I just received a K&B 7.5 (.45) 9100 D/F series engine from a fellow modeler. My question is , At top dead center, it feels as if there is a flat spot on the bearings (I know there isn't however), It just feels like it. Is this normal for this engine. It also does it at bottom dead center. Just wondering if this is normal. Is it the sleeve or the piston , or should I just inquire about returning it???
Thanks all
Don

hi!

i also own a 7.5 kb. add a bit of fuel or after run oil and a propeller and flip it a few times .if you still feel the flat spot the engines have a problem. that " flat spot " it not the bearing or piston it the rear intake systems that is a bit rouch.

hope it helps !

pecojets

The K&B .45 is an awesome engine - I know, I blew up dozens of 'em in competition when I was a kid. The connecting rod is probably the one-and only- weak point. There should be, I believe .003" clearance in both holes, top and bottom. This is the "optimum" number for glow engines at high rpm (approaching 30,000 rpm- this from I believe an RCM/MAN article some years back)
K&B is the only engine manufacturer that holds to this number.

I had two marine engines exceed this to the tunes of 32,900 and about 33,500 - used to kill one set of bearings and a glow plug per heat.

I shudder to think what I could have done if ceramics were readily available and affordable back then...

If you can still get them, spend the money and get an "RPM" connecting rod. This is your best investment.
A good tuned pipe makes these engines killers....

Don,

It sounds like you could be feeling the interference fit that an abc engine has when the piston is at tdc. If so, that's perfectly normal. You wouldn't want it any looser, because as the engine warms up, it the fit gets a little less tight and the engine develops its maximum power. I agree with couch-pilot; the K&B 9100 is a great little engine.

b/r,
DFF