valve train

[redflash]

I have a 420 that I want to incrementally build up. Thinking of champion rockers, chromoly push rods and 50 lb springs. Vegas Karts tells me that the correct length push rod is 6.74 inches.....They are out, and I can't find that length anywhere on the interweb. I realize that there are "cut your own" rods, but my cutting ain't always perfect. Is there anywhere I can order push rods cut to my spec for a predator 420 ?

Da Flash

 

[Functional Artist]

If ya can't find what your lookin' for "off the shelf", maybe get the "cut your own" rods
...& have a local machine shop, cut them to your specs.

 

[madprofessor]

If ya can't find what your lookin' for "off the shelf", maybe get the "cut your own" rods
...& have a local machine shop, cut them to your specs.

Ditto on cut-to-size pushrods and having a machinist do it.
Questions: If "incrementally" hotrodding a motor, do you plan on changing rockers again later to ratio rockers? That could require a different length pushrods. Do you plan on a different cam? Again, maybe different pushrods. Planning to remove governor? That higher rpm is the only reason for stiffer springs, and if you do that, then a billet piston rod and flywheel would be called for also. Know what a squish test, aka clay fit test is? You'd need to do that any time you change the pushrods' or piston rod's length or the head gasket thinness or the cam, in other words the clearance of the valves from hitting the piston.

 

[redflash]

No roller rockers..possible mild cam like mod2. ( pushrods are fine with that one)...governors long gone. My intent is to keep it under 5000-5500
rpm, this is off road application, so absolutely no sustained high rpm issues. Nor racing competitively , just beating around the rocks and hills of california. I'm already stage 1, and just want add a little...so maybe 32-34 mm mikuni, or blueprinted stock carb, and mod 2 cam at some point.
It makes sense though to beef up the valve train, ain't taking off the head though !

Da Flash

 

[karl]

I would buy a pushrod length checker tool, assemble the motor, and see whatcha really need,
then go from there.

Just a thought, if one push rod is a couple thousands shorter than the other, nothing bad will happen.
That's why the lash is adjustable. But I still understand the concern, I myself would way rather use the correct size off the shelf,
but have made my own.

 

[madprofessor]

My intent is to keep it under 5000-5500
rpm,

I sent my 12+hp Predator 212 minibike (DragginBallz) off to Ohio (had the round silencer) with the OEM flywheel still on it, and didn't lose a single minute's sleep about sending the buyer a timebomb. The valves with the OEM 10.8# valve springs will start to float at the rpm you say is your intended limit, and the cast iron OEM flywheel won't explode at that point.
Much has been said about using valve float as a rev limiter, but the other options always have a possible failure variable. Valve float can't ever stray into an over-rev state, because valve float won't let the motor spin any faster no matter how much gas you give it.
Just beware of stiffer valve springs, it's the most common factor for increasing rpm. Plan for that with the proper safeties in place, like billet parts in the rotating assembly.
ADDENDUM: My personal choice for a 15+hp Predator 212 (has the rectangular silencer) with cost being a concern has been a PVL 28-degree flywheel with a 7.2-degree advance key, a .265 low/mid torque cam, and an ARC billet .020-over rod, and a .012 S.S. head gasket. No valve/piston contact in that mix.