Upgraded Conrods?

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Oct 28, 2024
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Greetings all!
Finally getting to the rebuild. Cylinders are off and on their way for a 20 over bore. Looking at the conrods. I'm in my 60s and just want to ride the bike casually to enjoy it. Backroads. Nothing crazy.

While I'm in there, should I replace the conrods after 50 years? New conrods? MAPs?

Thanks in advance!
 
How long have you owned said bike? Depends on how it was ridden, but generally speaking casual riding won't break stock conrods. That's my opinion though and I'm thinking the doom a gloom crowd may disagree. :)
 
Excellent point! I have no idea. Bought it on EBay as a basket case. It's pretty clean looking but I'd rather be safe and do what needs to be done while I'm in there. Maybe just new stock rods?
 
Excellent point! I have no idea. Bought it on EBay as a basket case. It's pretty clean looking but I'd rather be safe and do what needs to be done while I'm in there. Maybe just new stock rods?
Given your intended use, polished stock rods will be more than enough.

New stock rods would be an extra precaution if you’re that way inclined.
 
As a rule, Norton rods have Reputation of being plenty stout. Polish out any nicks.
You can also have them shot peened.
Except for the hard corp guys, stock rods usually go right back in.

You can make yourself crazy on this rebuild, remember this.
A properly assembled, well tuned stock Norton can run plenty strong.
 
Your rods are more likely to break from the big end seizing than any weakness in the rod. As long as you use new shells, the journals are sized correctly, good oil is used and the oil pressure is good then the old rods will be fine.
 
Stock rods will probably hold up but they put more strain and vibration on the cases and crank than the longer rods. If you buy aftermarket rods you might consider the quality and where they are made (US, UK, China)
 
Why do stock rods put more strain and vibration on the cases? Let's assume they have been balanced before being re-installed.
 
While I'm in there, should I replace the conrods after 50 years? New conrods? MAPs?

Thanks in advance!

MAP rods are good if you are set on getting new rods. They also sell long rod and piston kits.

Alternatively, get new conrod bolts for your stock rods if you do clean them up and reuse them.

A dynamically balanced stock crank, rods, and pistons Norton engine is as smooth as anything else for casual riding.

By the way, I've had MAP, JSM, and stock pistons and rods in the same 750 engine. Can't tell the difference at lower RPM and cruising. The long rod stuff is nice at really high RPM.

Edit: MAP Cycle long rods and pistons are/were made in the USA as far as I know. JSM long rods and standard rods are a better design than the MAP long rods. Everything in a Norton engine rattles eventually. My current 750 engine has the JSM long rods and pistons in it. It is not quiet, but being a old Norton seems typical to me especially with a SS profile cam in it that requires more lash than a stock cam. The engine sounded the same with the stock rods, pistons, and SS cam in it. And the stock rods and +.020 pistons I took out of it when I started playing around with the engine 5 years ago look fine and the engine was pushed as hard it would go back in the 1990's. Why did I start messing with it? Because I know how to mess with engines and it gave me something to do with the last motorcycle in my garage.
 
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Lighter pistons are better for higher revs, and help save rods. But Norton pistons are not heavy. Jim Scmidt's longer rods and lighter pistons are a good idea. But the standard items are pretty good.
 
Why do stock rods put more strain and vibration on the cases? Let's assume they have been balanced before being re-installed.
The stock rods are too short for the 89mm stroke and the rod to stroke ratio is too low. Out of all the Brit bikes - BSA & Triumph etc the Norton has the worst rod stroke ratio. The angle is too severe at mid stroke and it puts too much side load on the piston skirt to cylinder. The reveral jerk is too vicious with the short stock rod. The longer rod smooths out the reversal jerk because there is more dwell time near TDC. The ideal rod stroke ration is closer to 2 to 1 as you see with the Triumph and this is why stock solid frame Triumph and BSAs are smoother than solid frame Nortons. See the rod stroke ratios below. Note that the lightweight JS pistons for the longer rods are much lighter than stock at 185 grams for a JS lightweight piston compared to 315 grams for a stock 850 piston. The JS pistons are US made high quality and do not collapse and rattle like some cheaper pistons for long rods.

*******************************

ROD RATIOS



Norton 750/850 5.875” rod with 89mm/3.5” stroke = 1.67 to 1 ratio

Norton JS long 6.4” rod with 89mm/3.5” stroke = 1.83 to 1 ratio

Norton factory short stroke 6.2” rod with 80.4mm/3.16” stroke = 1.96 ratio

Norton short stroke JS long 6.57” rod with 80.4mm/3.16” stroke = 2.1 ratio

Norton 1007 stock 5.875” rod with 93mm/3.66” stroke = 1.6 ratio

Triumph 650 6.5” rod with 82mm/3.23” stroke = 2 to 1 ratio

Triumph 750 6” rod with 82mm/3.23” stroke = 1.86 to 1 ratio

BSA 650 6” rod with 74mm/2.91” stroke = 2 to 1 ratio

Harley XR 750 6.4” rod with 76mm/2.98” stroke = 2.16 to 1 ratio

Modern F1 race bikes use rod ratios between 2.2 and 2.8 to 1
 
Yeah but the stock set up works just fine in a road bike. My mk3 engine is all original internally other than some replaced head studs and head gasket. At 50,000 miles and fifty years old, no probs at all, it's a nice running machine.
And those stock rods are the lightest rod available, last I looked at least.

Glen
 
Same as Glen have owned my Norton for 50 years soon and was an everyday rider for most of it's life average 6k to 10k miles a year and (till 2013, now semi retired but ridden regularly) I am still running with the original stock conrods with 40 oversized Hepelite pistons, 2S cam grind balanced crank and port work, also running original valves and I don't pussy (have never pussied it since new) I always ride it hard when out and about.

Ashley
 
Stock rods will probably hold up
"Probably" is an understatement given the life of our racer's conrods. To top it they survived two (2) crankshafts that broke their ends off in 30 seasons of racing, both times with no further damage to other parts in the engine.
I say the standard production forged conrods in a Commando engine are virtually indestructible in any road Commando and changing them out of caution is completely pointless.
All broken conrods I have seen in 45 years in the trade were due to no oil, normally the usual "I forgot to open the oil tap", or very rarely because the bike was started with no oil in the oil tank. Yes, saw that, too!
 
Your rods are more likely to break from the big end seizing than any weakness in the rod. As long as you use new shells, the journals are sized correctly, good oil is used and the oil pressure is good then the old rods will be fine.
Always bears repeating.

The rod breaks AFTER the bearing fails.
Stronger rods delay that catastrophic failure a few milliseconds.
 
With a Commando engine, we are working with torque - not top end power. Longer rods give less torque, however to discover how much torque you have, you must raise the overall gearing. Close ratios with high overall gearing work best when you have a lot of torque. Jim Scmidt's long rods and light pistons are a good idea for a road race bike. Light pistons improve the balance factor and help the motor to spin up faster. Longer rods work better at higher revs. My motor operates between 5,500 RPM and 7300 RPM. The standard rods are excellent. If you lower the gearing with a Commando motor, the motorcycle will not accelerate faster - it is counter-intuitive. With the heavy crank, it is possible for the rider to use too much throttle, when using quick taper needles in the carburetors. Intuition tells us that whacking the throttle open feeds more mixture, but more mixture does not mean faster, if the mixture is too rich. I use slow taper Mikuni needles in my Mk2 Amal carburetors. I ride the bike as though it is a two-stroke, except for the handling. You cannot accelerate very hard in the middle of corners with most two-strokes, because of their power-band. Most two-strokes adopt a lot of lean in corners -the older ones handle better when they have more travel in the rear suspension, and reed valves. If I race, I take notice of what is on the start line around me. Big four cylinder bikes often end-up struggling in corners, and are usually out wide at max lean in corners. Being passed near the end of a straight is not the end of the world.
 
"Probably" is an understatement given the life of our racer's conrods. To top it they survived two (2) crankshafts that broke their ends off in 30 seasons of racing, both times with no further damage to other parts in the engine.
I say the standard production forged conrods in a Commando engine are virtually indestructible in any road Commando and changing them out of caution is completely pointless.
All broken conrods I have seen in 45 years in the trade were due to no oil, normally the usual "I forgot to open the oil tap", or very rarely because the bike was started with no oil in the oil tank. Yes, saw that, too!
And, it's amazing that the small end runs directly in the conrod, and they don't seem to wear out! There could be other reasons why you would want different, but old is not one of them IMHO.
 
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