crank tolerances - need some help

[EthansDad]

Hey,

how much "play" should there be on the lower end of the crank and in what directions? I've got a fresh top end on one of my bikes and getting a nasty "tapping" noise coming from the motor.

I checked squish and its fine. even brought squish up a bit, no help on tapping.

If I take the intake (carb boot) off, I can get my finger on the crank and get the rod to go left-right about .5-1mm. I can not get any play in the rod up-down (in direction of piston rotation) though. Is this normal, or is my crank shot?

Any other ideas to check for this tapping sound? nothing stator or variator side hitting, and its definitely not a pipe or other vibration rattle.

-EA

[nastynotchback1]

i have heard strange noises when the crank bearing is going out.

[mmsoup]

Is it a fresh cylinder as well or just piston and rings?
Is there a new wrist pin bearing in?
Was c clip in groove on both sides?
If all OK but cylinder is used, I have three Malossi BB cylinders and all three sound slightly different with a new ring/piston. One that I cleaned up after a seizure makes a good deal more noise than the other two but I get just as many hours out of a ring on it as I do the other two (One new and the other a new replate) I could swear the ring was snagging every time I put a fresh one in and all works out.
Cylinder is probably slightly out of round on mine and causing the noise because it never actually snags a ring
I have a Malossi Team crank that has a good deal of lateral slap new in the box and I have two Stage 6 cranks, one of which has roughly the same slap that you describe as well. The other was rebuilt to tighter tolerances by a reputable shop with Stage 6 parts and it's the one in the bike that makes all the noise........go figure

[EthansDad]

same cylinder as came off of it and was run prior with no tapping.

yes, when I changed the piston, I also replaced the piston pin and top end bearing. with the pin (no piston) in the top end bearing, there is no play up/down on the top end bearing.

I flushed the crank and had it upside down shaking it - nothing came out.

yea, at this point, I'm going to replace the crank. its got a season on it anyway. I had another similar motor that I checked by pulling off the intake boot and yes, on that one too the lower end of the crank rod does move left/right just a tad, but no up/down motion.

-EA

[EthansDad]

**SOLVED***

Thx for those that gave some input here. turned out, it was not the crank. was a two part issue really, but the bottom line was the piston was hitting the head. what through me off is it was only hitting on one side, just barely and if I did a squish test (piston/head tolerance) it showed just fine. I brought my squish up several times, like 3x normal and it still tapped.

Problem was two part - one, DRR OEM 50cc Pistons - seems like they are shipping something *slightly* different than before - not sure if this was the only problem, but suspect.

Other part - the head was mis-aligned on the cylinder. I've done a lot of top end jobs and never seen this, BUT when I put the head on, it was not 100% aligned to the cylinder jug. I actually had to put the cylinder on, move the piston to TDC so it stuck out a bit, put the head on, then pull on the head hard to the left and hold it there while I bolted it. could tell as I tighten it if it was still hitting.

Very odd, but just thought I'd post it up here in case anyone else ran into this. I finally figured it Thursday night in my race trailer trackside at the nationals at about 12:30am and had to wait until Friday AM to get parts - like 1 hr before practice. nothing like getting it done at the last minute, eh?

[mmsoup]

As long as it started, ran and performed............priceless