long and short on pistons and rods

How much of the perceived wear increase caused by the short skirt J&S piston will be offset by the longer rod that will decrease rod angle & side thrust of the piston in the bore?

beng said:

The solution is to run a production class where the rules prohibit the use of silly parts. If someone wants to do something impressive, then lets see who can build the most powerful Norton twin using all stock specification parts, cam, valve springs and all that will run on pump gas. That will take a lot more art and brains than buying a wheelbarrow full of new design racing parts and bolting them together.

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A worthy aim, but if the buck stopped with you, how would you police it in practice, while at the same time run meetings efficiently? Workability is one of the main reasons why the regs (here, at least) focus on external appearance. Once you introduce rules for internals such as pistons, cams, valve springs, etc, you are in a minefield. Cubic capacity can be policed at race meetings, if the stewards order an engine to be stripped after a race. Pump gasoline varies from country to country, so you must have an octane limit, which I think the CRMC in Britain does already. Exhausts must meet modern noise regs, otherwise you won't be allowed out, so we have to live with non-stock cans or specify restrictive production silencers. The capacity for cheating is endless, so the rules have to be workable if they are to be respected.

BitchinBeezer said:

How much of the perceived wear increase caused by the short skirt J&S piston will be offset by the longer rod that will decrease rod angle & side thrust of the piston in the bore?

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The increased rod length will help a little.

The root of the short piston problem lies with the loose clearances required when running a forged piston in an air cooled iron liner.
A forged piston requires at least .006 inch clearance in a Norton. That leaves very little margin for wear before piston rocking is beyond the point where ring sealing becomes a problem. That happens at about .008 or .009 clearance.
The modern engines with water cooled aluminum bores and short pistons usually run piston clearances of under .001 inch so piston rocking is not a problem.

That is why I usually suggest short skirt pistons for race engines that will be rebuilt regularly but stick with full length pistons for street use. Jim

comnoz said:

BitchinBeezer said:

How much of the perceived wear increase caused by the short skirt J&S piston will be offset by the longer rod that will decrease rod angle & side thrust of the piston in the bore?

Click to expand...

The increased rod length will help a little.

The root of the short piston problem lies with the loose clearances required when running a forged piston in an air cooled iron liner.
A forged piston requires at least .006 inch clearance in a Norton. That leaves very little margin for wear before piston rocking is beyond the point where ring sealing becomes a problem. That happens at about .008 or .009 clearance.
The modern engines with water cooled aluminum bores and short pistons usually run piston clearances of under .001 inch so piston rocking is not a problem.

That is why I usually suggest short skirt pistons for race engines that will be rebuilt regularly but stick with full length pistons for street use. Jim

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I'm not a big fan of forged pistons due to the rocking when the clearances are miximum before the engine reaches normal operating temperature. However, a forged piston expands more than a cast piston. Therefore the "cold" clearance will close up to far less than the .006" when the piston/cylinder warm up.

I would prefer a longer skirt version of the J&S forged piston that could be utilized W/the longer steel J&S rods. That would give us the best of worlds, a longer skirt & a longer rod. That would make a nice, hot street set-up that would have better service life.

I've sold a lot of pistons. Most go on street bikes and a lot of them went to members of this forum. I haven't been hearing of any unusual wear and no one has reported it on this forum. I've had mine in four years. Run it hard and cruise it on the highway. The bike is a workhorse and I ride it every chance I get. The piston clearance is .0055" and you hear it when its cold but its always quiet as soon as it warms up because forged pistons expand more than cast pistons. When they are warm the clearance is reduced to about .001" That hasn't changed since I first put them in. Some modern pistons are super short with tiny little tangs for skirts. JS pistons are not like that, they are a compromise in length and no one has reported any wear problems on this forum. I've checked them for wear after 4 years and found them normal. I didn't even change the rings. The stress they take off a Norton motor will save your crank and cases and reduce vibration all around.

Below see the oldest JS pistons on the road (the 1st set & drilled for extra lightness). Those are polish witness areas on the skirt.



Below is a modern super short skirt Honda piston

Beng, I agree with much of what you have said. I believe Norton Commandos should be loved for what they are and we shouldn't try to turn them into something else. The motor in my own bike is near standard, and it is still enough to win races. I've found the commando engine to be very strange. I never believed in it, and s o the bike sat around for many years un-raced. The motor pulls extremely high gearing and still accelerates extremely rapidly, however I am deathly afraid of revving it past 7,000 RPM. I believe that nobody ever needs to do that. Throughout my lifetime I've believed that the way to high performance has been short stroke, radical cams, more valves, big ports and centre bearing cranks - 'top end motors'. It would be interesting to build a 1000cc Paton twin, however it would probably be horrible to race. The commando engine stuffs those theories. When we consider what Peter Williams achieved with the JPNs, it is really a joke that he got so much out of them. I've ridden a few racing bikes during my life time, and I really love my Seeley 850 - there is no anxiety with it - safe and fast, and you can do so much with it. It doesn't matter where it is on a race circuit, you think where you want to be , and it is there.

JS, Love the thought of your long rods and forged pistons. When I'm rich again, you have another customer.

daveh said:

beng said:

The solution is to run a production class where the rules prohibit the use of silly parts. If someone wants to do something impressive, then lets see who can build the most powerful Norton twin using all stock specification parts, cam, valve springs and all that will run on pump gas. That will take a lot more art and brains than buying a wheelbarrow full of new design racing parts and bolting them together.

Click to expand...

A worthy aim, but if the buck stopped with you, how would you police it in practice

Click to expand...

The AMA ran races for decades in the USA called class "C" where the motorcycles were supposed to be all production parts that were available to the public off the showroom floor or over the parts counter.

They enforced the rules by allowing other racers to either request a teardown or to PURCHASE any of their competitions motorcycles for a standard set price, this was called the "claiming" rule. It produced some comic moments, like when the BSA company had one of their works triples claimed by a relatively unknown rider I think at Ontario Motor Speedway a bit over 40 years ago.

I would not use a claiming rule for whole motorcycles, but one just for engine/gearbox units would be good. If any opponent could take your engine/gearbox for $2500 or have you banned from competition for two years you may be motivated to put few exotic parts into it. If someone protests you and has your engine torn down and exotic parts are found in it, two year ban. Along with this the negative publicity as a cheat would be unwanted.

beng said:

The AMA ran races for decades in the USA called class "C" where the motorcycles were supposed to be all production parts that were available to the public off the showroom floor or over the parts counter.

They enforced the rules by allowing other racers to either request a teardown or to PURCHASE any of their competitions motorcycles for a standard set price, this was called the "claiming" rule. It produced some comic moments, like when the BSA company had one of their works triples claimed by a relatively unknown rider I think at Ontario Motor Speedway a bit over 40 years ago.

I would not use a claiming rule for whole motorcycles, but one just for engine/gearbox units would be good. If any opponent could take your engine/gearbox for $2500 or have you banned from competition for two years you may be motivated to put few exotic parts into it. If someone protests you and has your engine torn down and exotic parts are found in it, two year ban. Along with this the negative publicity as a cheat would be unwanted.

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Yes, in principle it's not a bad idea if it was for a particular class (and not across the board). The devil is in the detail, though. It would be interesting to hear reactions from competitors here in Europe. It would help to level the playing field for the average guy.

Sorry of this is off topic, so back to pistons and rods.

One of our top racing car drivers was killed about 4 years ago. Before he died, a friend asked him what was the most important thing he took to race meetings. He answered ' MY LAWYER' ! - Claiming rules are bullshit. Racing is about development, and you don't have to do much to make a commando competitve against other old four stroke twins. If you want to race against four cylinder Japanese bikes, buy one or a good 250cc two stroke, and stop being cruel to commandos. Alternatively you could learn to ride your bike.

There is a friend of ours who still occasionally races in historic events. He hits the slops a bit these days, and is not in a very good mental state. A couple of mates went To Eastern Creek with him where he rode a 650SS Norton against the vastly oversize CB750s in Period 4. At the end of the main straight there is a decreasing radius bend. On lap one he rode around all of the leaders going into that bend. I once saw him throw a TZ750 away at 140MPH + halfway down the main straight at Philip Island when it chucked a rod. You do not need the extremely developed top end motor to win, however you need the mentality to handle the scary speeds in various situations. I was never sponsored during my early racing - I did not ever want to be in the situation where I was forced to ride at Bathurst. It has to be a matter of choice, and it would take a lot of effort to get your head straight about racing there. I imagine the IOM would be even worse.

What drives me is the insane handling capacity of the sloppy constructed and somewhat compliant-weak head and breast links combined with the game changing Bob Patton robust rump rod and hobot fork mods. It erks me a bit that others suggest a modern power plant to take on the elite moderns, which makes perfect sense - if not knowing the wonders of tri-linked isolastic power planting like I do. I expect it'll take over 130 hp to take on the elites in more technical tracks speed don't top 160. No doubt about moderns are missiles once they get past apexes so I expect to have to burn up a set of JMS pistons on shoot outs and track day circuit times. Its encouraging Schmidt has gotten 4 yrs fun fast street use out of his pistons but every reference i can track down online and some face to face with racers on and off roading tell me to expect to lose power and bores if racing around with short skirts for a few seasons. I'll harden Peel's bores then see how long JMS kit lasts trying to best them modern corner cripples but am limited in money so once Peel's superiority proven or failed may have to return to cheaper long long term kit. We still have to get reports of time to over haul on winning racers using the JMS pistons.

Hobot, There is no easy win unless you ride a cheater, and 'how fast can you afford to go' is not always an answer. There are certain riders that if I see them alongside me on the grid, I know I have a problem simply because of their sheer ability. Sometimes the faster you try to go, the slower you go.

Hobot, There is no easy win unless you ride a cheater, and 'how fast can you afford to go' is not always an answer. There are certain riders that if I see them alongside me on the grid, I know I have a problem simply because of their sheer ability. Sometimes the faster you try to go, the slower you go.

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Cheater! Who Sir Me Sir No Sir Not me Sir. I ain't a racer collecting points just wanting the maxim G's and still freaking out on what Peel delivered so easy. No way did I expect to become a sports bike hunter on a Commando and w/o the wonders of the tri-link likely would of given up on pressing a Cdo with more power. Big Huge point being missed in all my messages is Peel is so good it doesn't matter how good the other pilots are as they are on corner cripples which can kill them pressing into unpredicable zones where handling suddenly transitions dramatically. One partial advantage/cheating on Peel is she'll be my daily rider though thick and thin so becomes second nature after a while to just know how she'll react in various states. this is what concerned me with the short skirts as the off roader trials and MX's I've played with tell me how di$$sappointed young riders are when they realize their hot little 2 smokes and 4 tokes ate up their light pistons and valve gear in a season or so. I'm taking Jim Schmidt at his word on how he's used his Norton as would be similar to Peel in daily practice modes. I figure it'll take about 6 months to get Peel programed into my bones and blower power figured out, then throw down gaunlet for the private outlaw bikes shoot out, ya know the kind so modified they are break out of race rule books just for the hell of it. Peel should not have to rev her rings out to make big torque so may get a few years out of the JMS kit to remember the rest of my life. I got part of what ya saying about going to fast it slows ya down, and so I approach turns a bit slower than the racers dudes, so they sometimes passed me as I finished my slowing - till they were then hard on brakes with Peel hard on throttle to flash right past em. A human pilot can only twist forks and press a lean only so fast and hard to transition but engine power can do power trip outs that snatch the forks right out of your hands for nil effort power steering. Similar to dragsters with a bit choppy acceleration with tire break and grip - so Peel is steered in the skip outs to leap into better line on the regrips with more total time spend in harder acceleration than the hesitations reductions. Heat and swelling will be Peel's limiting factor so JMS fordged pistons with enough clearance and ring gap are saving grace to me.