Demon con rods

Any comments on the failure of the 850 MK 111 con rods around 20,000 miles?

Practically unheard of. We use the standart conrods in racing engines that give the power of the 1970s works engines, and they have been in one engine for a decade with that motorcycle being ridden by five different riders. Must have done thousands of miles on the track by now! A friend of mine has repeatedly revved his 750 Commando to 8.500RPM in anger on the track, no failure either.

Standart production conrods are very strong, being forgings, far stronger than billet ones, and I have seen them twisted 90° and bent nearly 90° in customers engines that ran out of oil without them breaking.

Suspect are the pattern big end bolts that are being sold by big players in the spares scene. How to recognize them against propper forged ones you can find on http://www.andover-norton.co.uk/Pirate%20Parts.htm

Joe Seifert

Unfortunately the majority of spares buyers are likely to buy on price alone, and most are simply not interested in quality! In the case of something like a big end bolt, replacing the OE high standard part with a shoddy Indian made item, is far more likely to result in failure than re-using the original, and is an obvious example of the likely final cost of saving a little money!

Demon con rods.....funny
I guess at least you have heard of the infamous "D" rods.
Made by a different maker (during the decline of norton and installed in MKIII's) ....than the prevoiusly reliable supplier....
Les Emery told me the story of the one that broke the head off by smacking it on the edge of the counter. A two strike forging caused a stress riser at the joint...
The customer no longer was willing to use them in his norton build.
THe "D" rods are identified by a D in the forging mark .

dynodave said:

Demon con rods.....funny
I guess at least you have heard of the infamous "D" rods.
Made by a different maker (during the decline of norton and installed in MKIII's) ....than the prevoiusly reliable supplier....
Les Emery told me the story of the one that broke the head off by smacking it on the edge of the counter. A two strike forging caused a stress riser at the joint...
The customer no longer was willing to use them in his norton build.
THe "D" rods are identified by a D in the forging mark .

Click to expand...

It's all a myth.....according to ZFD.
post81531.html

Well you do eventually learn who to believe and not believe.
I do believe....
When in 1971, my 70 commando roadster blew up in upstate new york going approx 100MPH.:shock:
I do believe that was oil dripping out of the hole in the split crankcase, It locked the motor and rear wheel which promptly wore through 3 of the 4 thickness' of cords.
Only by finally being able to grab the clutch did I not go down....the speedo popped back up registering 55MPH then I coasted to a stop.
The crank and head were the only items that the dealer and Berliner Motors salvaged out of the pile of mechanical rubble...FWIW the rod snapped ...

This singular incident (which came so close to killing me) is one of the prime motivators for me to have taken on with a vengance, starting in 1988 to learn for myself as much as I could about norton heavy twins. I very quicky learned that there is a LOT of BS out there, which is why I'd rather research for myself rather than just hear and repeat what joe blow says...
Since then I've worked on almost every NHT variant 1949 thru 1975

I will use aluminum con rods if I know their history and am willing to replace them every season. I have seen a couple times where the con rod snapped and left the big end intact on the crank. Skin grows back slowly. I prefer Carrillo. Jim

Two piece D rod needs to be checked if that type that broke. As reported about unheard of for rod to break first, but all too common for some other item to let go before rods suffer as rest of engine. In order of over built Commando Items the rods top the list, then the Z-plates, them maybe the frame spine.

I've lost a couple rods, first by oil comma slotted piston let go, then a fractured rod bolt exam showed corrosion had entered a scratch and followed the metal grain interfaces across more than half way. Took me a while to figure this out. Close exam showed differences in color-sheen of both bolt fractures, 2nd one was shiny new all the way across. This seizure rolled the steel end cap up like chewing gum, much to the sorrow of the TS case integrity. BTW both of my rod damage events occurred under 55 mph and on low cruise power/rpm. First kept running-turning horribly till shut off the other .5 sec tinkle sound then instant rear lock up that clutch freed next instant.



There is an issue about the bolts seating down and shaving off a sliver of rod material that then cause stress riser that eventually pops. Solution is to exam and relieve the area around rod head to assure full clearance and flat seating clamping. I'll bet Old Brits site has tech article on this.

According to my own experiences, the experience of Nick Hopkins (with Andover Norton since it was founded in 1977), and another dealer I respect very much, and who is not unknown in racing circles, as a racer as well as a tuner, the "D"-rod myth is just that.
Spreading it makes one wealthier, as one will sell rods to people who in reality don't need them. According to Nick the myth was spread by a ham-fisted owner/racer in early days who needed an excuse for his inadequate spannership and the resulting blowups.
Neither I, nor Nick have a problem with using "D"-rods in our own Commandos.
Carillos are counterproductive in that they weigh too much. Balancing the crank with them is a nightmare, and we try too keep moving components as light as possible, don't we? Hence the standart rods in our current short stroke Commando project.
Joe S.

ZFD said:

According to my own experiences, the experience of Nick Hopkins (with Andover Norton since it was founded in 1977), and another dealer I respect very much, and who is not unknown in racing circles, as a racer as well as a tuner, the "D"-rod myth is just that.
Spreading it makes one wealthier, as one will sell rods to people who in reality don't need them. According to Nick the myth was spread by a ham-fisted owner/racer in early days who needed an excuse for his inadequate spannership and the resulting blowups.
Neither I, nor Nick have a problem with using "D"-rods in our own Commandos.
Carillos are counterproductive in that they weigh too much. Balancing the crank with them is a nightmare, and we try too keep moving components as light as possible, don't we? Hence the standart rods in our current short stroke Commando project.
Joe S.

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I agree about the d rod myth. At least in my experiance. The biggest problem I have seen has been bolts.
I like aluminum rods because of their weight although I have not found any problem with balancing a crank for the carrillos. A little more reciprocating weigh and a little less rotating weight. The aluminum rods are undeniably strong enough when new.
I have raced with both and ran the same motor back to back on the dyno and on the track with both and found no significant difference in performance and a very small increase in vibration with the steel. There is a lot more to be gained by using short- lightweight pistons in reducing reciprocating weight than there is with the change from aluminum to steel rods.
I worry about the aluminum rods just due to the fact that new forged rods are hard to come by and running used rods leaves me wondering just how many times they have been stressed before I got them.
I don't worry about aluminum rods in a street bike because the normal operating speed does not stretch them as much so they can endure many more cycles. I do however have Carrillos and steel flywheels in both of my streetbikes and like not having to worry about bouncing them off the rev-limiter. Jim

"I worry about the aluminum rods just due to the fact that new forged rods are hard to come by"

How so? We (Andover Norton) produce them all the time- latest batch just in this week.

Don't know if it applies to our low CR low rpm low power motors, but drag racers with maxed out engines prefer Al rods d/t the elastic compliance over steel to help protect rest of the power pulse transfer components.

After reading all the reports form decades back onwards, seems Norton rods are build so well they don't even suffer the expected Al stress cycle fatigue fracture lessor Al rods are know for.

ZFD said:

"I worry about the aluminum rods just due to the fact that new forged rods are hard to come by"

How so? We (Andover Norton) produce them all the time- latest batch just in this week.

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That is good to know. I have not bought a new set for probably close to 20 years and had been led to believe that they were hard to come by . Probably should have asked John H. How is the price in comparison to carrilos? Jim

Old rods sets available form many vendors yet. I had one set of Norton rods magna fluxed or similar, then cryogenic chilled with hope to thermally compact the metal grains, then shot peened by shop Geoff Collins used in Canada. Don't know if a benefit or not for long term factory Combat sane safe public usage. Jim's Carrello rods are installed in Ms Peel big block now but prior to JIm's offering plan was to re-use Al rods on boost.

I use to have a picture of my racebike hanging on my office wall after an unfortunate incident in Michigan. It shows the cylinder and head hanging from the head steady. Two con rods hanging out of the bottom of the cylinders with one half of the crank hanging from each one. No flywheel and no cases except for small pieces still clamped between the mounting bolts. The chronometric tach was stopped at just over 8200 rpm.
I always figured that was a pretty good testament to the strength of aluminum rods. jim

Latest rods are 168.50 Pounds Sterling nett. Not sure what Carillos are and really that was never a question for me- I saw no need for them.
Joe S.

Well that would put them a little less than last time I bought Carrillos. Glad to know you can supply them. Now I wonder if my old supplier was pulling my leg or if they were hard to come by in the late 90's. Jim