Guaranteed To Blow Up... when??

I like to go fast. At what point am I really gonna blow it up?

At 7000 rpms, my bike is doing 122mph according to the calculator. Can I actually do 122mph without blowing up my bike?? really?? I'm just wondering if there's a "line of death" in the sand regarding the average stock norton commando without any lightened pistons, rebalanced crank, or short stroke conversion...

Once I burried the needle at 135mph on my 79 honda 750F going down a long hill on the hiway. (with a girl on the back if you can believe that... I was young and crazy)

You are no more likely to blow the engine at 7000 rpm in top gear, 122 mph, than blowing it up at 7000 rpm in 1st gear , 45 mph.

When the valves float, the engine reaches peak rpm. The worst damage that will likely occur is a piston nicking a valve. That can be bad enough. but blowing the engine connotates a rod letting go, or a piston slinging the crown off. Our Nortons can can hold together for more than 7000 rpm, but it requires mods to keep the valves from floating to reach the rpms that can do severe damage.

Think of valve float as a "safe" limitation to over-revving the engine into the range of severe mechanical failure.
If one modifies the valve train to get higher rpms, then one should (must) take the next step and control the centrifugal forces by employing lightweight pistons and rods, or short stroke mods.

For most of us running basically stock engines, up shifting at 1 rpm less than valve float, will keep our engines intact.

Slick

o0norton0o said:

I like to go fast. At what point am I really gonna blow it up?

At 7000 rpms, my bike is doing 122mph according to the calculator. Can I actually do 122mph without blowing up my bike?? really?? I'm just wondering if there's a "line of death" in the sand regarding the average stock norton commando without any lightened pistons, rebalanced crank, or short stroke conversion...

Once I burried the needle at 135mph on my 79 honda 750F going down a long hill on the hiway. (with a girl on the back if you can believe that... I was young and crazy)

Click to expand...

Sure, if it's in good condition it will be safe at 7000...for a while.

At engine speeds over 5000 rpm there is a lot of flexing of the aluminum cases and rods. The more they flex, the sooner metal fatigue will set in and they will fail sooner or later. At 7000 rpm the life will be pretty short.

Somewhere between 7500 and 8500 the cast iron flywheel will explode. The last one I exploded up was at 8300 rpm. [tattle tail tach] Jim

I would suggest that it should be safe for the odd squirt up to 7,000. And provided you 'pamper it' with top class oil and regular changes, are fastidious with your maintenance, and happy to accept replacing things more regularly than a steady rider, then it should do so for many years to come. Enjoying the bikes acceleration in this way should be fine IMHO.

Sustained high rpm is another matter though. It has an exponentially higher impact due the cumulative effect of the flexing and stressing that Comnoz mentioned, plus, of course, temperature.

That's why there is SO much difference between building a 'normal' short circuit race bike vs a race bike for the Isle of Man TT, or Bonneville. Building an engine to sustain WOT, in top gear, with the engine under maximum load, for a prolonged period, is a different ball game.

The Commando engine is essentially a 1940's design. In !940's Britain (no motorways etc) it was simply not possible to ride a bike like that on the road, there were no roads where you could sustain max speed like that, so designers didn't design them to!

Don't forget that the manual sez/recommends no more than 5800 rpms for continuous cruising.
Whatever that means....

You don't say what year you have either, not that it makes much difference.
The stock cam runs out of puff much above 6500 rpms though, so unless you have a Combat,
or a steep hill, that 122 might only be a barely reachable zone on the speedo.

When I was young and the new owner of my 75, I was going for top speed to see what she would do. As I approached 7k it made a terrible clacking sound when the piston hit a valve, bent it and broke the cast iron valve guide. Had to call my wife to come get me.

No redline for me since then and absolutely no trouble.

Jaydee

Aluminium alloy will only take so much strain before failing, once it hits that then it's over. Have a Briggs engine on a Lawn tractor, blade hits stone and tractor engine stops dead, blade is bent, so restart engine and go 100 yrds and park in storage, remove and replace blade, go to restart engine and con rod breaks before it fires. So the rod was taken to just before failure and then lasted for 3 mins before failing at 200rpm not the 3600 rpm running speed. So you can take a Norton to 7000 + rpm and sow the seeds of con rod failure but the time of failure may be just when kicking over the engine.

The magazine Motorcycle Classics had an article two years ago, IIRC, about a Texas tuner that brought a '72 Combat to Bonneville. The Norton was run in a stock class, open the link for the story:

http://www.motorcycleclassics.com/class ... x?PageId=1

I totally agree with Fast Eddie, so I'd be very interested in knowing what the builder did to provide the reliability they got at Bonneville.

I suspect that some number of posters must know of this team/tuner??

A buddy of mine bought a new 72 Combat Interstate in England and rode it through Europe on a vacation. He had main bearing failure somewhere in Italy. The factory covered it under warrantee of course. This was a brand new bike, barely broken in.

But then, I have 40,000+ miles on my 850 and haven't yet checked the gearbox layshaft bearing sooo... I guess we sometimes takes our chances.

If you run a pre 76 brit bike use this handy calculator:

"If you beat it will break. If you REALLY beat it, it will breaker faster and more extensively.
If you drive it like it might break, it will last quite a long time"

Trust me on this.

Suggest you enter a 24 hour endurance race and find out!!!
I do not believe the factory ever managed to put one together to complete a 24 hour endurace race but a factory employee (Mr Negus) did and his private entry Commando finished 2nd(averaging over 100 mph?) to John and Charlie Williams on a 750 Honda 4 in the 1973 Spa 24 hour endurace race .... a feat which apparently upset a few people in the factory and one headline in one publication was ' Employee beats factory'..... I heard that the factory race team manager would not speak for some to Mr Negus and others involved with the private entry bike!! Bet 'some' people stuck up copies everywhere to **** off the race shop!! (I would of done..BIG copies!!)...... I was told by one Gentleman involved that the factory could build motors that produced lots of power but which would not do so for very long!! The Richard Negus motor broke the crank shortly after the Spa 24 hour race.....not an uncommon failure as the factory often left a stress raiser inside the cranks to help them break more easily and I can assure you when they do fail they do so BIG TIME and make one expensive mess of the motor.....
Our experience with our short stroke 500 orton twin decades ago showed that the std Norton valve gear will work happily to 8,000 in cojunction with a std 650ss profile cam IF the valves are std size, the std springs are in good codition and set up correctly although for how long I have no idea....they were NOT originally designed to do so in the 1940s when the Norton Twin was designed as a 500cc twin to get people to and from work along with use at weekends for a bit of gentle touring etc revving to 5000 rpm max on the pool petrol then available..

Fast Eddie said:

The Commando engine is essentially a 1940's design. In 1940's Britain (no motorways etc) it was simply not possible to ride a bike like that on the road, there were no roads where you could sustain max speed like that, so designers didn't design them to!

Click to expand...

Hmmm, I think your forgetting racing which was a constant for Norton.

A lot less knowledge of materials and construction rather than design...a lot has been learned in 75 years. Use might have been a factor, but if they would have known how to do I think they would have...would have been a huge competitive advantage.

dennisgb said:

Fast Eddie said:

The Commando engine is essentially a 1940's design. In 1940's Britain (no motorways etc) it was simply not possible to ride a bike like that on the road, there were no roads where you could sustain max speed like that, so designers didn't design them to!

Click to expand...

Hmmm, I think your forgetting racing which was a constant for Norton.

A lot less knowledge of materials and construction rather than design...a lot has been learned in 75 years. Use might have been a factor, but if they would have known how to do I think they would have...would have been a huge competitive advantage.

Click to expand...

You are quite right, but their racing focus then was predominantly the Manx. The Dominator was a 'built to a price' road bike. A different kettle of fish...

Fast Eddie said:

dennisgb said:

Fast Eddie said:

The Commando engine is essentially a 1940's design. In 1940's Britain (no motorways etc) it was simply not possible to ride a bike like that on the road, there were no roads where you could sustain max speed like that, so designers didn't design them to!

Click to expand...

Hmmm, I think your forgetting racing which was a constant for Norton.

A lot less knowledge of materials and construction rather than design...a lot has been learned in 75 years. Use might have been a factor, but if they would have known how to do I think they would have...would have been a huge competitive advantage.

Click to expand...

You are quite right, but their racing focus then was predominantly the Manx. The Dominator was a 'built to a price' road bike. A different kettle of fish...

Click to expand...

I think it's a mix of all the factors, for sure they didn't need to rev high and go fast because no one else could...so goes the design evolution. It's funny when we look at our bikes and they were designed for the purpose and needs of the time, and at the time they were quite good...served the purpose longer than anyone probably thought they would...now they are pretty ancient designs compared to modern bikes...and then we want them to do things they weren't designed to do. It's fun and amazing what people have done to improve the old design...but then there are limits as this thread points out. Should we run a basically stock bike at 7000 RPM for an extended period? No. Heck I'm afraid to rev mine above 6000 for very long. I sort of look at it as respect for the old girl.

I've found that with my 850, it will go straight out the top of the 'safe' rev range. On a race circuit, I always try to change up below 7000 RPM, however usually see a bit over as I make the change. What worries me is the piston weight and the aluminium rods, - not valve float. From what I have read and the experience of my friends, the crank bearings and cases will cop 7000 RPM regularly, as long as it is not sustained. The cases I have which are cracked through the drive side main came from a bike which often saw 8,000 RPM. My Seeley 850 doesn't get ridden very often, which is fortunate - I believe it just has to explode one day. What I would say is that for extended crank life, the balance factor might be important. If the motor smooths out at high revs, the consequences might be better - even with isolastics.