JS Motorsport Pistons/Rods First Impressions (2024)

Thanks for posting. Keep us informed.

Slick

CanukNortonNut said:

I will be doing the same. Cylinders are at Machine shop to match +0.020" bore.
Thanks for the comments.
cheers,

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The engine you state is beside me now ready to go into place so good to hear . .020 overbore too with me . I have 2 MK 111 engines . I'll ride out the upcoming season then the large job of the transplant . Good to hear this . Enjoy .

Torontonian said:

The engine you state is beside me now ready to go into place so good to hear . .020 overbore too with me . I have 2 MK 111 engines . I'll ride out the upcoming season then the large job of the transplant . Good to hear this . Enjoy .

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Too funny, I too built two MK3 engines when I rebuilt mine. A buddy picked up a MK3 when I was about to tear mine down. Both ended needing full rebuilds, mine from a worn out camshaft and his from years of neglect.

We did the JS Rods, Pistons, cam and light weight lifters in his as well. He elected to go with lower stock compression where I am closer to 9.5:1 and he went wtih a JS1 cam. I am very curious to see how his runs compared to mine now.

That’s great to hear, and those engines look lovely !

I’ve had JS rods and pistons in two engines now, 850 and 920 and am also impressed with the overall smoothness.

I also think the longer rod makes the engine more responsive, ie it picks up revs quicker / easier.

I‘ve only ever used the JS#1 cam in the 850 though, and for me, I found that an excellent cam for fast road and track use.

Despite lots of hard use on the track, the cam and lifters in mine look completely unworn.

Hope yours beds in nicely and gives good service.

Do keep us posted.

Here are a couple pictures from the build. Comparing the old cam to the new and getting the bearings back from PolyDyn in Texas. The new tappets and blocks were a breeze to install. And don't worry I blasted and painted the cylinders after I did the tappet block install. There were so many prior layers of paint on these cylinders took forever to get them clean.

+1

Stock stroke Carrillo rods and lightweight 750 pistons with a Combat cam and stock lifters and valves. Never really riden a stock Commando for comparison, but this set up is fantastic

Digging through my photos over the last 6 months it took me to acquire parts and find time to build and get the bike running. Some comparison shots of the stock parts and the new JS Motorsport parts. These are the parts in the second engine I built for my friend. These are the standard compression pistons and JS1 cam.

maaseyracer said:

Here are a couple pictures from the build. Comparing the old cam to the new and getting the bearings back from PolyDyn in Texas.

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What's the story about the bearing shells - some kind of coating?

maaseyracer said:

The new tappets and blocks were a breeze to install.

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You should take notice of the problems Schwany had in securing the lifter blocks. His problem may be related to thermal expansion of aluminum, but I am not so sure. Even though you have an iron barrel, I suggest boring a shallow hole in the lifter block at the desired location, providing a positive arrest by virtue of the grub screw. Repeat for the second lifter block.

- Knut

mdt-son said:

What's the story about the bearing shells - some kind of coating?

You should take notice of the problems Schwany had in securing the lifter blocks. His problem may be related to thermal expansion of aluminum, but I am not so sure. Even though you have an iron barrel, I suggest boring a shallow hole in the lifter block at the desired location, providing a positive arrest by virtue of the grub screw. Repeat for the second lifter block.

- Knut

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The coating on the rod bearings is designed to retain oil, so the bearings always have oil, which helps to prolong the life of the bearing and the crank. The coating is cheap insurance.

I am not sure how the other guy installed his lifter blocks of what his problem was. However, the point of drilling a hole in your cylinders is to install a set screw which requires a locating hole drilled into the tappet block. Please note the point on the set screw on the photo I posted above, this point locates into the hole that is drilled into the tappet block. The set screw then gets loctite to keep everything in place. The instructions provided with this kit clearly state this. The lifter blocks are pinned from the set screws and the the tappet plates.

Read page 10 of Schwany's thread here:

I didn't know JS had prescribed a locating hole into the tappet block. That's good then.

- Knut

A few thoughts from my commute to work today. First off I took the long way in as I do not commute much more than a mile and a half or so each way. What is impressive is how well this build combination holds an idle and settles back to an idle. I have the idle set at ~1000 rpm and to be honest I am not sure where a Norton should idle, but this does not seem obnoxious and feels like where I had it before. City traffic speeds are not an issue mostly 1st and 2nd gear.

Still working out the tune, it is close, I moved the timing from 26 degrees when I got home this evening to 28 degrees. I raised the needle a position (Keihin CR), it will take me a day or two to figure out where I am tuning wise running CA 91 Octane. Can't wait to be done tuning so I can really open it up.

Parking lot photo:

mdt-son said:

I suggest boring a shallow hole in the lifter block at the desired location, providing a positive arrest by virtue of the grub screw. Repeat for the second lifter block.

- Knut

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That is actually stated in the JS instructions.

My personal hypothesis is this: with a pointed grub screw, drilling into the lifter block only slightly allows the point to seat. Without drilling, I hypothesis that the point will basically ‘dig in’ and will therefore not hold torque very well over time. Thus allowing the block to come loose.

Add to the above, the thermal expansion that you mention and it’s easy to visualise an issue arising.

For the lifter block drilling, I suggest that making a ’jig’ that ensures the blocks remain perfectly in alignment is a good idea.

maaseyracer said:

Just rebuilt my 850 MK3, got the engine installed and fired it. I am completely shocked how much smoother this bike is. I know Jim has made claims here, but seeing/riding/feeling is believing. I own several vintage motorcycles, 3 BSAs, 2 Triumphs, this Norton, several 305 Hondas. I thought my Triumph was a smooth, I thought my Norton was smooth prior to this. This is shocking. Yes, this motorcycle is much faster than it was, but in a different way, cam profile (JS2) is completely different than before. However, that is not the story here, the story is how smooth the ride is on this thing. I have read Jim’s site and I have seen his posts here, but I did not realize how much of a difference the rod and piston change would make. I am very impressed. Looking forward to getting some more miles on it.

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I put the JS pistons and rods into my Mk11 850 when I refurbished it about three years ago.

I also had the crank dynamically balanced and fitted CNW isolastics front and rear. The headsteady is Dave Taylor because the CNW one was out of stock. I also went over the bike following the "World Straightest Commano" article.

So with all these changes done at the same time it is not possible to attribute an improvement to any one item.

However I can definitely state the bike is absolutely totally smooth above 2400 rpm. I have had the bike over 40 years and the improvement is unquestionable.

Mark Savage said:

My thoughts on this are:
The tip on the grub screw is to give an accurate centre point for drilling the lifter block for engaging the grub screw. If I was to do it again, after drilling the lifter block, I'd grind the point off the grub screw so it would fully engage in the hole rather than being held off by the point in the bottom of it.

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+1

Mark Savage said:

My thoughts on this are:
The tip on the grub screw is to give an accurate centre point for drilling the lifter block for engaging the grub screw. If I was to do it again, after drilling the lifter block, I'd grind the point off the grub screw so it would fully engage in the hole rather than being held off by the point in the bottom of it.

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Yeah, I like your thinking.

johnm said:

I put the JS pistons and rods into my Mk11 850 when I refurbished it about three years ago.

I also had the crank dynamically balanced and fitted CNW isolastics front and rear. The headsteady is Dave Taylor because the CNW one was out of stock. I also went over the bike following the "World Straightest Commano" article.

So with all these changes done at the same time it is not possible to attribute an improvement to any one item.

However I can definitely state the bike is absolutely totally smooth above 2400 rpm. I have had the bike over 40 years and the improvement is unquestionable.

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I had the 920 crank dynamically balanced before assembling that engine. The 920 goes smooth at a lower speed than my 850, somewhere around 2500 rpm for the 920 and 3000 for the 850. Above 3000 I can't feel any difference in bike to bike vibe levels as they are both very smooth.
The 920 just has stock rods and the RGM 920 kit pistons. Those are the same weight as stock pistons.

I would say that the dynamic balance is well worth the $180 spent, especially if the engine is already apart for some reason.

Glen