Finding parts for 125cc watercooled motorbike engines for project use.

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Dec 26, 2021
Reaction score
Kawerau, New Zealand
Relevance to engine build projects- I could cobble up a high performance engine, using some commercially available parts.
I don't think I will be getting into aluminium castings, though.
Pistons, rods, cranks, gearbox, clutch, primary drive; are examples of readily available parts that I can use.

I see I can buy a complete watercooled 50cc motorcycle engine for very little money,($500-$600) but 125s are restricted to gearbox gocart engines for quite a lot more money.
- or crate engines for the Yamaha YZ125 motocross bike-
Are 50cc 2-stroke engines still legal for road use?

This cheap 50cc was for motocross use.

I have seen others with radiator and tuned pipe deals as well.

Individual parts sets seem to be restricted to YZ125 motocross engines, and they are quite expensive.

A problem with building a 125 engine is that I need a suitable engine dyno.

If I geared it down to 4000rpm and put a 3 ft prop on it, that would be really breezy! - a 23 inch prop at 6000 rpm is windy enough!
loads are of the form mw(sq)r, so

the big 11 foot props are limited to 1500 rpm, mainly for speed of sound limits.
For a similar tip speed, a 3 ft prop could turn at 5,500 rpm.
how does this compare with 8000 rpm at 20 inches, for a linear relationship?
that is 4500 rpm at 36 inches.
Force is related to speed(sq) , but
Standard RC model plane props are generally speed-rated directly from diameter and rpm.
A wooden prop may possibly need a slightly lower rating than a fibre-reinforced resin prop.
Pitch for the 3 ft prop can be increased quite a bit.

This, I suppose, wold be an ultralight prop. - they generally use around 500 cc engines, but still around 40-60 hp.

Any ideas on small engine dynos?

I expect an eddy current design would be good.

I am trying to get an engine project going without actually spending the earth!

I think the most I spent on a small engine plus muffler was about $1000.

If I stuck a motocross engine in a road frame, could that be registered, or only suitable for offroad use?
I can see I can get a complete running road bike for around $1800 in the 125-250 4-stroke range.

The 125 4-stroke frame and running gear actually looks substantial enough, even if it is only making about 10 hp.

I see that people are putting 80cc engines in bicycles, and they can ride those legally in New Zealand.
- different areas probably have different rules.

If I can find a base water-cooled cylinder and head set, I could modify that using J-B weld to hold modified external transfers in place.
The exhaust passage needs to be part of the main casting, I think.

Cooling passages don't need to go below the level of the transfer tops.

It seems to be best to integrate the cylinder into the water jacket, and use Nikasil bore finishing.

A dry liner is no good in a high performance two-stroke, for cooling.

What do you think?
Do I have a prospect of getting into watercooled 125 two-strokes without extreme expense?

A benchtop lathe/mill combo would be good to have, though, at around $3500 nz. I can get a dedicated stand for this.
- This kind of thing is more worth actually spending money on.
-plus a suitable slip-roll set. The cones don't need to be longer than one foot.