Rod bolts and nuts?

jrbeltz · 02:29 PM

I understand the need for new rod bolts but why new nuts? I understand that the bolts stretch and therefore cannot be re-used, but nuts don't stretch...they apply the force. Pelican is currently out of stock on nuts so my rebuild is on hold or do I use the old nuts...any help?

1984 Carrera

KTL · 11-26-2013, 11:51 AM

I think its just good insurance to use new nuts since they are such a critical fastener? I agree the threads themselves are not highly stressed by the torque you apply to tighten them.

I would say use red high strength threadlocker if you choose to reuse them. Or else unfortunately look to another supplier if Pelican can't stock them in your timeframe.

ajwans · 11-26-2013, 02:01 PM

What engine? Pelican Parts - Product Information: 928-103-172-02-M260 works on an early engine and has the shake proof design under the flange.

Hat tip to Henry Schmidt as this was his suggestion some time back.

andy

Roslin · 12-01-2013, 12:51 PM

There should be no need for new nuts as long as the mating surfaces are ok.
I would not recommend any use of Loctite as well.

Roslin

Walt Fricke · 12-01-2013, 02:08 PM

I've never seen use of a thread locking compound recommended for rod bolt nuts.

I don't think rods fail due to the nuts backing off under normal use. If the bolts are overstressed, and elongate, then yes, the nut can back off some. But that just means you have a blown up engine a little sooner, because once the cap can separate from the rod body due to elongation you lose the pressure at that journal, you wipe the bearing, and kaboom,

On the other hand, I have not been afraid to reuse the nuts. For instance, the ARP and Raceware rod bolts can be reused. You have to measure them before first use, and if they are within a very small tolerance of that measurement when it comes time to reuse, both manufacturers say you can reuse the bolts. If you can reuse the bolts, for sure you can reuse the nuts.

I don't really know why Porsche recommends (or did) replacing the nuts when you replace the bolts. Replacing the stock bolts has been the deal for a long time, at least with the air cooled motors.

porterdog · 12-02-2013, 07:30 AM

Quote:

Originally Posted by Walt Fricke

I don't really know why Porsche recommends (or did) replacing the nuts when you replace the bolts.

I suspect you do and are too polite to say.

Same reason the engine tins are made from steel.

Same reason this part costs $140 vs. this one at $25.

KTL · 12-02-2013, 08:07 AM

Let me correct what I previously said. I mentioned red loctite for peace of mind. It's something that has made its way into the Porsche rod bolt urban legend over the years and has not proven to be a harmful technique. Unneccessary? I believe so.

The threadlocker seems to have a somewhat similar level of lubricity as basic engine oil, which is the typical recommended lubricant applied to the threads before torquing the bolt, so the effect on torquing & stretching the bolt itself is not drastically altered. I've actually seen some people put assembly lube on the threads followed by the threadlocker and then assemble the rod on the crank. The threadlocker actually works- locks the threads.

KTL · 12-02-2013, 08:11 AM

Quote:

Originally Posted by porterdog

I suspect you do and are too polite to say.

Same reason the engine tins are made from steel.

Same reason this part costs $140 vs. this one at $25.

I don't follow what you're saying? Porsche recommends replacing nuts to pad their wallets? Because i'm not following the comparison of OEM vs. original Porsche window trim parts.............

One can argue it's good practice to replace the nuts simply to ensure you've got a clean load bearing surface on the nut face. But I agree if the nuts are nice & clean on the threads, contact surface and on the hex tooling, no reason not to reuse them.

AlfonsoR · 12-02-2013, 09:50 AM

Yeah, i too believe it's an insurance thing. Nuts that are part of a critical fastener have the faces ground for flatness and more consistent friction factor.

I would suspect that of the mating surfaces, the nut face would be more likely to take a set or deform/conform to the rod due to parallelism tolerance for the rod in the rod bolt areas. This is more on the microscopic level and probably not worth worrying about on a street engine. New nuts however are cheap insurance on a critical part, that has in the past (with OE style stretch-to-yield) fasteners, come into question.

porterdog · 05:26 PM

Quote:

Originally Posted by KTL

I don't follow what you're saying? Porsche recommends replacing nuts to pad their wallets? Because i'm not following the comparison of OEM vs. original Porsche window trim parts.............

Yes, without question. Compare those two parts, both original Porsche. Pretty similar parts, but the $125 one rots away to nothing because it has a significant steel component.

Engine tin could easily be made from aluminum, but that doesn't rust away and require replacement.

They know where their bread is buttered, make no mistake.

AlfonsoR · 12-02-2013, 07:20 PM

^^^ I have no doubt Porsche knows how to pad their bank, however, new nuts to go along with new rod bolts is pretty standard fare. i believe you would find that recommendation in most cases from other OEM's and most professional builders would probably say the same.

KTL · 12-03-2013, 01:47 PM

Aluminum engine tin would be nice but I bet that too would corrode some, just like the impact bumpers do? I think we got a good dose of aluminum in these cars considering their age. Transmissions, trailing arms, engine, front hubs, steering crossmember and so on. And in all honesty, its not like the steel engine tin doesn't last a long time. You may be a bit biased because you live in the rust belt region of the country like I do?

There's cars that are over 40 years old with tin still fairly well intact. That's pretty good durability IMO. Much better than most domestics can tout. The bean counters have to draw the line somewhere or else the car goes from being quite expensive to plain crazy expensive?

porterdog · 12-04-2013, 05:37 AM

Yeah, my tins (esp the front) are in rough shape except where the external lubrication feature has prevented them from rotting.

And truly, there's no way I WOULDN'T replace my rod bolt nuts.

At the same time, I expect that a continuous flow of replacement part sales is and has been part of Porsche's recipe for long-term success. In all fairness however, the parts quality on my P-car kicks the living snot out of anything else I've ever owned (which is probably why confronting stuff like the cited parts is so galling).

Walt Fricke · 12-04-2013, 08:59 PM

One argument for replacing nuts along with new bolts might be that a used nut might have a different tooth surface coefficient of friction from having been used. I'm recalling charts discussing different torques to achieve the same fastener stretch depending on what sort of plating was on the fasteners (along with different torques depending on lubricants used).

This wouldn't matter if using a stretch gauge. And I think the same would be true of the method of torqueing to a low value, and then turning an additional specified angle. In neither case is the actual torque applied a substitute for stretch. Which is why I suspect that most who use reuseable (if measured within spec) rod bolts don't worry at all about reusing nuts. Though when a bolt gets out of spec and is replaced, typically it comes with a new nut anyway - because last time I looked you had to buy sets of these aftermarket bolts, and they came with nuts. No singletons unless you got one from a shop which had broken a package for a motor or two.