HONDA XL BORE OUT

[DGBCustoms]

Hi

I am interested in doing my first engine bore out.

I have a Honda XL 125 1979 and I saw on ebay a cylinder and the head from an XL 250 R bike with 4 valves.

Can I mount the the cylinder of the 250 on the crankshaft on the 125 with out any modification.

Because if so keeping the stroke of the 125 which is 49mm and the new piston with 74mm will bring the engine up to 210 cc with a 4 valve head.

Is this achievable or it requires more modifications on the cranckshaft.

I appreciate your time and consideration for this post.

Regards

[G]

I would expect you'd have to make some significant changes to the crank cases.
If you want a faster bike, I'd suggest just getting a faster bike .

[_Iain_]

[G]

Cylinder wall won't be a big issue as it's a new one. He's not talking quite 100%.

However, a 170cc kit on a 125 2 stroke often isn't a massive issue if the kit was designed for that. But as you suggest, 4 strokes have more moving parts and so on - so you've got more worries about everything fitting without basically building a new engine from scratch.

[stevo as b4]

Not a chance in hell and a very stupid question to ask, that the OP would have realised if he had an XL125 and XL250 engine stripped down to look at. The 250cc crankcase will be totally different, and have obviously much bigger stud spacing and more room for the piston at BDC.

It's never going to work! You can get 150cc/160cc big bore conversions or even foreign market 150cc motors that would probably have interchangeable parts.

The OP either needs a 150cc or 200cc clone engine, or a complete XL250 engine and frame surgery to fit it.

I dare say Teffer's might have some input and data here for various 125cc OHC Honda lumps, and what is compatible top end wise.

[G]

[Fizzer Thou]

The XL125 and the XL250R engines in question are very similar if the 250 is like the one below.The American version was based on the UK version of the XR200 but with the RFVC head.But the UK version XL250R had a horizontally split crankcase and was based on the XL250SA engine.


It was a well known modification to put the XL185 barrel and piston onto the XL125 bottom end,having bored out the crankcases so as to accept the larger diameter sleeve.
When I owned a 185 I wanted a bit more oomph so bought a piston for a Honda XR200 and had the barrel bored out.Worked quite nicely

https://www.cmsnl.com/honda-xl125s-1979-englandmph_model20140/partslist/

https://www.cmsnl.com/honda-xl250r-1984-usa_model841/partslist/

https://www.cmsnl.com/honda-xl185_model354/
https://www.cmsnl.com/honda-xr200_model374/
____________________
Just talk bikes.What else is there?

Always have a 'Plan B'