1983 XS250 to 400 conversion - My Build

[CadeXS]

Right hey guys. Let me start off by saying this is my first bike. I am a amateur.

I have picked up and old 1983 xs special. It came with a 250 and 400 spare including whole engine.
I am replacing the top half, cylinders etc with the 400 and so far I have run into some problems. (By the my father who is handy with mechanics is guiding me through the process.

The 250 motor has bad rings and leaks compression terribly thus the change. And I want a bigger engine

First problem is that I was planning on putting the 400 block onto my 250 crankcase/gears or whatever, but the 400 pistons don't fit the crankshaft (I've included a picture)

So I am plannig on taking out the lower end changing the crankshaft. will the 400 crankshaft fit straight into the 250 lower end/gearbox. I wana know if anyone else knows before I go ripping into it and finding out it wont work and I should've just bought new rings......

I apologise for getting any terms wrong. I am new.

Thankyou

 

[CadeXS]

oh any advice to what I am doing would be appreciated. Cheers

 

[gggGary]

Don't do it. Get on Craigslist and find a complete running bike or post a WTB ad. Really, that's my advice. If you can't do that go get rings or whatever. But small engines are not worth working on, go find another one that runs and bolt it in. conversions or swaps from different models are NEVER easy. Unless someone has documented a "this works" swap there will be a BUNCH of gottchas that will make it undo-able.

 

[inxs]

- i thought the 250, 360 and 400 were all variations on the same building block, the stroke all being the same with the bore, valves and cam varying-250:360/400 -?-

 

[txpowdercoater]

I agree with the motors are cheap and swap it out , get to searching, I think you will be amazed at what you find.

 

[CadeXS]

inxs- yeah that's what I thought too. I would've thought this would be an easy process considering that on the frame it says 250/400 hence it being bigger than most 250s. and aswell the hole for the piston in the crankcase fits the 400 piston. So it all seems interchangable EXCEPT the conrod.
what I am wondering really is that will I be able to fit the 400 con rod onto the 250 crankshaft?
(finally got my terms sorted, bare with me haha)
although looks as though I'll be going for the rings

 

[gggGary]

The key word is variations. You just have no way of knowing what those variations are typically there will be 50 to 200 detail changes from one engine size to another. Any of them can be show stoppers. differnt size bearings larger counterweights (very likely) so a bigger cut inside the crankcase to clear it. Like I mentioned unless someone has documented a swap you are headed down a very time consuming most likely fruitless path. Take an XS650 for example their are huge changes in the same size engine year to year and lots of parts don't interchange up and down the years. This a voice of experience on swaps, been there, given up on that.

 

[ivor]

I've only just discovered this thread, and I'm surprised not more people commented on it.

As far as I am aware, the XS250/360/400 are the same bike and engine, only main differences are head, barrels, pistons, carbs, (32 and 34mm), and crankshaft. I guess the crank is different for the balance factor. Oh, and the gearbox and rear wheel sprockets.
If you want to keep the original engine number, you just need to swap the internals and top end.
This is all easily checked by using a parts book, you can see where the part numbers are different.

 

[drewpy]

Ivor, it's a DOHC we're talking aboot. different animal

 

[ivor]

DOH

I should've looked at the thumbnail