Alternative rods and pistons

[Diesel]

And to add to 50gary's excellent response... when using longer rods the acceleration curve of the piston through mid stroke is less dramatic and helps reduce the likelihood of piston ring flutter as rpms increase.

[1crazypj]

OK, for use with Honda CR500 con-rods only---
you want pistons?
Some are originally designed for 4 valve heads but are easily modified.
Honda Hurricane, I got some 82mm forged, think 79 or 80mm may be stock size
Suzuki VS700,750,800cc V-twin, 80,83mm.
Yamaha XV750, 83mm
Honda XR400, 85, 86,87, 88mm(big bore pistons available)
Yamaha XT350 86mm
Honda CM450 twin (around 1982?) 75mm (long rod 650 racer?)
Suzuki GSXR1100 78mm
As the CR500 rod needs small end bushing to size it isnt a problem to fit any pin you want. the only real criteria is the crown height, centre of pin to top of piston. If its around 25, 26, or 27mm your good to go! The compression may be a bit low with the smaller sizes cos they are designed for pent-roof combustion chambers but it is possible (not easy) to mill head and reposition cam sprocket. If a piston is available aftermarket( J.E. Wiseco, etc) you can probably get a part machined blank and cut top to fit combustion chamber ( I havnt made any enquiries cos I'm still experimenting with used pistons, except for the XT and XV ones)
If you have a european motor with the 140mm rods you can use XT500 87mm pistons(880cc?) but they have to be heavily modified and they are the absoulut maximum you can ge in stock case with boring, to go bigger you have to weld cases, reposition studs and pretty much make a new cylinder and head
Suzuki SP 350 85mm (840cc?)
Thats all from me for now.
PJ

[eric123]

thanks and great work...please keep us advised of your progress.

p.s. any theories as to how low the compression will be with the 25 to 27mm crown heights...judging by stock pistons, the compression may be 8 to 1 or so with some of these alternative pistons...

[1crazypj]

With a high crown piston you can machine a squish band into piston top to fit it into the combustion chamber. the greater swept volume raises the compression. 82mm flat top pistons will give about 8 to 1 CR, thats why I machined head. When I went to 83mm from XV750, the pistons needed a flat surface machined into top above top ring, 5mm wide, angled to match combustion chamber, sticks up about 4mm, had to make a 20thou aluminium base gasket (plus the standard one) to get CR back down! I'm still messing with valve timing so I havnt measured actual CR yet.
Will keep you all informed
Bye for now,
PJ

[bigmatt]

Just to bring this old topic back to the surface again;
It sounds like the Low-dollar Do-it-yourself Grassroots way of making a big bore kit with a decent compression ratio (10:1) is to use xv750 pistons with cr500 rods?? I like this because of the lower piston accelerations, and I'm assuming that all of the other pistons suggested are flat-top/semi-domed, which means low compression ratios. I think this is the route I'm going to take, but I still have a few questions.
1. Can the stock liners be bored or are new ones required (I'm assuming that new ones are needed and LA is the place to get them)
2. Are the valve reliefs in the xv the same as in the xs?? I can't find many pics of the pistons so I can't really compare the two.
3. How much can be taken of the top of the xv750 piston before the crown gets too thin/weak?? Can I get 10:1, or will I need to raise the head with spacers and thus screw up the cam timing??
If I'm wrong about any of the other pistons, please let me know.

Thanks for the help. Bigmatt

[jack]

I think i'll add to this also,a longer rod allows the the piston to dwell at TDC longer,which in return gives a more complete combustion, which means more power overall,less piston speed as Gary mentioned and the torque is of a flat curve with alonger peak as compaired to a short rod with a short peak curve and falls off quickly.Thats why Outlaws are so deadly in the curves,they use a short rod to build their torque quickly for coming out of the turn but have nothing on top end. the perfect rod ratio would be in the 1.80 range. the higher the ratio the less of a load on the thrust side of the cylinder ,which equals less friction = HP Jack

[bigmatt]

Just an additional thought, would longer rods reduce the amount of vibrations?? If acceleration of the pistons is reduced, you would think the vibrations of the motor would also be reduced. Just a thought.

Bigmatt

[jack]

A longer rod will not reduce vibrations being your adding weight but you could reduce the weight of the piston to counter act the weight addage of the longer rod or have H 11 tool light weight wrist pins custom made along with the light weight piston to really reduce the rotating mass to reduce vibrations. To reduce vibrations you either balance the motor,rephase it, weight reductions in the motor, exhaust system and the way the motor is tightened up to the frame. This being a parallel twin motor it's difficult to reduce vibrations with exception of rephasing in my opinion Jack

[bigmatt]

I had originally planned on just rephasing, but then I found I'm only getting 130lbs of compression. I guess it's not bad for a bike with 25k miles on it. Anyway, I figured I'd go big bore instead of just a rebuild, and I might as well do it right (use cr500 rods). Anyone have the xv750 piston information??

Thanks

Bigmatt

[crazypj22]

I didnt realise computer had been down so long!
I'm going to do a 707cc with Suzuki GSX-R pistons. (78mm) just bore stock cylinders and do the CR rod thing. I guess by now you've found out CR crank pins are 3mm longer than XS, no big deal to grind them shorter ( they have hole in centre as well so they will be lighter than stock 650). If anyone wants to try changing balance factor slightly, the GSX-R piston pins are correct diameter, just shorten to fit your pistons ( they are lighter weight, taper inside) going to do a few other mods as well. should have the 800 finally done by end of Feb. taken far too long to get to the stage its at but sometimes you cant do what you want when you want
( the crazy formerly known as 1crazy pj)
PJ

[jack]

PJ........can you PM about this 707 combo and Rod combination,now you've got me thinking bigger is better Jack

[bigmatt]

crazypj22 wrote: I guess by now you've found out CR crank pins are 3mm longer than XS, no big deal to grind them shorter ( they have hole in centre as well so they will be lighter than stock 650).

I did find that durring final assembly of the crank. We assembled it, and the cr500 pin will fit, they just stick out a bit from the crank cheeks.

I have the xv750 pistons, and am planning on machining the squish zone into the domes. Any suggestions on how much to take off?? I was planning on just following the contour of the existing piston, but you had said that there were problems with really high compression??

Also, what would you recommend for the valve reliefs?? are they necessary when using these pistons?

thanks,

Bigmatt

[crazypj22]

If you use XV750 valves (43mm) you have to machine a bit off pistons.
I shortened the top about 2.5mm to the diameter and angle of the combustion chamber to get a squish band (or quench area if you prefer)
The main bearings may hit the extended part of the pin if you dont grind it to length, that will either stop the engine turning over or break something when you try and bolt it together.
i shortened pins before assembly as I didnt want grinding grit/dust in mains and big-ends.
PJ

[jack]

PJ............where can one get a hold of the crank pins and pistons pins and pistons, you're talking about? There're a great solutions for lighting the bottom end and the con rods. What is the cost here also .............Jack

[bigmatt]

Jack,
The Con rods are from a honda CR500 and come with the big end and small end bearings, and the big end pin (hollow). I forget who I got the con-rods from but they were about $70 a piece. Weisco also offers the kit for $92 each.
The pistons were out of a wrecked bike with 3k miles off of ebay for $20.

The pins will fit without modification, and have about .020" clearance between the outer bearing races and the end of the pin.