JS Motorsport
 
 
motorcycle
HIGH-PERFORMANCE
PARTS
FOR NORTON TWINS
JS Motorsport
4480 North Academy Avenue
Sanger, CA 93657
Phone/Fax: 559 875 0659
Email: jim@jsmotorsport.com
JS Motorsport Technical
Lower vibration = increased reliability

The best piston/rod combination on the market. Our lightweight pistons (1/3 lighter than stock) are forged aluminum, making them stronger than original cast aluminum pistons. The combination of lighter pistons with our longer lightweight rods makes for a much smoother and reliable ride whether you are cruising down the highway or pushing it to redline on the racetrack (demo video).

Rods

We have worked with Carrillo Industries to develop a longer, lightweight proprietary rod. Several years of research, design, and negotiation with Carrillo has gone into our new rod's production. The traditional bronze bushing in the small end of the rod has been eliminated and our .708 diameter pins use DLC (Diamond-Like Carbon Coating). This is on the extreme edge of racing NASCAR technology, as the tremendous loads these racers subject their engines to (9000 rpm for 600 miles of practice and racing) will deform and wear out the bronze bushings. Coating pins and eliminating the bushing solves this problem and is the only way to get a more dependable yet lighter weight steel rod. I have personally tested these rods beyond 8000RPM. Our new bushless 6.4" length rod weighs only 105 grams on the small end (the end that causes vibration). The older version stock length (5.875") Carrillo rod with bronze bushing weighs 138 grams on the small end. This amounts to a total pin-end savings (for 2 rods) of about 60 grams on the small ends (about the weight of a stock wrist pin). Note that aftermarket aluminum rods are also heavier on the small end than our proprietary rod. The CARR bolts are the highest qualtity and do not need replacing for the life of the rod (if not overtightened). Use original style shell bearings. Balancing is not required for Commando engines (balance info). Use stock ignition timing (compare weights).

MORE POWER, LESS STRESS

Our custom Norton rods are 6.400" long (.525" longer than stock), increasing the length of rod-to-stroke ratio from 1.67 to 1 (stock 750) to a more ideal ratio of 1.83 to 1. The advantages of a longer rod are:

A) More power: The increase in power is a result of more dwell time at the top of the compression stroke, allowing more time for the fuel charge to burn completely and exert maximum force on the piston throughout the power stroke, This translates to more power for the amount fuel you burn.

B) Less Vibration: More dwell time at the top and bottom of the piston strokes means less "snap" due to the pistons not having to stop and reverse direction as quickly and abruptly as they would with a shorter rod.

C) Less stressfull wear: The longer JS rod has less angle when traveling through mid-stroke than the short stock length rod. This means that less side forces are being applied to the cylinder wall - thus reducing friction, heat and wear.

Reducing stress is very important for Nortons as they have a tendency break cranks & cases and vibrate everything to pieces. Our longer rod also reduces the side pressure that the piston exerts on the cylinder, causing power-robbing friction, and because of this helps the engine to run cooler and more efficiently.

Flat top pistons for street and racing - compression specs

Because of the forged piston's higher expansion coefficient, they require more cylinder wall clearance when cold than traditional cast pistons. Once the motor warms up the extra clearance disappears. Anything tighter than .0055" piston-to-wall cold clearance will result in a piston that is too tight in the bore when hot and will cause scuffing. We recommend .0055" clearance for 750 street riding with gentle break-in (bore = exactly 73, 73.5mm etc). 850cc motors need .0055 to .006" clearance (bore = exactly 77, 77.5mm etc), and 920 or 1007cc need .006" clearance (bore = exactly 81mm or 83mm). All racers without break-in need .006". JS Motorsport pistons have a deck shorter than stock, so require our longer rods.

NOTE THAT ALL COMPRESSION SPECS REFER TO UNMILLED HEADS WITH A .120" STEP AT THE SQUISH BAND.

SEE LEAK DOWN TEST

750 PISTONS
Our Medium Compression 750 pistons are stock height with approx stock 9:1 C.R. and will accept standard size as well as 3mm oversized valves.
A drop in kit with no modifications necessary.

Our High Compression 750 pistons have the deck height raised .050" for a compression ratio of approx 9.5:1. These High Compression pistons will drop right into your stock street motor if your head has not been milled, leaving a squish band clearance of approximately .050". Milling your head .040" with our High Compression pistons will bump the compression ratio up to approx 10.5:1 This will require checking valve to piston clearance and machining an outer ring relief in the top of the piston to give you the necessary squish band clearance - or instead of milling your head you can use our .003" (.1mm) thin head gasket ring. All our Racing/High Compression pistons have deep valve pockets to accommodate big-valve heads using up to 5mm oversize intake and 3mm oversize exhaust valve, as well as stock diameter valves.

LINK TO FASTEST 750 PUSHROD PRODUCTION NORTON AT BONNIVILLE

850 PISTONS
Our Medium Compression 850 pistons have stock compression of approx 8.5:1 with shallow valve pockets to allow the use of our pistons with a milled head. They will accept standard size as well as 3mm oversized valves. The deck height is raised .020" higher than original 850 pistons.
A drop in kit with no modifications necessary.

Our High Compression 850 pistons have approx 10.5:1 C.R. with the deck height raised .110" higher than the original 850 piston. These High Compression pistons will drop right into your stock street motor if your head has not been milled, leaving a squish band clearance of approximately .050". Milling your head .040" with our High Compression pistons will bump the compression ratio up to approx 11.5:1 This will require checking valve to piston clearance and machining an outer ring relief in the top of the piston to give you the necessary squish band clearance - or instead of milling your head you can use our .003" (.1mm) thin head gasket ring. All our Racing/High Compression pistons have deep valve pockets to accommodate big-valve heads using up to 5mm oversize intake and 3mm oversize exhaust valve, as well as stock diameter valves. You can adjust compression with our various copper base and head gaskets (compression info).

920 PISTONS (81mm)
Our Medium Compression 920 pistons have approx 9:1 C.R with the deck height raised .010" (compared to original 850 pistons) and shallow valve pockets to allow the use of our pistons with a milled head. They will accept standard size as well as 3mm oversized valves. A drop in kit with no modifications necessary with unmilled head (not necessary to enlarge the combustion chamber unless the head is milled).

Our 920cc High Compression pistons have approx 10:1 C.R with the deck height raised .090" higher than original 850 pistons. Modification of your head's combustion chamber will be necessary to fit the larger diameter Hi C.R. pistons. Milling your head .040" with our High Compression pistons will bump the compression ratio up to approx 11:1 This will require checking valve to piston and squish band clearance, or instead of milling your head you can use our .003" (.1mm) thin head gasket ring. All our Racing/High Compression pistons have deep valve pockets to accommodate big-valve heads using up to 5mm oversize intake and 3mm oversize exhaust valve, as well as stock diameter valves. Use JS copper head gaskets to adjust compression.

1007 PISTONS (83mm)
Because of the large displacement, you can order High Compression up to 12:1 with flat top pistons. To lower compression below 10:1 you must special order head gaskets (available up to 1/8 thick).
All our Racing/High Compression pistons have deep valve pockets to accommodate big-valve heads using up to 5mm oversize intake and 3mm oversize exhaust valve, as well as stock diameter valves. Modification of your head's combustion chamber will be necessary to fit the larger diameter Hi C.R. pistons.

LINK TO HIGHLY SUCCESSFULL NORTON ROAD RACERS

MODIFYING FLAT TOP PISTONS FOR HIGHER COMPESSION
You can raise compression of our Hi C.R. flat top pistons approximately one point. For example - from 10:1 to 11:1 for racing by using our thin .003" head gasket ring (or using a milled head) and cutting a step to clear the squish band (see diagram).

Extra high compression domed pistons for racing only

Domed pistons designed to fit standard and big valve heads and give approx 11:3:1 C.R. for 750 long strokes and 750 short strokes(short strokes see below). 850s will have approx 12.5:1 C.R. for alcohol only unless combustion chambers have been enlarged with big valves etc to at least 51cc (approx 12:1 C.R.). Beware that over 12:1 C.R. will create too much heat on gasoline. Fits stock heads with no machining required except that bores greater than 78.5mm will need larger diameter combustion chambers. All our Racing/High Compression pistons have deep valve pockets to accommodate big-valve heads using up to 5mm oversize intake and 3mm oversize exhaust valves as well as stock diameter valves. Big valve heads may have different size combustion chambers. Large displacement 920s and up do not need domed pistons because the C.R. would be higher than 13:1 (OK with alcohol).

In the photo below you can see the underside of the dome is milled 3 dimensionally for consistent thickness. This gives an even distribution of material and keeps the weight down to the levels of our regular lightweight flat topped piston. The extra machining raises the cost.

It is assumed that every race engine builder will check and set piston squish band, and valve pocket clearances. Heavily milled heads often require machining to clear the squish band or use thicker copper base or head gaskets (compression info). However, no special machining or modifications should be necessary when using our pistons with a stock Norton head except that bores greater than 78.5mm will need larger diameter combustion chambers.

Piston options for 750 & 850 Short strokes

It takes something special to get the compression high enough for a racing 750 short stroke.

Options are:

77mm Flat top HI CR pistons. Use .003" head gasket ring and cut piston crown periphery approx .040" to get 10.3 CR with stock unmilled head.

77mm domed pistons give approx 11.3:1 CR with stock head - NO MACHINING NECESSARY. Big valve heads may have different size combustion chambers.

81mm bore x 80.4 short stroke (840cc) will give approx 12:1 C.R. with domed pistons & stock head.

To reduce bore wear in high performance motors and get ultra high milage on street bikes we recommended that you send your cylinders to Bore Tech in Ohio for silicon carbide impregnation (ph. 513 625 8374 they also do boring) or Laystall in the UK. To avoid scuffing, racebikes should use Castrol R40 or straight grade racing oil with zinc additive.

Dished pistons with squish band

Large diameter 81 or 83mm pistons with a tight squish band can have too much compression for the street unless you dish them as in the photo below. These pistons are special order.

Reliability and long life

The photo below shows a used piston after a season of racing. There is minimal wear on the skirt and no evidence of wear or rocking at the top of the piston.