JS lightweight pistons and rods

This is a real world shout out for JS pistons and rods…

I’ve now built 3 motors with JS rods and pistons, 850, 920 and the most recent 1007.

As a Triumph and Nourish guy in the past, I have always been of the uneducated opinion that the Norton rod is too short for the stroke. Triumph only went from 6.5” to 6” to fit the motor in the OIF, and that was with a 82mm stroke. Nourish definitely preferred 6.5” in his bigger motors.

Under the tutelage of Dave Degens I’d spend hours shaving grams off of pistons and gudgeon pins, he was adamant that anything saved here led to more power at the wheel, just as he was taught by Jeff Monty and Sid Lawton.

So, with that background, the weight saving with JS parts is really quite mind blowing for me.

My 920 pistons are actually lighter than stock 750 pistons. In my recent 1007 build, the entire reciprocating weight (that’s the small end of the rods, pins, pistons with rings) for both pistons was actually over 1/2 lbs less than the ‘stock’ Maney pistons and normal Carrillo’s.

HALF A FREAKIN’ POUND… going up. Stop. Down. Stop. 116 times per second !! Of course that necessitated significant weight reduction from the flywheel, which I also had dynamically balanced.

Needless to say, the effect on vibration and the way the bike runs in general is pronounced. It’s smoother, of course it’s not totally smooth, but for a rigid mounted 83x93mm 360 degree twin, it’s good! It also now revs higher than before. It revs up on the throttle much quicker. It produces more power at given revs than before, and because it revs higher, it produces quite a bit more peak BHP, an extra 7BHP in fact !

So there you have it, an exceedingly beneficial modification in my experience.

Jim, send the cheque to the usual address please…

Every time the pistons and rods are at the top and bottom of their stroke, inertia comes into play. It must affect acceleration. With rod length, angularity comes into play aas well as the rate of reversal at the top and bottom of the stroke. Force=mass X acceleration. With longer rods there is less acceleration of the piston mass at top and bottom of the stroke. My 500cc Triton motor was made outof 650cc parts. It had 63 mm stroke with 6.5" rods. It was good, but really bad. I didn't usually lower the gearing because it badly needed a 6 speed box. It could either be really fast when coming out of corners out of corners - or fast enough towards the ends of the straights, but not both.
You might find this video interesting :

Yeah they work well. Must definitely be nice at high RPM combined with your other parts.

How much HP does a 1007 make at 6800 RPM? And does it climb or start to fall off after 6800 RPM?

Schwany said:

Yeah they work well. Must definitely be nice at high RPM combined with your other parts.

How much HP does a 1007 make at 6800 RPM? And does it climb or start to fall off after 6800 RPM?

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It peaks at 6500 at the moment, but I’m certain there’s more available between 6500 and 7000 with a little more work to inlet lengths and exhaust.

You know as well as I do that there’s nothing to be gained by stating Dyno numbers on t’internet…

But I will say its bloody impressive and already makes my other Norton builds look rather lame !

Fast Eddie said:

It peaks at 6500 at the moment, but I’m certain there’s more available between 6500 and 7000 with a little more work to inlet lengths and exhaust.

You know as well as I do that there’s nothing to be gained by stating Dyno numbers on t’internet…

But I will say its bloody impressive and already makes my other Norton builds look rather lame !

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As Sargent Shultz used to say on Hogan's Heroes, "I know nothing." I've never put a vehicle on a dyno.

I think dyno numbers spewed out by a machine are intriguing and useful. It is a good reference if you always use the same dyno according to those that do it. The AFR charting is going to show something if nothing else. Somebody is always going to say dyno numbers are inaccurate. If I listened to all the negative comments on the internet, I'd never do anything.

More than 85 HP?

Schwany said:

As Sargent Shultz used to say on Hogan's Heroes, "I know nothing." I've never put a vehicle on a dyno.

I think dyno numbers spewed out by a machine are intriguing and useful. It is a good reference if you always use the same dyno according to those that do it. The AFR charting is going to show something if nothing else. Somebody is always going to say dyno numbers are inaccurate. If I listened to all the negative comments on the internet, I'd never do anything.

More than 85 HP?

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Exactly, they are a great tool if used appropriately, a proper way to evaluate changes made objectively. And of course, having an accurate AF ratio is incredibly helpful.

They are not a tool that enables the final set up though, riding yields other issues / needs / benefits. For example the dyno cannot show you that the engine spins up faster. It shows power against rpm. Not rpm increase against time.

What matters is that when I first dyno’d it, we played around a bit and left with 5bhp more. After the rebuild it had 7BHP more than that but we didn’t play around. I’m expecting another 6-8bhp from further fiddling that I have in mind, namely inlet manifold extensions, exhaust pipe changes, and then final fiddling with fuel n sparks.

If you get another 6 bhp at the top end of the rev range, is that as good as having it in the middle of the rev range - or do you presume it is bigger there too ? With most modern motors, they are probably designed to rev to gain power. If you race a Commando against two-strokes and big four cylinder bikes, you need to accelerate hard as you go through corners where they can not. The power characteristic is important. The later advances in two stroke technology were about geting more torque, and they have always had 6 gears. The gearbox is a torque converter. Witn most motors, lowering the gearing increases the acceleration. With a Commando you need to raise the gearing to realise and increases in torque you have achieved. Close ratio gears give better acceleration.
How do you measure the torque a motordevelops in the middle of its rev range. In every viseo I have seen fof a bike on a dyno, they are revving the tits off the motor. Bigger inlet ports tend to give more power higher-up the rev-range. But we are all now aware that 30mm ports in a Commando engine are better.
Whatever combination of power and geaing you come up with has to suit the circuit. And it helps to observe where the other guys are getting on the gas. A more modern two stroke probably allows the rider to get on the gas much earlier coming out of corners, than the older piston-ported type.
When I built my Seeley, I could not get the Laverda 750 motor for which I had paid, I chose a Commando engine because of Gus Kuhn's achievement when racing against Peter Williams. It is quite good enough with only slight modification. But gearing and handling are everything. I am lucky, because I raced a bit years back, and I usually know what is happening with my bike. I take note of the parts of a race circuit where it is faster or slower.
When you can flick your bike into a corner and accelerate flat out from beginning to end, less horsepower does not matter. Smooth strong and consistent power delivery is more important.
You do not need a bike which might grab you by the throat and jump on you. My first race bike was like hat.

Fast Eddie said:

This is a real world shout out for JS pistons and rods…

I’ve now built 3 motors with JS rods and pistons, 850, 920 and the most recent 1007.

As a Triumph and Nourish guy in the past, I have always been of the uneducated opinion that the Norton rod is too short for the stroke. Triumph only went from 6.5” to 6” to fit the motor in the OIF, and that was with a 82mm stroke. Nourish definitely preferred 6.5” in his bigger motors.

Under the tutelage of Dave Degens I’d spend hours shaving grams off of pistons and gudgeon pins, he was adamant that anything saved here led to more power at the wheel, just as he was taught by Jeff Monty and Sid Lawton.

So, with that background, the weight saving with JS parts is really quite mind blowing for me.

My 920 pistons are actually lighter than stock 750 pistons. In my recent 1007 build, the entire reciprocating weight (that’s the small end of the rods, pins, pistons with rings) for both pistons was actually over 1/2 lbs less than the ‘stock’ Maney pistons and normal Carrillo’s.

HALF A FREAKIN’ POUND… going up. Stop. Down. Stop. 116 times per second !! Of course that necessitated significant weight reduction from the flywheel, which I also had dynamically balanced (the crank had been correctly balanced beforehand too, so this is not a crank balancing topic).

Needless to say, the effect on vibration and the way the bike runs in general is pronounced. It’s smoother, of course it’s not totally smooth, but for a rigid mounted 83x93mm 360 degree twin, it’s good! It also now revs higher than before. It revs up on the throttle much quicker. It produces more power at given revs than before, and because it revs higher, it produces quite a bit more peak BHP, an extra 7BHP in fact !

So there you have it, an exceedingly beneficial modification in my experience.

Jim, send the cheque to the usual address please…

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Hey! Cut it out! I'm enjoying just rebuilding bikes and you keep tempting me to build specials. I'm sure Jim would like that but I'm getting too old for this nonsense Besides, I'm dreaming of the space to have a lathe and mill, where in the devil would I have the space for a dyno?

Click off the internet and stay away, stay far away.
Must be strong,

Crank is currently at Marine Crank getting Checked and Dynamically balance.
Be strong, Stay AWAY......

MichaelB said:

Click off the internet and stay away, stay far away.
Must be strong,

Crank is currently at Marine Crank getting Checked and Dynamically balance.
Be strong, Stay AWAY......

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You guys! I can't even find anyone reliable to simply bore cylinders - unhappy with each I've tried and those I know would do a good job - well lets just say I probably can't live long enough for them to catchup and get the job done!

Schwany said:

Yeah they work well. Must definitely be nice at high RPM combined with your other parts.

How much HP does a 1007 make at 6800 RPM? And does it climb or start to fall off after 6800 RPM?

Click to expand...

some more info , some years ago ...

marinatlas said:

some more info , some years ago ...

https://britcycle.com/bikes/customer/BrentF_Nor_920_Comm_Visit.pdf

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Thank you for that.

Good easy to understand read about what Maney was up to. Enjoyed it.

What really amazes me about the Commando engine, is you can do so much with so little. It is extremely responsive to tuning, but everything around it needs to be adjusted to suit it. When you have made a gain, it is slightly more difficult than with other motors, to realise it. I have been raising my countershaft sprocket one tooth, each time I rebuild my primary drive.

MichaelB said:

Click off the internet and stay away, stay far away.
Must be strong,

Crank is currently at Marine Crank getting Checked and Dynamically balance.
Be strong, Stay AWAY......

Click to expand...

So is mine, I had them do the Atlas also. Amazing how much they took off, I think it was around 100 grams?