XL250 -350 conversion ??

[Mick22]

Just got given a XL250K0 in poor cond but complete, I want to build into a race bike but I've already got a couple of Pre 75 250 race bikes so thinking of building it up for the open class. I guess I'll have to base the engine on an XL350 to get it big enough ?? Is it just a matter of swapping the crank and top end to make it 350cc + or do I need to start with a complete 350 motor

[jimg1au]

in past posts this has come up before.i thing the best way is make your 250 a 305 better power on a lighter engine
this was my understanding.all stuff to do this is buy off the shelf stuff from usa.i was looking at this type of project but got a tt500 motor instead in a running slider.
cheers
jim

[Freakshow]

What about a 412 kit ?

off the shelf and ya can use titianum valves and all that other secret stuff i know you boys are using

[Tim754]

You cannot just swap the crank and top end ,different crank case halves. Complete  XL350 engine bolts into 250 frame.

[jimg1au]

check this out previous thread
http://ozvmx.com/community/index.php?topic=4748.0

[Mick22]

Thanks for that fellers, answered all my questions   Thank god for the internet, that would have taken me weeks to find out 10 years ago

[cxs58]

Hi Mick22 - 305 is the way to go and I might know of someone in brissy who has a spare kit.

[cxs58]

I think he still has a stroked 350 crank as well.

[amaiconda]

I have a 74 XL 250 with a 350 crankshaft and gearbox. There were a few things that had to be done to accomodate them- I had to have the Crankcases milled out to accomodate the 350 crank bearings. The oil transfer to the head via the Tacho drive had to be altered because of the larger bearings, and rebore locating holes for bearing locating pins. The splash plate in the bottom half of the crank cases had to be relocated (lowered). I had to addapt a retainer for the gear that links the kick start to the oil pump on the end of the countershaft. I also had to swap barrel studs to 350 ones, and bore out the top crank case to aqccept the 350 barrel Appart from those few things I had very little trouble addapting 350 parts to my 250. I did this basicly because I am cheep! At the time here in Australia registration for a 250 was much more affordable than a 350. I was able to retain the serial number for a 250on the motor.

[firko]

I've had a bit of experience with these engines over the years and can guarantee that the bigger/hotter the engine becomes, the gearbox life shortens accordingly. While a good 410-420 big bore 350 does indeed produce the HP, I found that a 305 kitted 250 does everything the biggies do but the gearbox survives a lot longer and is 12kg lighter.

One little thing I learned the hard way is to use a cush drive rear wheel. I was using a MK8 Pursang rear in my Champion 350 Honda and was going through gearboxes at a rate of knots. When I changed over to a cush drive Yamaha wheel the gearboxes lasted a shitload longer.