Pistons & Con Rods

When I raced my 500cc Triumph years ago it still had the long 650 conrods in it with the ultra short stroke of 63mm. In later years I thought about fitting shorter rods to get a bit more torque into the picture, and make it less of a nasty bastard. However I'd already cut 12mm off the 650 barrels, and it would have meant making new aluminium barrels. Recently I started looking for light pistons for the commando engine, and found that with a bit of machining of the crown that a 73mm Honda Fireblade piston might fit a 750 commando bore. Again I came up against the barrel making problem, but worse because of the integral pushrod tubes. From this forum I found out about JWS pistons being used with longer carillo rods. I believe that the Japanese pistons have been spun cast and forged to get the grain flow right, and I suspect the JWS pistons are similar. The Fireblade pistons are 100g lighter than the standard 850 pistons, and the JWS are prrobably similar. The higher gudgeon pin raises a small question in my mind, as it probably slightly reduces leverage. I'm wondering if anyone has gone in the opposite direction and made shorter barrels to use light pistons with standard length rods ? The fireblade pistons are very cheap ($100 complete), they are teflon coated and have very thin rings.
Years ago I read about the way to calculate optimum rod length for various stroke motors, and it obviously changes with the way the motor is to be developed with respect to torque. - does anyone have any clues as to a reference about this ? The JWS items probably help the motor tolerate higher revs, and will also help it get there quicker due to inertia considerations. But the standard commando rod length might already be the optimum for a torquey motor of that stroke.

Hi, before he get his new pistons/rods made , jim Schmidt used some GSXR pistons in a modified barrel (cut the top , with at least one fin less..........) you could see that in his race manual, a very interesting book , that's the same way you want to go with the fireblade pistons..............

I would say a Commando is on the lower limit of rod to stroke ratio and would benefit with a bit taller ratio.

Suggest simplifying your life and going with the JS pistons and con rods package unless you can get the fire blade arrangement to drop in easy. The JS package offers considerable weight reduction compare to most alternatives I know of.

Rod ratio calculator says factory Cod have ~1.62 and JSM rod on factor stroke give 1.74, whlle JMS with 63 mm stroke ~2.45, which is excessively hi ratio and rod may foul cylinder bore bottoms.
http://www.actionimport.com/Tech_Files/ ... ulator.htm

In our ole Chevy V8 like engines the longer rod or higher rod/stroke ratio is a benefit to some degree for a number of reasons you can fine here.
http://www.hotrodders.com/forum/stroke- ... 32680.html

With all other things equal, introducing a greater rod to stroke ratio in our beloved Nortons should reduce the need for ignition advance a bit due to dwell in and around TDC. In other words, there a bit more time in and around TDC not only for the fuel to ignite but to achive peak pressure, which is a very good thing. I read that this enhances high speed performance.

With the initial 500 cc ultra short stroke (59.6mm stroke) we used a Steve Maney barrel cut down by Steve to allow (if I recall correctly) stock rod lengths. This resulted in an obscene and unheard of to me rod to stroke ratio yet it did one of the best dyno pulls of 64 RWHP. Not sure what positive contribution the long rods made but I do know from logic that we were reciprocating a lot more mass than we do now with the shrot rods and billet cylinder.

My street XR makes 72-75 RWH and I only run it to 7.
see ratios here:


Harley-Davidson
COPYRIGHT © 2000-2012 Stan Weiss — World Wide Enterprises
Cubic Inches Bore Stroke Rod Length Rod / Stroke Ratio Bore / Stroke Ratio
X-750 1972-2011 45.8 ci 750 cc 3.125 2.9835 6.438 2.1579 1.0474

Sounds to me like the JWS pistons and rods might be a good way to go. Although I don't believe I will be making a billet crank. I can probably afford the slight loss of torque, if the rev range moves up a couple of hundred revs safely. Ever since I started campaigning the Seeley, I've had the uncomfortable feeling that I might be sitting on a bomb. It regularly sees over 7000 revs, and I believe that most of the old British designs were not intended to go over about 6,500 rpm. I know that sounds a bit low, but my main opposition years ago ran a Triumph 650 while I was riding the short stroke 500. His bike was set up for torque and was never revved over 6,300. I rode his bike once, and it was really very fast. I could never convincingly beat him with the 500 and I used to peddle it a lot faster. Almost every one of the old pre-unit Triumph 650s in our historic racing has blown up. We don't see them racing here any more. My commando motor has been balanced to 72 %, so it is very smooth at high revs, it is just that I don't trust it. The only reason I kept using the Triumph 500 for 12 years, long after it's time, was that it had the short stroke billet crank and would cop 10,500 revs. Other than that it was a nasty piece of shit !
I haven't raced the Seeley for about 5 years, because of the disadvantage the 4 speed CR gear box caused at the clutch starts. And I've only ever had one really good start with it on our local circuit. It really surprises me after riding that old featherbed Triumph for so long. I did not know how good it could be. I took it t o the old farts meeting at Mt Gambier in 2003 and won a couple of races, but there was really nobody there. In my last historic race, I got a fairly decent start, and it gave me a good close look at the fastest guys before it popped a fuel line and stopped. I intend having another go at them next April. They are all running 1100cc CB750s on methanol, but it is all 'point and squirt' stuff, and I'm convinced I can ride the Seeley around them.

I have lost the link to Jim Schmidt's web site, would someone please post it here? I need to keep track of it, so I can buy rods and pistons when I can afford them

http://www.jsmotorsport.com/

L.A.B.,Is there any possibility of having a sticky thread list of reputable Commando parts/servces vendors? Ta.