Fiat TC rods - standard and race

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Fiat TC rods - standard and race

Post by Guy Croft » Sun Dec 03, 2006 11:06 am

Top left
Standard 2 liter Fiat TC con rod from period 1973 to around 1981, made from forged steel, harness about 19-20 Rockwell (HRc) around the tensile strength of En8, 500 MPa tensile and heat treated by carburising to develop additional strength around 7-800 MPa.

Centre left
Later generation bolt-only TC rod from late generation 130TC Strada (hardness about 15-18 HRc) and typical of rods used in 8v Integrale and later 16v Integrale although the two types have different bearing locating lug positions. Some of the engines with later rods eg: Evo 2 Integrale have provision for bolt pretensioning by strain (stretch) method using micrometer, in other words cone drillings in both ends of bolt, for what it's worth. Years ago after catastrophic failure of a set of new fully lightened, polished and shot-peened rods of this type at a very low 8000 rpm in an NHRA short oval engine after only 20 laps on test I had very detailed metallurgical tests done on this late (and rather heavy rods) though unfortunately after publication of my TC book. The tests said that the bolt-only rods I sent in were actually weaker than the early nut & bolt rods due to inferior heat treatment. It's certainly true that it's possible to enhance the strength of a rod by using bolt-only - but only if the material of the threaded end is sufficient. These rods are OK in compression and bending but I would never run them over 7500 and preferably less. Run them higher if you want to, I would not.

Lower left
Early TC rod lightened by machining around the big-end bore, significant lightening but not much use really as the reduction in mass is at the crank end and let's be honest - has little effect on reducing stresses anywhere. The place to reduce rod weight is really at the small end region, by grinding (not machining - it can disturb the small-end bushes) either side of the small end region. Not that there is much reduction in all conscience. And any machining MUST be followed by the most diligent polishing of and around the machined region or the rod, whilst lighter, will be full of potential crack-propagation areas generated by the machining, especially at the weak points below the small-end region or where (in this case) the big end region meets the I beam section. What do you get for all that work? Well, you get a lightened and polished rod made of En8. Don't waste your time with it, is my advice because no amount of prep like polishing and shot peening is going to increase the tensile or bending strength of the core material, it's just going to reduce the likelihood of crack propagation due to an imperfection in the metal surface.

At right
Left-hand rod
Genuine Abarth prepped rod from a works 131 16v unit from the 80's, badly damaged because it 'ran' a bearing. It looks just like a polished lightened early generation TC rod albeit it with bigger bolts, but it's not. Hardness tests (32 HRc) show it is a far stronger steel, race material with the right combination of ductility and tensile strength, but interestingly still made in the same forging press as the production rods and carrying the Fiat forging imprint at the root of the I beam.

Right-hand rod
True race rods from a world authority. I beam rods are the strongest. H beam? Sorry, not as strong. Do the bending calcs. Don't be seduced by the glamorous shape of the H beam rods. This is GC spec Cunningham race rod, 34 HRc chrome moly steel, split cap forging (upper and lower halves forged separately to ensure grain flow in optimum direction in lower section, Cunningham are the few firms in the world - maybe the only one - who make them that way), super light, non-directional polished, shot peened, honed to flawless perfection in big and small ends, balanced to within 2 gms per set total and end-over-end. Cunningham super-high tensile bolts. Will run to over 9500rpm in 2 liter Fiat all season and probably forever. Need I say more?

Well, maybe...

The world motorsport market is being flooded by cheap and very poor quality rods (and other parts) from China. I could name names, sure. Look for second-grade rod material, impaired polishing and machining, pin bushes too tight, big-end bores out of round and out of size and honed with a house-brick. If you need race rods buy from a traceable source like Cunningham.
If you choose rods on the grounds of cost - and many do - frankly you need your head tested. You must have absolute confomance and traceability on race rods or you're wasting your money at any price.
Std and race rods weights.JPG
Rods referred to in text above. The one you want is the one on the right.
Std and race rods weights.JPG (149.25 KiB) Viewed 11426 times
GC Cunningham race rods 2 liter TC.jpg
Cunningham race rods. I'm a great admirer of Cunningham, I like the no-nonsense approach of old-school Hot Rodder & company founder Greg Cunningham.
GC Cunningham race rods 2 liter TC.jpg (113.78 KiB) Viewed 9958 times
std 2 liter TC piston rod assy wt.jpg
The weight of a 2 liter TC std assy of rod, nuts, piston and rings.
std 2 liter TC piston rod assy wt.jpg (112.07 KiB) Viewed 9941 times
GC race 2 liter TC piston rod assy wt.jpg
The weight of a 2 liter TC GC race piston and rod
GC race 2 liter TC piston rod assy wt.jpg (111.73 KiB) Viewed 9930 times
wt of 2 race big ends.jpg
..the difference between the 2 - 324gm - is more than the weight of TWO race big ends - contributing massively to the inertial stress on the rod..
wt of 2 race big ends.jpg (111.21 KiB) Viewed 9954 times


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