Scooterotica

hi, just fitted a 60mil crank and now finding that the piston skirt is bottoming out before bdc. Am I ok to trim a mil or two from piston skirt to rectify this problem. its an Imola top end by the way.
cheers,Baz.

What size packer did you add under the base to compensate for the extra stroke????

0.7mm packer and a selection of gaskets to play with.

packer will not stop the piston from bottoming out on the casing, it is stroke, rod and piston length below the pin that matter. On a reed valve cylinder it will not hurt to trim the skirt a little.

I was going to say that rich . il be trimming my gt piston at same time as that's going on back on at some point in future and that was catching when I checked. cheers ,baz.

I guess you've no choice, it won't do any harm. On piston ported motor you need to watch the inlet duration biput on reed motors shouldn't have an impact.

not up to speed on reed motors so when they say 360 degrees inlet timing what exactly does this mean and what goes on here ???

rosscla wrote:I guess you've no choice, it won't do any harm. On piston ported motor you need to watch the inlet duration biput on reed motors shouldn't have an impact.

cheers mate , your right there isn't much choice here so ive taken about 1.5mil off skirt and this clears casing ok.

bazza3004 wrote:not up to speed on reed motors so when they say 360 degrees inlet timing what exactly does this mean and what goes on here ???

Bazza,

This means that the inlet port is always open, and the flow of gas is controlled by the reed. In other words, the inlet port is open either because the port is not obstructed by the piston skirt, or there are windows cut into the inlet side of the piston that allow fresh charge to reach the crankcase. This means that the engine can draw fuel at any point during the rotation of the crank, either because the crankcase pressure is demanding it as a result of piston moving up the barrel, or the expansion pipe is demanding fresh charge by creating it's own negative pressure in the crankcase.

Of course, the engine does not draw-in fresh charge for 360degrees: it will only drawing in fresh charge while there is a negative pressure in the crank case. The important role of shutting-off the inlet charge, controlled by the piston skirt on a piston-ported engine, is controlled by the reed valve in 360degree engines.

The advantage of such a set-up is that the reed valve reduces spit-back, which reducues double-loading of the air with fuel, improves fuel consumption, improves bottom end power etc etc.

The down-side of a reed valve is that it is an obstruction in the inlet charge. Pure top-speed motors often use piston ported inlet (Kursaal Flyer) as mid-range tractability is not a priority.

Here endeth the lesson (sorry if I have patronised anyone)

Many thanks for your reply knowledge it is very informative an easy to understand (while always helps were im concerned).