New product - Domed pistons

Domed racing pistons for approx 12:1 C.R.
Extra machining (and expensive - I know). But at least they are available - in sets of 4.
more info at:
www.jsmotorsport.com

Artwork.

How about lower compression pistons for those who just want a bike that is easier to start ?

Jean

Lower compression is easy - I have thicker copper head gaskets or base gaskets for that.

Jeandr said:

How about lower compression pistons for those who just want a bike that is easier to start ?

Jean

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On the other hand, why buy expensive forged pistons for a low compression engine? Cast pistons will work just as well, even better in some respects, and cost much less. Nothing wrong with Jim's pistons, in fact they are really great for high performance engines, and I like them a lot. Jim has put a lot of effort into making them avaialble. But there isn't really any benefit to just replacing the cast pistons with forged ones in a lower performance engine. The real benefit from Jim's pistons comes when you use them as part of his long rod, light piston, package. In that case you are actually getting some benefit for the extra cost. Just my opinion, as usual.

Ken

I hope Jim will correct me if I am wrong. But the advantage of the forged piston in lower compression would be that because they are stronger they can be shorter and thus coupled to the longer rod for better geometry. You are still spinning less weight so get less stress and less vibration.

I am getting ready to tear down my 850 project and am considering these pistons (not domed but light weight) just for the above reasons. I want for this bike to be a road bike with lots of low RPM torque and able to burn gas that is low in octane or full of corn. All advice and information is welcome. I would also appreciate a conversation about how the light pistons would influence balance factor and how that would fit into the above goal.


Russ

PS-I realize this is not the intent of this thread, but it is sort of relevant. I would be happy to start a thread on this topic if it suits.

I am getting ready to tear down my 850 project and am considering these pistons (not domed but light weight) just for the above reasons. I want for this bike to be a road bike with lots of low RPM torque and able to burn gas that is low in octane or full of corn. All advice and information is welcome. I would also appreciate a conversation about how the light pistons would influence balance factor and how that would fit into the above goal.

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Put on extra mass on flywheel and in such a way to increase the BF into 80'%.
Put on-adapt smallest port head and valves and carb you can and get a non over lap cam profile. Put in cross over tube or 2-Y-1 small long headers exhaust out single silencer. Bump ignition up just below knock-ping-detonation. Bumping CR up is better for low end that hi end btw. Corn booze fuel has higher Octane than same gas w/o the ethanol so might be shooting self in foot lowering CR. Better study up on this before spending. I got to christen me and Peel on first ride to pavement to village to group under shade tree passing around a jar of moonshine,
took me a glup and gave Peel a shot in her tank and off we went high as sh-t
more knock resistant.

hobot

rvich,
I have just fitted Jim's 9:1 piston and rod set into my std 850, I have not been able to get any substantial miles on her yet, only about 50 miles so far. But I wil be taking the bike to the Aussie MotoGP a trip from where I live of about 2400 miles home to home. So I should be in a good position to give you a report when I get back.

Cheers Richard

rvich said:

...the advantage of the forged piston in lower compression would be that because they are stronger they can be shorter and thus coupled to the longer rod for better geometry. You are still spinning less weight so get less stress and less vibration.

I want for this bike to be ... able to burn gas that is low in octane or full of corn. I would appreciate a conversation about how the light pistons would influence balance factor and how that would fit into the above goal.

Russ

Click to expand...

Russ

The lightweight pistons were designed both for street and racing - but they come in different compression heights so you have your choice. The increased comfort of reducing the vibes for street is something you appreciate more and more as you log more hours on the bike. My standard pistons have compression that is low enough for street gas. On top of that you can install one of my copper base gaskets to lower the compression further - you wouldn't want to lower it any more than that. Stock 850s are about 8.5 to 1 and thats already pretty low. The balance factor is the next big improvement. The lightweight pistons bring the balance factor up from the stock 52% to 65% and thats a better balance factor for Norton's. Changing the balance factor may not make much improvement in vibration because of the isolastics but you know that the stress on the crankshaft is reduced and that's a good thing.