Reverse configuration of a RX125

[Mrs Busa Pete]

I got my self a wobbly custom bult rotex 125 chamber for $67.00 deliverd of trade me for the ysr/rx 125 .I was going to cut and fit it normaly but it went in the too hard basket about 10 minites after un rapping it so i pulled the head and barrel of and had a look both sides looked near exact so i turned the pistin around and then reasembled it in reverse flow. Fliped the carb mount to level up the carb throw on an exhaust and it fired in to life reved high sort that out later after wobbly is fitted .May have to lift the rear sub frame to clear tyre and get seat and tank mounted but so far so good .

[speedpro]

I'm not sure how much differance it will make but the hole in the piston for the gudgeon pin is offset. Turning it around will affect rod-crank geometry and thrust face pressure. Due to the geometry changes it will have an effect on the power delivery but as to just what and how I don't know for sure. If the rod ends up being more vertical at ignition the motor will be better at higher revs and vice versa. All to do with rates of combustion pressure change and simple geometry. Just thinking that it will also affect piston acceleration either side of B & TDC which at TDC could change squish velocity though again not too sure of the amount or significance.

[speedpro]

Also with the exhaust being rear exit it is going to be a short run to the back of the bike. You can save a bit of length by sliding the stinger into the rear cone to the point where the coin meets the middle section. I've done this and didn't notice any change in performance. Another friend had the stinger exiting from the side of the centre section, again with no noticeable performance change.

[TZ350]

Piston pin offset changes the geometry of the main bearing, big end, little end, piston and cylinder center line relationship. The piston pin is offset so all of the above can not be in straight alignment all at once and in effect lock at TDC.

With the piston pin offset you can see that if the piston and bigend are at TDC and the little end pin offset to the thrust side, the rod will be on a slight angle to the bigend pin.

This gives the rod some helpfull leverage at the TDC transition, in getting the crank moving and most importantly it allows the piston to gently rock from the non thrust face that was rubbing on the cylinder wall during compression onto the thrust face for the power stroke. With piston pin offset this happens just before TDC.

This is the important bit! with this rocking just happining before TDC the transition "piston slap" is much reduced.

Pin offset can easily be seen by fitting the pin and looking inside the piston you will clearly see and measure any offset. If an offset piston is turned arround the transition slap will be greatly increased.

Some motors offset the cylinder in relation to the crank centerline and some others don't use offset at all so not all pistons have their pins offset. Pin offset also has an effect on how the motor runs and can have performance advantages but was not the primary reason for its use. In the old days of clunky internal cumbustion engines offset was used to help them run quieter and extend piston life.

[speedpro]

In the old days of clunky internal cumbustion engines offset was used to help them run quieter and extend piston life.

I learn something new every day.

[Mrs Busa Pete]

had to much trouble with power delivery so decided to put rs 125 motor in the rs 125 frame in the original configoration. Plan to get hubs respoked to some 17" rims for beter tyre choice work in progress it will only be a spare bike for mates to have a go at buckets unless it is faster than the fxr.
Thinking of a 100cc 4 stroke for the ysr so can run in f5 and f4 the `100cc4 stroke does comply for f5 dosent it

[Pumba]

decided to put rs 125 motor in the rs 125 frame in the original configoration.

Shit Pete go to work, your brain is going to mush, I think you are talking bout your RX (or I hope you are)