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I need advice? I am tuning a 1965 650cc Thunderbird engine,I've got new barrels, pistons, reconditioned twin carb splayed head, two new AMAL monoblock and the breather mod from the later engines.
Here’s the problem do I use the original con rods or go the final bit and get billet rods? I’ve polished the original rods and they look fine, but I intend to do a few ¼ mile sprint’s on the bike. Some one has suggested I get the original rods ultrasonic tested or get billet rods?


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Stock rods are billet. Billet only means new material. Did you mean steel rods? It depends upon how often you intend to replace the rods and how often you red line the engine. You should have an RPM limiter for safety reasons anyway. Steel rods are heavier and will slow the engine pick-up and increase vibration.
Aluminum is usually crack tested using Magnaflux Zyglo ultra violet dye. Shot peening and liquid nitrogen freezing can be used to increase strength.

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Many thanks for taking the time to reply and information more food for thought. I was thinking of Lightning Conrods, made from 7075 T6 alloy?


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Gday DM. As I understand it "billet" is a term used for making something from a line of extruded metal. For example the ABC aluminium Co may pour Ally and before it goes completely solid it is force fed thru rollers to form its shape. This would form a billet.
For example, if you went to a metal supplier and bought a 12" by 1" block of alum that had been forced thru rollers (not as cast) then this could be classed as "billet" alum.
A current example would be like comparing MAP rods that are machined from a "billet" run of alum V's a forged alum rod such as the Morgo rods or std triumph rods.
As billet rods would basicly (spelling!) be made and machined from a strip of rolled (read forced and or forged) alum that has gone thru a heat treatment process to grain refine its structure they can be considered very strong and reliable.
Triumph and Morgo rods are drop forged Alum, which uses the same runs of Alum coming out of the rollers, but are then roughly forged into the conrod shape, heat treated to make sure the grain runs in the same direction then machined to the desired shape.

7075 ally comes under the wrought Alum class (such as 6061, 2520 etc). The 7000 series Alum has been alloyed with zinc. It is bloody strong, but the grain structure of zinc is huge. This can cause the alloy to laminate and fall apart. It wont happen overnight but it will happen. It will take years though. When your boys inheret the bike tell em to change the rods when they get the chance!
MAP do make a really nice rod the Phantom 309 has tested. I use MAP rods as well. Bloody rippers they are too. I also run Morgo rods in another race engine, they have not failed despite the thrashing they recieve.
You cant buy Morgo rods any more.
For a road bike stick with STD rods.
If you're gonna thrash it go MAP.
FWIW.
Chris.


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I may have been a bit simplistic. From Aluminum.org:

"Ingot and billet are cast from molten aluminum. In the cast house, crucibles of molten aluminum empty their silvery liquid either directly into molds or into a holding furnace where the metal is kept molten at temperatures between 1,200 and 1,500 degrees Fahrenheit. Alloying elements are then added.

Before the alloyed metal can be cast, it must be purified by forcing mixtures of gases through the hot metal. Impurities (dross) come to the surface and are skimmed off.

Most metal is cast by the direct-chill (DC) process, which produces huge sheet ingot for rolling mills, round log like billet for extrusion presses, or square billet for production of wire, rod, and bar.

In DC casting, alloyed aluminum flows from the furnace through troughs to the casting station and into molds. For sheet ingot, weighing several thousand pounds each, three or four molds or a dozen or so billet molds may be poured simultaneously. The mold itself is only a few inches deep and forms the cross-sectional shape of the ingot. In the mold, sprays of cool water rapidly solidify the metal, which is gradually lowered into the casting pit. The process continues until the desired length is achieved. The process can be done either vertically or horizontally."

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Con rods are an element of an engine where failure causes a very great deal of collateral damage.

If one breaks it will destroy many of the other expensive and new parts you have put in your motor.

Your original rods are now 45 years old. You plan to use this machine in some competion events.

Aluminium has difference properties to steel. With age and use aluminium rods will eventually fail.

I replace the alloy rods in my race bike about every five years. That would be about every 50 hours of race use.

If you wish to use the bike for competion and have the money I would replace them. I use rods from Thunder Engineering in the UK but expect that the after market rods mentioned above would also do a good job.

I agree with Chris's recomendation but would add I would use new standard rods in a street bike if I had the money

John


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hmmm ... 'made from billet' is when a part is carved/milled/lathed from a piece of steel, alu, etc. bar, round, or plate stock.
A part made from billet is not as strong as a hot forged part because the alignment of the 'grain of the metal' follows the curves of the forged part.

This is not to say a well designed rod carved from billet would not be adequate and the aircraft structural grade 7075 T6 you mention is one of the strongest alloys avaiable to us.

I certainly don't believe alu connecting rods are merely cast and not forged as has been described. hmmmm


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Originally Posted by DMadigan
Steel rods are heavier and will slow the engine pick-up and increase vibration.


I wouldn't dismiss steel rods so easily. My experience with carrillo rods has been very good and, when combined with other changes (lightening the flywheel and balancing the crank), they did nothing to slow pick-up or increase vibration. Besides the increase in reciprocating mass is negligible over stock...at least on the A65 rods.

Oh...but I would like to try some MAP rods.


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I was not dismissing steel rods. Certainly the crank can be lightened to account for the increase in inertia but without a balance shaft there are few options to account for the increase in reciprocating weight. Strength to weight ratio between aluminum and steel is approximately equal so you could make the steel rods the same weight as aluninum if you only want the same strength rods.

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I would use steel rods, if i was building a tractor.:-)


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