S-4 rod bolt rating

90 S-4 · 08-06-2008, 11:58 AM

Hello,
Does anyone know what the tensile strength is on a stock S4 rod bolt.
Thanks,
Joe

Barbapapa · 08-06-2008, 02:44 PM

Hi!
Good question. I could only guess based on typical recomended torques of similar bolts of known materials and the WSM torque value implies something in the 160 kpsi range. That's just a wild guess though. it may not matter though...
The yield rating is not as important as the capability of stretch and the design to allow stretch. Porsche rod bolts give me a warm fuzzy feeling when being torqued, they inspire confidance. I would wager that you could take a similar ARP bolt of higher yield strength and it would not function as well as a Porsche one.
BTW, I've known some really cool Joes that have '90 S4s and are prone to serious projects, are you one?

90 S-4 · 08-06-2008, 03:26 PM

Hello Mike,
Yea its me, how you been ? I'm glad I bumped into you, I definitely value
your experience and opinion. My situation is, I'm trying to determine if the rod
bolts that came on my rods are going to be up to the task of what there going
to have to do. ( You'll probably remember ) anyway, the rods are Carrillo & the
the bolts are also made by Carrillo, at least the Carrillo name is stamped on to it.
When I talked to the tech guy (Scott) at Carrillo he told me the ones I have are a
3/8" WMC with a tensile of 220k They make a Carr bolt with a tensile of 285k but
I'd have to resize the big end in order to use them. So, the question is will the
WMC 220k bolt be stronger than a stock S4 piece. tech/Scott also said that the WMC 220k is stronger than the weakest part of the rod itself & I'd break the rod
long before I broke the bolt. I told him what the general hp target was & he confirmed that their rod was up to the task. I to have a lot of faith in the German
steel & think an oem piece is probably as strong or stronger than our aftermarket
stuff. I wish I had the rating on the oem fastener so I could have some sort of
comparison ability and be able to make a comfortable call/guess to move forward
with. But more importantly, how's your "special project" coming ??????????????
Thanks, Joe

Barbapapa · 08-07-2008, 08:13 AM

That sounds reasonable that the existing 3/8 bolt is probably stronger than necessary and that would keep you from having to modify the rods which entails risk. I would be comfortable using them as is.

For fun you can calculate the forces! Your engine is probably pulling the piston down at 3500 Gs max. The small section of an S4 rod bolt is .085 in/sq so IF that were as strong as yours one bolt could take 18,700 lbs which is well within what it would see even with the heavy S4 rod and piston, you could probably figure out exact numbers but math is hard!

What hurts rod bolts the most is not being tensioned properly! A bolt not tight enough will fail in fewer cycles than if it were tighened to close to the yield point.

Anyway, it's great to hear from you. I've been busy building a racer and maintaining it. Even with a stock engine it's some work to keep it going

I just rebuilt the engine and loading on the trailer today for its first race at Road Atlanta.

Last March the car was a forgotten street car, I took it all apart and did that to it. Now that the chassis ready and sorted I can stick the 7 litre engine in but then it will be in the unlimited class along with 900hp tube frame cars. Right now I have actual competition. With the HP/weight classing of NASA GTS, the stock 79 engine fits nicely into an active class. One competitor has a 944 with a Chevy V8 that got grandfathered in, most of the GTS racers here in the SE are really fast, it's daunting!

90 S-4 · 08-07-2008, 10:30 AM

Mike,
Thanks for the calculations, it's always good to have another set of eyes check over the
critical parts. Your car turned out fantastic ! probably a good idea getting the chassis dialed
in with the smaller motor, it'll give you a chance to really get acquainted with the dynamics of the type 928 chassis.
I'm extremely curious how all your new suspension parts and brakes and custom
bits are going to work out, I'm sure they have leapfrogged your dialing in curve. I'll bet you find
yourself pretty close to the front of the pack before you even know it. I still want to buy a set
of those rear upper adjustable camber/control arms you designed, I'll bet there going to come
in very handy while trying new track set up's & those wheel's are a very nice choice, they
are suppose to be very strong and light & look good to

Best of luck on the maiden voyage!
Looking forward to hearing how things went, keep me posted eh
Joe