Reconditioning titanium rods

[chris_seven]

Originally Posted by Speedy Squirrel

Why do people think it is safe to re-use the GT3 rods when they have reached the end of their service life? Is there a body of experience to suggest that they are removed too soon?

1. Hours are not a very good indicator of fatigue life, unless it is hours of defined usage (25 hours on a couple of specific tracks). Otherwise, it is time for Rainflow counting and the Palmgrin-Miner rule, which requires more data collection than just the four over-rev ranges.

2. The large load reversals cause most of the damage. That is why there are the four over-rev ranges counted in the vehicle. Staying below those lofty RPMs causes MUCH less fatigue damage.

3. Re-annealing and/or shot peening does very little to restore fatigue life. It is like putting the smoke detector in after the house has burned down.

4. Re-sizing takes away material from the rod cap, making it even thinner. Is that safe on a highly weight optimized rod like the GT3?

The thing with the GT3 rod is that, because it is a high reving stroker, it needed bigger bolts, but there was no way to make the big end larger to accommodate them. No room relative to the case.

That being the case, if you are building something less that 3.4 liters, I wonder if you would be better off with a custom titanium rod, even if it is cut from annealed plate, rather than forged. It will have a much more reasonable fatigue life, due to more material in the right places, of course with a weight penalty.

I do agree that '40 hours' is a poor way to define fatigue life and may be a better means of analysis could be provided using modern data loggers.

I do tend to believe that the 40 hours life is simply because there is no real data to hand.

I would suggest using Inverse Power Law - Weibull Models to allow more realistic accelerated testing if budgets were available.

The use of shot peening for the enhancement of fatigue life has been well established for many years as has the use re-peening to restore and improve fatigue life to the levels of that found in new parts.

Much of the early published work was carried out using steel components but there is a reasonable body of work to also show re-peening Titanium does re-establish the compressive residual surface stresses that can be lost due to stress relaxation.

I would agree that if surface or sub-surface defects have already initiated there would be little or no point in using this technique.

[MST0118]

Interesting discussion and have a follow up question. With the Pankl rods, I'm getting the used titanium bolts and assume that I can't reuse those. If my assumption is not correct or if one or more of the bolts can be checked for possible future use, let me know your thoughts.

If not, the question becomes whether I should purchase new titanium bolts (which are about $75 per bolt) or whether I should just replace with ARP steel bolts of equivalent. It seems that some argue that titanium bolts may not be strong enough in a high HP air cooled application since typically the pistons are heavier than those used in the GT3 motor. I'm trying to lighten the reciprocating mass on the engine as much as reasonably possible so would like to know the options on that. Thanks.

I'm sure there are different viewpoints on this, too, but would be interested in what you all are doing and if you've seen or heard of failures with titanium bolts.

[chris_seven]

Rod Bolts are always designed to ensure that they are never subjected to fatigue loading and thefroe should never fail.

When correctly tightend the preload introduced really mans that the bolt only needs to withstand the static load applied.

It is possible that rod bolts do fail in fatigue but this is normally associated with them having been incorreclty tightened.

If the preload is insufficient the bolt will be subjected to fatigue loading and in my view failures would be much more common due to the stochastic nature of fatigue process.

I do here comments aboyt Ti Rod Bolts galling in service but this also seems to imply that there is insufficient preload.

As an annealed 6AL4V has a tensile strength of around 160ksi it would seem that Ti Rod Bolts need to be larger diameter than steel bolts.

SPS have recently introduced TITAN 180 Bolts which are 180 ksi.

The early GT3 rods seemed to us a 'though' bolt with brass washers and a Ti Nut but later rods seem to have the bolts screw directly into the rod.

The early rods seem to be use once parts which suggest that they are preloaded beyond thier yield point.

I would consider using a steel bolt but this probably means the rod will need to be re-sized.

[Neil Harvey]

Lots of confusion here. The easiest and most effective way to lighten the masses going round and round and up and down are the rods as they go in both motions.

Late GT3 rods take a bolt only and are steel. ARP make these but these are a custom size and not sold through the ARP outlets.

I don't think there was a Ti bolt for any of the GT3 rods. The early rods used a bolt, washer and not but these were super-ceded with just a bolt only.

Ti bolts were used in earlier race engines only.

[faapgar]

Rich Jascic in Blairstown N.J. 201-362-8163 has rebuilt many Pankl Ti rods for Peter Dawe who stocks the Ti small end bushings.Ciao Fred

[Speedy Squirrel]

The 2001 GT3 R used a Ti throughbolt, hardened Ti washer, and Ti nut. This bolt also aligns the caps together. There is a large diameter near the center of the bolt that fits tightly in the rod and cap to align them. The bolt should turn freely, but the clearance is small. ARP can make a steel replacement, but it is not off the shelf.

The bolt is the least of your worries. You are running rods that are probably near the end of their fatigue life. That was more concerning to me, and I evaluated the same rods for my project. Ultimately, I decided to buy aftermarket Ti rods. Same 6Al-4V, but from billet instead of die forged, with steel bolts. The thing is, there is really no point in buzzing a 3.4L engine to 8500. The intake valve area is just not there for it. I'm going to be happy with 7800 or so. For that reason the lesser rods were OK with me. They are a little beefier (and heavier), but they won't be timing out any time soon.