just got a Z400 with the thumper racing 470 big bore piston in it. the quad has a very audible knock sound at low speed idle. the original owner had this to say "because of the big piston, the machine shop bored the cylinder a couple thou. of an inch big to allow the piston to swell and get longer ring life. so what you are hearing is piston slap, not rod knock. it has sounded like this from day one, but has not given me a bit of trouble and goes away under throttle and when warm."

does this sound right at all? are the big bore motors noisy? i am afraid to ride this thing or even rev it up for fear this other guy isn't telling me all the story. have only run it a couple minutes a couple times.

please help!

 

Thats the problem these days,every machinist has there own tolerances.Instead of following manufacturers specifications for proper piston to bore clearances,they take it upon themselves to re-invent the wheel.

Different pistons expand at different rates and sizes,but no piston should slap the bore when cold.Nor should a piston need that much room for expansion,unless maybe the engine runs temps in the 240 range.

Just because it quites down when warm up doesn't mean its reached maximum expansion either.Its probably on the high-end for running clearance when its fully warmed up.

Regardless of what they tell you,the knock is not normal and they are not engineers or metalurgists,they are machinists.
If they did there job right in the first place,then they could blame the manufacturer,instead they have to take the blame.I'd at least pull the head off,drop the piston down in the bore a bit and try to get some bore sizes.Then check with the piston manufacturer for there recommended specs.

Check to see if the specs are within reason,if not,contact them and ask them to explain why.

On the other hand,you said that you bought it off of someone else who had the work done,maybe the motor is already whupped.

 

i went and got an opinion from the top notch shop and let them listen. they are convinced the sound is not good, but not piston slap either. sooooo..... as of tomorrow morning, the process begins and the motor is coming out. they are going to work their way down noting everything that is not right and do it up the way it should be (or should have been, in this case) and they guarantee it will be flawless. yeah, i got a great deal on the quad, but this will eat into my bonus cash a bit. oh well....

 

Hey when you get it together let us know how badass that 470 kit is...I'm gettting one later on or when my engine goes....