Engine component fatigue life

[tmvolumex]

Guy,
In your first tuning book, you listed some recommended fatigue life values for various engine components such as pistons, cranks and rods. From your recent posts it appears that your recommendations have changed on some components when used at various levels of tune and RPM. Also, there are a couple of guidelines you mention these days such as: Do not regrind the 2 liter cranks. Would you mind updating your list on the useful life of these Fiat / Lancia / aftermarket components? I am also interested in the RPM limits of your newly sourced rods. In addition, could you add your recommendation on the service life of valves, all types, such as: 1 piece stainless steel, 2 piece factory and sodium filled. I am in the process of building up a Volumex head that has a total of 150,000 miles on it. I am wondering if the sodium filled exhaust valves have reached the end of their useful life. The valves were new when I installed them and they appear in great shape, but I am concerned about their fatigue life. Valves don’t undergo huge stress reversals like a connecting rod, but I assume they do not have an infinite fatigue life. I had heard a rumor that “old sodium filled valves can fail”. I don’t have any details on this rumor and don’t know what “old” is. Is there any truth to this rumor? My valves are 8mm stem, Fiat / Lancia 8 valve VX valves. One more thing, on inspection of the head, the bronze valve guides were in great shape except for one exhaust guide. This guide was totally worn out, allowing 50 – 75 thousanths of lateral valve deflection at full lift. I was amazed the valve and seat were not beat up. I have no idea why this happened. Any thoughts. The bronze guides are factory bronze guides circa 1990.
Thanks,
tmvolumex

[Guy Croft]

I will reply when a get some time free, Tom!

GC

[tmvolumex]

Guy,
Thanks, Ill keep an out out for your post.
tmvolumex

[Guy Croft]

Tom, hi

some observations for starters:

I am also interested in the RPM limits of your newly sourced rods.

The longest stroke engine in the FL TC range is of course the 2 liter at 90mm and this has the highest rod stress; steel has a fatique limit and if you keep under it the rod will never fail, quite unlike aluminium rods. The survivability of a race quality steel rod is a near-impossible thing to specify considering that the life cycle varies from owner to owner. Without going too much into stress computations with piston weight etc., my rods will run comfortably at 9500rpm (and over) season after season. Naturally you can reduce the working life of a rod by rough handling - scratches and surface imperfections are perfect sites for crack propagation..



In addition, could you add your recommendation on the service life of valves, all types, such as: 1 piece stainless steel, 2 piece factory and sodium filled. I am in the process of building up a Volumex head that has a total of 150,000 miles on it. I am wondering if the sodium filled exhaust valves have reached the end of their useful life. The valves were new when I installed them and they appear in great shape, but I am concerned about their fatigue life. Valves don’t undergo huge stress reversals like a connecting rod, but I assume they do not have an infinite fatigue life. I had heard a rumor that “old sodium filled valves can fail”. I don’t have any details on this rumor and don’t know what “old” is. Is there any truth to this rumor? My valves are 8mm stem, Fiat / Lancia 8 valve VX valves.

It is very much a rumour. There is no chance of exceeding the fatigue strength of the valve material in a contemporary valve train in an engine like yours. The primary load is tensile on valve closure, a low stress in real terms.
Heat: Valves will change shape fractionally due to creep of course but generally they can be reground true. They are at risk from excessive temperature but those sodium cooled valves disperse heat way better than solid - and are thus more survivable at temperatures up to about 550-650 deg C than other types. Beyond that they can exhibit distortion of the valve head, at extremely high temperatures - 700 deg C plus (and higher) on turbocharged units the distortion can be extremely rapid and necking of the stem can occur too. If you, say, crack tested a valve, checked the stem diameter and it reground OK there is no reason at all not to re-use it.


One more thing, on inspection of the head, the bronze valve guides were in great shape except for one exhaust guide. This guide was totally worn out, allowing 50 – 75 thousanths of lateral valve deflection at full lift. I was amazed the valve and seat were not beat up. I have no idea why this happened. Any thoughts. The bronze guides are factory bronze guides circa 1990.

1. Worn valve tip and bucket can cause 'tip-rock'. That side load wears guides pretty badly
2. Similarly - misalignment of a valve guide bore (in terms of its perpendicularity to the tappet) tilts the valve under lift fractionally and damages the guide bore.
3. Abrasive material trapped in a bronze guide (dring manufacture or honing/installation) will become embedded and can cause rapid wear.
4. Carbon buildup and fouling on the exposed section of the ex valve stem definitely causes extreme abrasive wear, likely coming from over-rich or worn rings on that cylinder. If the valve is not seating well you get local overheating of the valve head non-firing side, stem and guide too. It can lead to valve head fracture too. A seat regrind solves that one.

Contaminated oil (by gasoline) leads to wear of bronze guides to bits in no time at all, mind you the same is true of cast iron guides and it affects all of them, not just one.

Hope that helps for now,

GC

[tmvolumex]

Guy,
Thanks for the reply.
It sounds like your new race rods are really something, 9,500 RPM in a 2 liter, that’s S2000 territory! I have to get some of those for my next engine project.

Glad to hear that the rumors of sodium filled valves coming apart are unfounded. I will check mine out (10 X magnification) and reuse them if there are no visible defects.

I think that my worn valve guide was due to one or two of the possibilities that you mentioned. The first being carbon build up on the stem acting as an abrasive. This head utilized 40-80 Alquati cams and thus has a lot of overlap. This overlap lets exhaust gasses back into the intake track carrying carbon along with it. The valves did have a lot a buildup, especially after 150,000 miles. I got my moneys worth out of that engine. The second possibility is a misaligned guide. I had the original cast iron guides replaced with bronze ones when I built the motor for a Volumex. Ill check the valve guide bore in the head to see if it’s straight and square with the cam box surfaces.

Thanks again for the reply,
Tom McGaffigan

[Guy Croft]

Actually, for a good rod, like GC/Cunningham I beam types, 9500 rpm on the 2 liter TC at 90mm stroke is nothing. It just so happens I have not dynoed or raced that motor any higher. And indeed why would I when it's very difficult to get it to develop peak power much over 7600 or so?

But my stress computations indicate that with a lightweight racing piston they will go to 12500 before getting anywhere near the endurance limit of the rods/threads or bolts, and my analyses are borne out in fact, Greg Demetriadis (site member) with his 1/4mile championship winning Lancia Integrale is routinely dyno testing at 10500 and racing higher still. Sure he has slightly shorter stroke and longer rod but I did the comparative stress evaluations ages ago and talked it thru with Greg and, unsurprisingly he hasn't broken a rod yet.

You have to remember that to survive at very high rpm when the stresses around the rod are borderline endurance limit (I work on 800MPa) a race rod must have an exceptional surface finish - a small scratch or imperfection would the perfect point for a fatigue crack to propagate. To be honest - at very elevated rpm it would be 'touch and go' as to what would break first, the piston pin boss region or the rod bolts. Not that you'd ever find out, there would be an awful mess.

I saw a cracked rod bolt in an F1 Pankl titanium rod once, yes, cracked but not fully fractured. Chance in a lifetime to see something like that..

GC