Con rod choice?

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jseng1 said:
.........alloy rod out of the "yellow peril" racer .....

Con rod choice?
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Wow is it just me, or is that about the most shallow piston ever? :shock: doesn't look as though the wrist pin bore could get any closer to the bottom ring groove, and looks like a pretty thin top land too, boy it's gotta be getting really hot up there doesn't it. Sorry for going off topic gillou, but that piston seems extreme to me.
 
okie-dokie then tintin, there's less skirt and well just ...less.... on that piston for sure, thx for posting. Cj
 
gillou said:
Hello,

I have to buy two new conrod to rebuild my engine race. I have seen R&R all alloy conrod. What do you know about this?

Gilles
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I installed a set in my Atlas when I rebuilt it. I had read that Atlas con rods were prone to failure so I wanted to replace them with Commando rods. Unfortunately I couldn't find a decent set of Commando rods at a reasonable cost and I discovered the R&R rods on the M.A.P. website.
They are beautiful things, finished like fine jewelry. The end caps fit so seamlessly that it's hard to find the joint.

The downside I later discovered is that the big end is larger than stock rods and will foul on the inside of the crankcase. I had to use my Dremel to slowly and carefully remove material from a couple areas inside the crankcase halves. It was a bit tedious. They haven't broken or shot through the crankcase yet, and admittedly I don't run the old girl very hard. I've no idea how they would hold-up in a racing engine.

One thing I noticed was the lack of an oil spray hole. I did a little research and read that the spray hole was unnecessary and that racers would often block the oil hole by swapping the bearing caps. It seems like Carrillos are the way to go for racing if you can afford them.

Con rod choice?
 
From what I've read about Jim Schmidt's long rods and light pistons, I think that is definitely the way to go, even if you don't race. The more weight you can remove from the little end the better. It improves both performance and reliability. Before I read about Jim's stuff, I'd looked at building a 750cc motor using 73mm Honda Fireblade pistons with the tops cleaned. Because I was too dumb to think of longer Carillo rods, it would have meant making shorter barrels to get the deck height right. The piston weight in my 850 is a big worry to me, the motor hasn't let go yet however I'm always expecting it to happen. My whole approach has been to improve mid-range torque rather than have to rev the motor high to get it going. As the acceleration rates within the motor rise, the increase in the internal loads is a squared relationship. It is interesting to ride a bike before and after fitting lighter pistons, their effect is quite noticeable in the way the bike accelerates, and vibrates less.
 
acotrel said:
' I've also had a stock steel Norton rod break in a short stroke 750, primarily due to the extra acid dip lightening that the factory mistakenly tried for weight saving.'

Are you saying the failure was due to hydrogen embrittlement ?
I use aluminium rods in my 850 motor, however the piston weight worries me. With the rebalanced crank and the way the motor is set up using methanol, it spins up extremely quickly as it comes up through the gears, even when the overall gearing is high. I always try to keep the revs below 7000 RPM, however it is not easy. Piston weight and those aluminium rods were the main reason I was always reluctant to race the bike. I simply did not believe in it. In fact the only reason I raced my 500cc short stroke Triumph for all those years was that it was almost indestructible.
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No. I think the failure was because they removed too much material from the rod beam when they lightened it, and it just wasn't strong enough for serious race use. Mick Ofield told me that it was a well know problem with these rods, as modified for racing by the factory race mechanics. Still, this particular pair survived many races, including several at Daytona, which seriously stresses engines, before they expired. Might have been ok if I had replaced them regularly. I didn't think that was needed, because they were steel. I did have them regularly magnafluxed to check for cracks. We used a 7500 rpm red line for the short stroke engines, as recommended by the factory, so I don't think it was a case of over revving.

Ken
 
I once got hold of a supper low mileage late crank and rods. I had the rods crack tested then polished them before having a kind of shot peen process done on them.
I fitted the crank and rods into a Triumph race engine that made around 75 rwhp. It had been successfully running a Norton crank and rods for a few seasons and I thought it deserved replacement.
The engines total use was a bit of running in and one hard day on a dyno. Then a rod broke on a practice lap!
Hence mine now has JS Carrillo rods !!
 
I've been keeping track of the rod failure reports to see every single one of them had some pre-existing known fault or racer lightening weakness over factory issue or had bad bolt or crank flex or over heated bore piston binding also attending BUT for Fast Eddie's repeated reports that leaves a mystery to me - suspecting some other component failed to control its loads so transferred excessive bind loads to rod that failed but not because of its own innate weakness if rest of engine behaved. Seriously just trying to understand how risky we are to spend beyond Norton rods even in 70 hp 8000 rpm 360' twins. Summarizing Norton racer rod reports seems like more life of hi stress survival use than any other brand engine rods so even if they are breaking d/t plain ole fatigue they are still about top of the world list in factory issue endurance-mass-cost quality.

I had Peels aluminium and non ferric parts cryo tempered in semi-educated
*belief-hope* that the severe thermal contraction might help pack metal grains tighter like a type of internal shoot peening & some years later find references that show indeed it does help ally toughness some and may be enough to get away with Norton rod savings.
 
cjandme said:
jseng1 said:
.........alloy rod out of the "yellow peril" racer .....

Con rod choice?
Click to expand...
Wow is it just me, or is that about the most shallow piston ever? :shock: doesn't look as though the wrist pin bore could get any closer to the bottom ring groove, and looks like a pretty thin top land too, boy it's gotta be getting really hot up there doesn't it. Sorry for going off topic gillou, but that piston seems extreme to me.
Click to expand...

Yeah those pistons are shallow all right - a little too shallow for me. The Yellow peril race team wanted to bring down the piston weight as much a possible. They actually contacted me for bushless steel rods but then they decided to go with the alloy rods which was too bad because I would have loved for them to have a set of my rods in their racer. The aluminum rod cap expands when hot and loses oil pressure and my guess is that the hammering with the extra clearance broke the rod bolts. Looks like the skirts were damaged when things came apart.
 
Hortons Norton said:
Must have been a really short stroke! :roll:
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I have also wondered about that. Do you need a longer piston skirt because the stroke is longer and the motor produces more torque, or is the skirt length an arbitrary thing ? Does the rock in the piston affect the maximum piston speed before the rings flutter ? The standard commando is close to that already.
 
If I decide to order con-rods Carrillo, where I can order them and which price?
 
Longer rods for less angle change and smaller bores need less skirt for the long haul but anything goes in racer experiments and the photo example looks un used as yet. Don't understand the short stroke 7500 red line unless was do to crank material worries. I've read of more cast iron flywheels breaking in past than rods. These were mostly street users reports, one at warm up idle that rods couldn't take.
 
John Hudson: I have never seen dural rods shot-peened at the work:. For racing they were polished all over and if the
steel caps did not match the contour of the outside of the rod they were filed to shape before polishing.
Everyone misses a very important point in Norton con rod bolts -as supplied they have a razor sharp edge on the
underside of the head and this should be carefully filed off and the bolt rotated in lathe chuck or drilling machine and
the head polished with emery tape. Otherwise, the turning torque of the Phillidas nut causes this sharp edge to scrape a
shaving of dural off the side of the eccentric recess in the rod as the bolt is pulled down which, of course, then remains
under the head of the bolt and prevents its seating and tightening properly. This can have happened on original
assembly at the works, so whenever bolts are removed from the rods, check that there is not already a shaving in the
-
bottom of the recess. > >
BIG ENDS: Big ends wear very slowly if oil is cared for. After a main bearing job the shells should be changed
because the hard bits of bearing circulating with the oil become embedded in the soft shells and can score the crank. At
the same time, of course. the crank should be dismantled and all the sludge scraped out.
 
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