Piston Weight and Balance Factor?

The nice thing about the engine is that for road use you really don't have to worry about these problems- unless you run at 90mph most of the time.
I read about one bike being used that way in the UK, 2-3 hours at a time at very high speed. The drive side case cracked.
My aging body cannot take that prolonged exposure to the wind blast, 70-75 sustained is about it for me. Also, here in Canada they will take the bike away if you ride at 90mph all the time.

It's interesting to read this stuff though. You start to realize that the engine is quite durable for normal road use but really doesn't fall into the "overbuilt" category.

Glen
 
We've all heard about over revved Notons blowing up with stock parts and many of us have experienced it personally. But if you reduce the reciprocating weight by 1/3 and reduce stress even further by improving the rod stroke ratio - then those problems tend to disappear.
 
Is there a list of parts that make a Norton engine overbuilt? Would a 920 kit be on that list? ;)

Using a little common sense with parts selection and 60 years or so of experience wrenching it's not hard to build a strong motorcycle or car engine that will hold up on the street. Especially with the parts selections one has to work with today. No engine can endure abuse for very long though.

My Norton engine is almost overbuilt for the old cases, so I can't run it at high RPMs for long. It does tolerate 7400 RPM, but now that it is geared up a bunch I probably won't see much over 6800 rpm anymore. And that will only be when getting on a HWY on a long entrance ramp with no cars on it. Not racing and using common sense on the street it should stay together.

Time to jump on the stationary trainer. An old man's got to get in some cardio.
 
"Is there a list of parts that make a Norton engine overbuilt? Would a 920 kit be on that list? ;)"


Here's my thinking with my 920, which may or may not prove to be correct.
Build the bike as light as possible. So far there has been no downside to that.
Forget about striving for more maximum bhp. That tends to mean big cam and maybe carbs, head work and higher rpm. This is ok to do with aftermarket stuff but , in my mind, not a good plan when on a budget and using mostly stock Norton engine parts.
If kept to 6500 rpm, stock rods are plenty strong enough for the job and they are very light. The rgm pistons are about 10 grams lighter than stock 850 pistons, so not a bad combo. Plus I had all this on the shelf.
When it comes to rod ratio, I go with this thinking - the stock Norton ratio is already ideal for street use.

The idea is to pull a lot more hp out of the midrange, and the stock 850 is already quite decent there.
I think it's the safest place to make extra grunt with these engines and it's also the most useful on road grunt.
So the comp is boosted as far as I dare go (mountain riding in mind) and the engine is 11% bigger than the 850.
The end result is good, if it holds together.
The bike is smooth and the light weight is a treat.
On Dyno hill the 850 , stock other than open peashooters, managed to accelerate from 100 to 110 kmh in top.
The 920 with the same gearing managed 100 to 135 kmh, 3.5 times as much acceleration.
That is a big improvement in upper midrange power, 4 to 5000 rpm. I added weight to the bike and determined that just over half of that boost came from the 850 to 920 modification and compression boost.
Now to see if it all holds together.


Glen
 
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In the dim distant past, a mean piston speed of 4000 ft/min was considered the max speed to run any engine at. Even the Formula 1 GP 3.5 litre v-10 engines revving to 18000 rpm kept to this limit. On a Commando it is just under 7000 rpm.
 
jseng1 said:
We've all heard about over revved Notons blowing up with stock parts and many of us have experienced it personally. But if you reduce the reciprocating weight by 1/3 and reduce stress even further by improving the rod stroke ratio - then those problems tend to disappear.
Click to expand...
What do you believe would be the optimum rod length to stroke ratio for a Commando engine. I am really impressed by my 850 motor. I never believed in it, but it does what it needs to do very well. I would never have thought that such a horrendous bottom end could spin so high reliably. My balance factor is about 72%. So the motor runs dead smooth at anything over 5000 RPM. But the rod length to stroke ratio must be good.
 
worntorn said:
"Is there a list of parts that make a Norton engine overbuilt? Would a 920 kit be on that list? ;)"


Here's my thinking with my 920, which may or may not prove to be correct.
Build the bike as light as possible. So far there has been no downside to that.
Forget about striving for more maximum bhp. That tends to mean big cam and maybe carbs, head work and higher rpm. This is ok to do with aftermarket stuff but , in my mind, not a good plan when on a budget and using mostly stock Norton engine parts.
If kept to 6500 rpm, stock rods are plenty strong enough for the job and they are very light. The rgm pistons are about 10 grams lighter than stock 850 pistons, so not a bad combo. Plus I had all this on the shelf.
When it comes to rod ratio, I go with this thinking - the stock Norton ratio is already ideal for street use.

The idea is to pull a lot more hp out of the midrange, and the stock 850 is already quite decent there.
I think it's the safest place to make extra grunt with these engines and it's also the most useful on road grunt.
So the comp is boosted as far as I dare go (mountain riding in mind) and the engine is 11% bigger than the 850.
The end result is good, if it holds together.
The bike is smooth and the light weight is a treat.
On Dyno hill the 850 , stock other than open peashooters, managed to accelerate from 100 to 110 kmh in top.
The 920 with the same gearing managed 100 to 135 kmh, 3.5 times as much acceleration.
That is a big improvement in upper midrange power, 4 to 5000 rpm. I added weight to the bike and determined that just over half of that boost came from the 850 to 920 modification and compression boost.
Now to see if it all holds together.


Glen
Click to expand...
I totally agree with your approach to tuning a Commando. A lot of people look at what has been done with modern bikes and try to do similar, when they really need to look back and understand what worked. You can go larger capacity, higher compression ratio and hot cam, but many people do not know how to get the most out of an unchanged motor.
In the olden days, in Allpowers A grade road races, Manx Nortons were the go. In 1963, I watched Jack Forrest trounce the lot at Bathurst with a Norton 650ss which was probably stock standard. He was an international rider, but with that bike he would have been right behind the 8-ball. At the time, mate was a top A grade rider - he hated Forrest because of his big mouth.
 
A light weight 920 would be fun.

The long rod kits work in conjunction with a bunch of other over engineered parts. I've used the JSM long rod version and the MAP long rod version for Norton 750 engines. Also used stock. There are real benefits to using the thinner stronger rods and lighter pistons with a lighter weight stronger crank in a smaller displacement engine. Not budget friendly though. It would more than likely be cheaper to build a RGM kitted 920 with stock bottom end parts and get all that bigger displacement torque on a street ride.

The warden is calling. Gotta go
 
By my tally, the current cost of a complete ultimate 920 engine would be around $27,000 CDN with a TTi box included.

I'm not sure it would be all that much more fun on the road than the low cost version I have here.
If we ever get a racetrack operating here again, different story.

Glen
 
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'Budget friendly' is to work with what you have, However I would start with the gearbox rather than the motor. A Commando motor has plenty of go, but with the wide ratio gearbox cannot deliver it's best. Most Japanese bikes have 6 gears, and an engine management system, these days. It is not really about hot cams, bigger capacity, and race cams. What is in a Commando motor is enough. The latest Royal Enfield probably has fuel injection.
 
Motorcycles for me have always been about making changes. I can't remember every leaving one as it came from the factory. Never sold one that anyone was unhappy with. My P11 however is a little different. I can understand how some, maybe a lot of somes, would hate it on the street. Can't please everyone, and wouldn't be having any fun if I tried.
 
worntorn said:
I quite like the standard 4 speed, thanks.
I have 2 modern 6 speed bikes. I must be lazy, don't care for all that shifting!
It's a treat to get back on the Commando with its 4 gears and wide spread of torque.

Glen
Click to expand...
For road use, I would rather ride a Commando with a 4 speed box, than a modern Japanese bike with 6 gears. The thing about a Commando, is when you change it wisely, you get a result. And that is fun. The reason I built a two-stroke, won a race with it and the immediately sold it, was because it had no soul. A Commando-based motorcycle has character. I like riding my Seeley 850 because of the way it behaves - it feels excellent. - 'Some things are so bad that they are good' ? It appeals to my sense of humour.
Yesterday when I was down the street, there was a Ford Falcon XA GT parked there. I had forgotten they even existed. Back in the 1970s, I actually drove one. It was a really pleasant experience. I feel sorry for our kids - that world has gone.
My wife has a friend who has a modern Ford Mustang. She lives in NSW. She sometimes gets it out onto a backroad and really gives it the berries. I understand why she does that. I think it might better if it had a Tremac gearbox.
 
acotrel said:
For road use, I would rather ride a Commando with a 4 speed box, than a modern Japanese bike with 6 gears. The thing about a Commando, is when you change it wisely, you get a result. And that is fun. The reason I built a two-stroke, won a race with it and the immediately sold it, was because it had no soul. A Commando-based motorcycle has character. I like riding my Seeley 850 because of the way it behaves - it feels excellent. - 'Some things are so bad that they are good' ? It appeals to my sense of humour.
Click to expand...

The Norton appeals to my delusional side. Plus, I get a thumbs up once in a while riding on the street. Must be because it is Candy Apple Red and looks like an antique. I think it is a thumb... Could be a middle finger I suppose. :)

The Japanese 4-stroke bikes I owned were mostly 5 speeds. I had one Japanese bike that was a 6 speed, a '90 FZR400. The short throw buttery smooth shifting gearboxes in the Japanese bikes I owned were better suited to my shifting style than the AMC 4 speed gearbox is. However, the AMC gearbox is a good box considering how long ago it was designed. I got results when I turned the throttle on all of them even the Norton.
 
The only modern bike I have ridden was a VFR400 Honda. It was excellent, but I did not realise why it would not go faster than 180 KPH. I was riding it at that speed on a winding country road. But I had lost all consciousness about speed limits. I when I approach a corner, I always know when I can get around it without crashing. But on public roads, that is not relevant. If I had a modern road bike, I would lose my licence very quickly.
When you road race, your mentality becomes different. I like the freedom it offers.

 
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