Std or lightened crank?

[wes]

Bought my race bike with polished and balanced crank and rods, and rode it until a rod let go at Brands GP.

Bought a std engine, and put my race head on with sp cams, but less compression as lost oversize and worked barrels, crank was as std.

When I checked the valves, the adjustment seemed miles out, which is the first time I'd checked, but shouldn't have altered swapping heads to different engines.

The "new" bike seemed easier to ride and pulled smoother, so could this be down to the std heavier crank or the valves?

Want to rebuild with new shells etc over the winter, but not sure whether to rebuild with polished crank.

Second decision is whether to go with stock rods, or a spare pair of polished rods I got with the bike of unknown history.

I have heard that rods go, regardless of mileage, but the more their used the more stressed/weak they must become...no?

Any real advice?

[Ewetea]

As an engineer I offer my comments: it has always been my understanding that rods were polished to remove surface imperfections that might lead to hairline cracks and ultimately failure of the rods. They are continually subjected to alternating forces of tension and compression as the crank yanks them down and then pushes them up the bores. This is only likely to occur under race conditions when the items are being subjected to extended high levels of stress due to the requirement to keep the revs up and make use of the powerband. Under normal road conditions they are more than adequately designed for the life of the bike. Years ago cranks used to be nitrided to provide a "skin" on the surface to again prevent the formation of stress cracks. They are also lightened to permit the revs to rise quicker as there is less weight to spin up. Ideally, material should be removed from the periphery as that results in maximum material removed at biggest diameter. Again, a road bike needs the mass to provide an energy reservoir at low speed ie tickover. That is obviously not a requirement for a race bike. Oh, and any removal of material should be done leaving a nice radius rather than a sharp corner, which would be a stress raiser.
In 1968 I had a 250 Royal Enfield, and I installed a 9,5:1 comp piston. After 500 miles the crank snapped! Aah, the good old days...thank goodness they're gone.

[cargo]

When I checked the valves, the adjustment seemed miles out, which is the first time I'd checked, but shouldn't have altered swapping heads to different engines.

The "new" bike seemed easier to ride and pulled smoother, so could this be down to the std heavier crank or the valves?

Any real advice?

I believe you have answered your own question there.

Your right valve clearances prob wont have changed as a result of sawpping the head.....but how would you know either way if that was the first time you checked them