Nick from Kent wrote:Iv'e recently bought a Monza 160 and although the engine has allegedly been rebuilt, it smokes a lot soon after starting. I have had the top end and barrel off and the rings seem to be new, there is no visible wear on the bore and the piston has no slack movement. The head was a bit coked up, so I gave that a clean and took a look at the valve seats, which seem fine with no sign of burning. The tappets are correctly adjusted to 0.06mm. Now I'm stuck! Anyone have any ideas where I should look next to see where the oil is getting in?
Hello Nick from Kent,
Ok, let’s get this one started ...
It's normally best to try an initial diagnosis of a smoker with the engine still assembled

. But seeing as we're past that stage:
There's really only two ways for oil to get into the combustion chamber: down from above, i.e. down thru the guides. Or up from below, i.e. passing the rings.
1. If it smokes AT start-up but then gets less, it's most likely guides (basically 'cos, while the engine was stationary, oil was seeping down the guides and collecting in the combustion chamber).
2. If it smokes AFTER start-up and continues blowing, it's most likely rings (basically 'cos the movement of the piston is continuously pulling oil up the bore and into the combustion chamber).
The fact that the rings LOOK new means nothing. They need to be the right size, be fitting properly in the piston grooves, and be correctly gapped. These are best checked by someone with the necessary equipment, knowledge and experience.
As for the piston being slack or not ... well, you can really only ascertain that by measuring the clearance between the bore and the piston (as well as checking for any ovaling in the bore).
The condition of the valve seats tells you nothing, I’m afraid. But:
1. You could try wiggling the valves in their guides - the specified maximum clearance is .057mm.
2. There should be an oil seal on the top of the guides. They should have been replaced; but even then, they can get damaged during an engine re-build.
HTH, and over to others!
Craig