New conrods for 650 (and other twins)

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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JEAN-NOEL
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New conrods for 650 (and other twins)

Post by JEAN-NOEL »

Hi all,
Ridgecrest Engineering are no longer manufacturing conrods for our twins !
I think that THUNDER ENGINEERING can do this job.
Just an idea : why not make new rods CNC machined on the existing pattern model but with superior materials, like 7075 T 6 ?
It is exactly what it has been done for the new crankshafts.
And the look will remain original !
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Merlin
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Re: New conrods for 650 (and other twins)

Post by Merlin »

And the look will remain original

No no no I am not going to say anything. :D :D :D
Chemists do it with test tubes
Don Madden
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Re: New conrods for 650 (and other twins)

Post by Don Madden »

I read that the "humidium" alloy used on our twin con rods was the same developed by Rolls-Royce & used by Triumph, BSA, Norton, etc. The metal is problably well up to the job but only AMC fastened the cap under tension insead of compression. Also, the stud anchor reduces the cross section of the joint somewhat. Most 650cc rods show stretch at the big end & I have seen severe cases with the anchor trunnion bores visibly oval from stretch. I have a couple that broke there. I believe the answer is not new metal but better cap fastening. Triumph & Norton changed the cap to steel with increased engine capacity & finally BSA followed suit just before production ceased but that would not address the fastening problem for AMC. The existing rods can be improved by scrapping the studs, boring through the cap & rod & counterboring the rod to fit an allen head screw, which needs minor grinding to clear the crankcase. This method places the joint under compression instead of tension & while not as strong as a proper joint design, appears to be better & suitable for most use. Ed Crowell in California has done many & found them suitable for normal street use but not racing. Cheers, Don.
JEAN-NOEL
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Re: New conrods for 650 (and other twins)

Post by JEAN-NOEL »

Hi Don,
I agree with you. I want not to fit my rods, even it they are fine, on my new crankshaft.

Cheers
Don Madden
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Re: New conrods for 650 (and other twins)

Post by Don Madden »

Then the best option I know is Carillo, located in Los Angeles. If the say they have no stock, send a rod for them to use as reference. They are very strong but expensive. Cheers, Don. 8-)

I have just uploaded a photo of one of the broken rods I removed from my G12CSR. Note that the rod failed at the stud anchor trunnion. Also uploaded a photo of Ed Crowell's CSR modified to 750cc by welding up a nodular crankshaft & grinding to stroke it. It is fitted with Carillo rods made shorter to compensate. Note the welded up inlets to mount dual carbs. The engine runs well & has been reliable. Cheers, Don.
JEAN-NOEL
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Re: New conrods for 650 (and other twins)

Post by JEAN-NOEL »

Hi Don,
thanks for your pictures.
Obviously the trunnion hole make a weakness point.
Cheers.
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GSAX1
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Re: New conrods for 650 (and other twins)

Post by GSAX1 »

According to the makers, in a 1950's interview, the conrods would eventually fail at the small end. After 50 years on full throttle :lol: :D :lol:.
Don Madden
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Re: New conrods for 650 (and other twins)

Post by Don Madden »

And not leak oil, too! :lol: :rofl: :lol: Cheers, Don. 8-)
mdt-son
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Re: New conrods for 650 (and other twins)

Post by mdt-son »

JEAN-NOEL wrote:I agree with you. I want not to fit my rods, even it they are fine, on my new crankshaft.
Hi Jean Noel,

i have thought of commenting this posting for a while. Prior to your comment above, Don Madden claimed AMC rods have a design which puts the caps in tension by cap bolts. I don't understand his statement, but then I do not have any genuine twin conrods for inspection (they had been removed from my engine by a previous owner).
Judging by pictures, the rods looks very traditional, except for the trunnion counteracting the cap bolt pull. Lower part of rod and cap will be under compression, unless there is something I have missed.
I do understand that the trunnion eye may fail though. The load path from the small end to the conrod cap is far from well designed. Furthermore, AMC tried to ease the stress by providing a trunnion for load distribution - by itself a good idea - but the rod material used didn't cope. It is not a creep resistant material.

Please let me know how you understand Don's statement. Anybody who wants to chime in are welcome.

Best,
Knut
JEAN-NOEL
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Re: New conrods for 650 (and other twins)

Post by JEAN-NOEL »

Hello Knut,
I agree with you :in both cases the tightening takes place in the same way. However, the solution recommended by Don Madden encompasses more material, but with the drawback of creating a new point of weakness by grinding the connecting rod body to support the BTR screws. According Don those who made the transformation seem to have been satisfied with it. On the photo posted by Don we can see that the rupture occurred at the level of the trunnion, but without knowing the cause. Over revving, lack of oïl pressure, ovalisation ? Do not forget that these connecting rods, with an excellent design, were designed for the G9 / M20, and remained identical in the G15 / 45, with much higher constraints ! Finally I have my engine rebuilt with original crankshaft and conrods, tested and refurbished by T & L.
Kind regards.



















































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