Rod Play

[Mike Lenz]

Does anyone know how much up and down pay you can get away with on a ktm400 engine? New rods will delvelop some play fairly quickly. I have even seen a new one with a slight amount of play. But how much is too much? How far can you go? Dont want to lunch a set of cases!! I realize they arnt suppose to have any, but they do. Every one Ive seen does after not much use. This is the weak point on the 400 engines so it scares me to just let it go. I have blown one up and lunched the cases back in the day. But that motor was ported to the moon and reved too high to begin with. Ive never had a stock engine let go but that always present play scares me.

 

[Rocket]

Mike
Are you measuring the play with a dial indicator or is this just by feel?
Rod

 

[Kip Kern]

Mike, I have seen a figure of .008 max in a KTM manual but I can't remember what manual I have seen it in.  I was building a 400 at the time when I had the same question.  This is rod play only, does not include piston being mounted.  Maybe someone out there will know for sure.

 

[Rocket]

.008 sounds like an awful lot.  Somebody should know for sure.
Rocket

 

[Mike Lenz]

I havent measured it with a dial because I have no idea whats too much. But Im guessing Im feeling one to two thousands. I have 3 400s I ride and they all have play,some a little more than the other.

 

[Rocket]

Mike
The only way I have ever got an accurate measurement is with the crank out and clamped in a vise with a mag base dial indicator on the vise.  Then you can move the rod up and down and tell what you really have.  I'm thinking less than .002 would be okay.  Long time since I've had to do a rod kit.  With the crank in the motor and trying to feel the movement is really hard because of the side clearance also.
Just my 2 cents worth,
Rod

 

[sachsmx]

According to my INA Bearing two stroke crank pin bearing design guide, the recommended radial clearance (play) when new for a crankpin of 18-30mm nom. dia. (this should encompass the 400 engine) should be 0.017mm (0.0007in.) to 0.026mm (0.001in.) when new.
My golden rule is if you can feel it, it's too loose. I'd say that no more than 0.0015in. max. to be on the safe side.

 

[Mike Lenz]

Like I said I have been able to feel it on a brand new one. UP and down, not side to side. I know CZ guys have said the same thing about new CZ rod kits having play. Id have to change rods more than pistons to keep almost all play out.

 

[Mike Lenz]

In a 78 engine repair manual I found not only the spec on the crank axial clearance, but also "crankshaft pin oscillation" of mzx.05mm. What is that??

 

[Kip Kern]

Mike,  I guess I would call someone like Euro Rod as they may know?  I think they moved to Florida recently.  Good Luck and see you at Mid Ohio.  Breakfast will be ready around 6:30-7:00!

 

[Mike Lenz]

Kip thanks for the generous offer but I will not be able to make it to Mid Ohio due to work reasons. Have fun!