Help!

[Andres]

What rod is better 38mm or 40mm? And if so why? Example:Z-L or FM80

 

[crassius]

I like 40mm stroke on a 38mm rod because this brings the rod square to the crank at a time when piston is higher in cylinder which means the pressure is still high in the firing chamber.

 

[Frankenstein]

I like 40mm stroke on a 38mm rod because this brings the rod square to the crank at a time when piston is higher in cylinder which means the pressure is still high in the firing chamber.

How simple is it to just swap rods... I would assume you'll need a press, is there an advantage to it?

Is it possible to find a 2 piece rod like those that are seen in many other engines? Would there even be an advantage to that other than easily swapping the bottom end bearing, which happens rarely if ever..

 

[Street Ryderz]

I like 40mm stroke on a 38mm rod because this brings the rod square to the crank at a time when piston is higher in cylinder which means the pressure is still high in the firing chamber.

Our rods are 85mm center to center for a 40mm stroke the con rod bottom pin is 1mm higher from center in the tdc position so that it totals 2mm through a full rotation,Fred at Cr Machine has done some video's showing rods and he discusses the length but never shows a measured rod length! I have every rod he showed all the same markings and every one is 85mm center to center with one exception the newest zeda engines do use a longer rod on a lower crank pin!

 

[crassius]

How simple is it to just swap rods... I would assume you'll need a press, is there an advantage to it?

I believe A80 motors are 40 X 40, PK80 are 40 x 38 while standard motors are 38 x 38 (tho some may be 38 x 40) - hard to tell looking at outside of motor, and hard to tell which your seller is selling as some just pick a popular name and hang it on their product.

There is a big advantage t making rod square to crank as soon as possible - see original engineering articles by Vincent (of black prince, black shadow fame) written n late 30s - or read about the 1/2inch rod shortening by triumph 750s around 1980 or so.

 

[Steve Best]

Besides the displacement advantage, most of these engine's ports seem to be designed for a 40-42mm stroke.

Longer rods reduce piston thrust and improve leverage on the crank.
This is balanced off against the greater reciprocating weight of a longer rod.
Gains are real but very small.

Pressing crank pins out and in is beyond the average hobbyist.
It takes a big a$$ press, some jigs, lots of knowledge and some $$$ measuring tools.

 

[Street Ryderz]

Pressing crank pins out and in is beyond the average hobbyist.
It takes a big a$$ press, some jigs, lots of knowledge and some $$$ measuring tools.

That is true if not done right you can kiss your engine good bye in no time!I am lucky and have the 6 ton press and lathe to do this procedure properly and quite a few years of practice at it!I will say though sticking the crank weights in the oven for awhile to 600 degrees makes putting a cold pin in much easier then just pressing it in!Truing it after the fact takes the most time and work for me I am not like Fred(CR Machine) I wont bash the crank while chucked in the lathe because my setup is no where near as beefy to take those impacts so I have to remove it to make adjustments reset it and set dial indicator up each time!