Be aware that on a
B rod cap, the nut spotface on the inboard side comes very close to breaking thru to the bore. Occasionally I find rod caps that have broken thru on this area and probably left the factory this way. You can't see it until the babbitt is melted out, but I usually set these caps aside and don't use them. My point is , if you have someone spotface your caps keep this in mind. Certainly use a counterbore tool with a corner radius if you do rework them.
 
I know an engine reconditioner who has used Nyloc nuts on the rods for years without a failure so I gave it a shot. Although I was prepared to drop the sump and replace them at short notice, I had no problem.
 
A.R.P. 7/16-20 nuts come in steel and stainless steel
There are 12-point with 1/2 or 9/16 wrench and 0.600" OD and 0.800" OD flanges.
They have fanged hex and regular hex with 5/8" and 11/16" wrench. URSUS, have you checked the various offerings?

My thoughts on A.R.P....
I looked for quality, high strength stainless steel Fasteners and found A.R.P. I was able to buy from stock stainless @ 180,000 psi yield to replace the 70,000 psi, 80.000 psi yield that I used before, allowing me to go down a size.
I investigated A.R.P. and found they make their fasteners in the U.S.A., totally in house including heat treat and finishes. The quality surpasses aircraft specifications and they patented many processes including special work hardening process for stainless to more than double the strength. They will make special bolts at your request with yield in the 300,000 psi range. This is from racing boys that made fasteners because others failed.

Model A boys don't need A.R.P. There are plenty of manufacturers qualified for Model A fasteners. I'm sure you will get recommendations of other fastener places posted here that are trustworthy. I'm just a proud A.R.P. man.
 
Dan McEachern said:
Its hard to picture what you are describing, but obviously the nut face should make decent contact with the cap spotface. Not sure if your rods have been abused or the caps distorted or stretched, but I don't think its a common problem. Be aware that on a
B rod cap, the nut spotface on the inboard side comes very close to breaking thru to the bore. Occasionally I find rod caps that have broken thru on this area and probably left the factory this way. You can't see it until the babbitt is melted out, but I usually set these caps aside and don't use them. My point is , if you have someone spotface your caps keep this in mind. Certainly use a counterbore tool with a corner radius if you do rework them.
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These are ARP #300-8394, 7/16-20, 12-point, in black oxide. I measured the spotface diameter on a dozen Model-B rod caps and found a typical diameter of 0.675 inches. The ARP nuts diameter is 0.820 inches at the flange contact end, hence the overhang on the ledge outside the spotface.
 
Synchro909 said:
I know an engine reconditioner who has used Nyloc nuts on the rods for years without a failure so I gave it a shot. Although I was prepared to drop the sump and replace them at short notice, I had no problem.
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And you should not have a problem.
A plain high strength rod nut torqued properly for the application will never come loose.

Modern race rods use NO locking device on the fasteners.
 
Nyloc nuts are only rated to 250 degrees F. Its not the nylon insert that keeps the assembly secure, its the nut torque and the tension developed in the rod bolt as Pete mentioned above.
 
Normally, the strength of the threads in the nut determines the strength of a joint in tension. However, the weak link in the connecting rod is the cap attachment studs, not the nuts. I found this out from experience. The studs have a finite fatigue life, and should be inspected for cracks especially where the studs enter the rod.
 
Phil Brown said:
You need ARP # 300-8333 they have smaller outside diameter than the #300-8394 both have 7/16 - 20 thread
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These are the nuts that I too recommend. I gently spot face the rod cap, oil them and torque to 45 or 50 lbs. No cotter pins are needed. Stock castellated nuts have only 4 or 5 full diameter threads, and will strip off in high RPM racing conditions, so I never use them. I don't use castellated nuts on the mains either, for the same reason. The last few engines that I built, I machined the acorn type of lug nuts flat and used them, they are strong and look cool. There were many aftermarket rods built back in the day, I believe stock A/B rods are plenty strong, the problem was the nuts.
 
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