Ask the experts

[CHOPPA]

Thinking about other options for bucket bikes, still like the idea of modern 2 strokes...

What is the upper limit for a water cooled 100cc 2 stroke? Is it like 105cc or something?

Looking at a Aprilia RS125, it has a bore and stroke of about 54 x 54. It would assume it would be easier to change the stroke and lower the barrel rather than resleeving.

Would a radical over square ratio work? So reduce the stroke by 5 or 6mm whatever the equation for 100cc adds up to?

Would it be better to:

1, make an offset crank pin
2, is it possible to shorten a rod?
3, see if a shorter rod is available from something else

Probably just dreaming here but if it looks easy enough to do then ill do it

[koba]

Bert looked in to this quite a bit, the 125 down to 100.

He did it with a Yamaha TZR125 as it already has a short stroke.
He got it going, badly at first but it was really starting to show some results but then he switched to an FXR.
Temporarily hopefully!

There have also been a few based on RG150's all seem to go well, the one that Nigel Duff owns (Wobbly had input) is still one of the most powerful bucket engines I know if, it was build, hell probably 9 years ago!

EDIT: Starting with a 50mm stroke would be a good idea in my mind...

[chrisc]

Aprilia RS125 has a 54mm bore and a 54.5mm stroke.

54x45 is 103cc so you'd be taking out 9.5-10mm in stroke.

[Yow Ling]

While you can shorten a rod it has no effect on the capacity

Resleeving is pretty easy process

Suzukis are 50.6 stroke, you can use a 50mm rm80 piston for square bore/stroke

There is a proposed rule change for 107cc currently 103cc i think

going way oversquare affects the time the ports are open in a bad kinda way

[Bert]

Aprilia RS125 has a 54mm bore and a 54.5mm stroke.

54x45 is 103cc so you'd be taking out 9.5-10mm in stroke.

Chopper. Your on a hiding to nothing with the Aprilia motor.
I think you could reduce the stroke by 1.5-2mm and resleeve down to a 80cc piston.

But I'd be going to the suzuki, kawa or my fav the Yamaha. Which all have similar stroke ~50mm strokes. I thought you were building a cheater anyway...

Yamaha issues, standard rod runs a 16mm piston pin.
Spent an age destroying pistons trying to make it work until I found a yz100 piston (50mm 0.25 steps to 52mm) but slowly getting harder to find (keep an eye on ebay). Early 80s RM 100 uses similar sizes.

[speedpro]

You are proposing a big job and in the end you will have an engine with an undesireable bore/stroke ratio. Better to start with something closer to the mark.

[diesel pig]

The Cagiva mito 125cc has a 50.6 stroke same as a RG150, Then you can sleeve down and use a 2mm oversize wiseco RM85 piston. Then you get 50 x 50.6 as close to square as possible to get without major engineering.
Square is want you want all the best GP two strokes were square for a reason.

[Bert]

The Cagiva mito 125cc has a 50.6 stroke same as a RG150, Then you can sleeve down and use a 2mm oversize wiseco RM85 piston. Then you get 50 x 50.6 as close to square as possible to get without major engineering.
Square is want you want all the best GP two strokes were square for a reason.

I recall the cagiva runs a 15mm piston pin, ktm85 big bore (95cc not the 105cc) offers a piston solution...

As DPig as outlined square is best...

[F5 Dave]

yet oddly uses a reed on barrel arrangement.

have to use the 7 speed model for trickness

[CHOPPA]

Thanks all! Yeah the mito would be the ticket but they are hard to find and pricey...

The RS125 I was looking at sold so another reason to flag that but have learnt a bit haha