Cb250rs + Xr500/xl500s

[xwhatsit]

Before we start, I'm not seriously planning this. I don't have the money for it, unless it turns out that my existing top end is so mangled that I might as well put an entire top end in, in which case I would consider it. However I do find the concept very interesting and it sounds like something I would like to attempt a little further down the track.

I'd heard about this conversion before, but lb99 said that instead of swapping the whole engine, you could just swap the top ends instead. So, I got a couple of questions:

What's the difference between the XR500 and XL500S? Most people seem to have mentioned XR500 motor, but I have heard of XL500S too. Is the entire model range compatible, or only the early ones?

Does the top end just drop straight in, fit in the frame perfectly, or does there need to be other modifications? I thought perhaps as well as that the carburettor and exhausts would need to be changed.

The last question is a legal one. Would I need to get COF, all that nonsense? I noticed that the engine number is actually on the gearbox/clutch/etc rather than the cylinder barrel and head. So would I be picked up for it at WOF? If not, I could probably get away with riding one of these on a 250cc licence.

Like I said, not attempting this yet, the plan at the moment is to fix my seized 250cc engine. But this is another option in the unlikely possibility that I have to change an entire barrel/top end on my 250cc engine.

Cheers.

[barty5]

as far a i remember there may be a difference in the valve sizes but most of it should be in the gear box ratios as for fitting in a 250 frame we once fitted a xr 500 to a xr 250 frame tight fit but it worked the wof issues would only be picked up if they took a close look or knew what they were lookin at get your wof at testing station they will proelry never pick it up. the top mount may need to have some mods done to fit and the top end i would think wont just bolt to your g/box as barrel sleeve will be different.

[Conquiztador]

These things have been done before. But normally you find that there is more to it then what you first expected.

- The stroke is most probably different. If not, then the piston would be double the size to allow for the doubling in size.

- Even if the stroke would be the same, you would find that the conrod is meatier on a 500cc one. And big end bearing is bigger.

- The oil pump for a 500cc would definetley be diferent compared to a 250cc.

- There was a Suzuki in the 80's that was done as 250cc and 400cc. by looking at it you would have thought that the only diff was that the barrells were sleeved for the 250. But as you started to investigate you did find that there was some differences. Oil pump, conrod etc. So it was not a straigt swap.

Best option would be to get the specs for both engines and then work it out.

Good Luck.

[tommorth]

xr/xl 500 both use a 7 plate clutch for a start i have a manual will get the bore stroke etc tomorow its up the road my suggestion would be get a bigger bore exaust made up get a weisco oversize piston max will take you to bout 265cc the one below is i think 262cc makes a huge difference heaps more comp is your cb 5speed or 6? what state is the head in? great little motors

[Mr. Peanut]

If I was a motorcycle tester, I'd know the difference between a 250 and a 500cc engine.

[xwhatsit]

Ah... it's all moot now, as I've rebuilt the engine to original specs and it is purring away reasonably happily.

@Tommorth: I didn't know you could get aftermarket oversize pistons -- then again, I want to keep this bike long term, not just for my 250 stage, and carving bits out of the barrel is probably not in my best interests. Some more compression would be nice though.

It's a 5 speed, I wish I had 6 gears, what's the story with that? I've hugely lengthened the gearing with sprocket changes to make it so it's not revving out too hard at 100kph, but that has dulled acceleration somewhat.

@Fumeux: Not so sure there -- the place I go to didn't even seem to notice the fact I have clipons on it, despite the handlebar mounts still sitting there -- which is something that should definitely not pass. And the 250 and 500 cc engines look damned similar, apart from a different engine number of course. Just a bit taller (apparently you have to `cut and shut' the fuel tank), and a bit wider, but having seen an XL500 and XR250 next to each other I think you could get away with it, if they didn't pay attention to the engine number. VINning would be a different story, however.

[Dim]

It's a 5 speed, I wish I had 6 gears, what's the story with that? I've hugely lengthened the gearing with sprocket changes to make it so it's not revving out too hard at 100kph, but that has dulled acceleration somewhat.

I hear you, wish mine had an extra gear aswell. I didnt think the revs were too high at 100kph...well not as bad as they are at 130 anyway.

(yay 1st post)

[xwhatsit]

I hear you, wish mine had an extra gear aswell. I didnt think the revs were too high at 100kph...well not as bad as they are at 130 anyway.

(yay 1st post)

Welcome, nice bike!

No, the revs weren't really that bad at 100kph, but the bike has more than enough torque to deal with that speed and above without revving so high. After rebuilding my engine once I tend to be a bit paranoid, and going to 130kph on Kiwibiker rides a few times where it's nearly redlining doesn't make me feel too happy ^_^

6 gears... awesome. I wish!

[Dim]

Ah theres my problem, mine doesn't seem to have the connector from the cam chain to the tacho cable, so im never entirely sure how many revs its doing (only guessing from the scream and vibrations). Plus it only manages 130 if the conditions are right, downhill + tailwind + 4wd/large car to slipstream behind

So what was involved in the engine rebuild? (just in case i have to do one)

[Ms Piggy]

Hey xerxesdaphat,
I am in no way shape or form a mechanic but I have an old magazine article I was given by a friend about a CB250 that had been converted to a 500. If you'd like I can dig it out, photocopy it & post it to you. Just PM me with your postal address - I'm not 100% sure if the article has any mechanical advice b/c I haven't laid my hands on it for a while.

Cheers CSL

[vifferman]

And the 250 and 500 cc engines look damned similar, apart from a different engine number of course. Just a bit taller (apparently you have to `cut and shut' the fuel tank), and a bit wider, but having seen an XL500 and XR250 next to each other I think you could get away with it, if they didn't pay attention to the engine number. VINning would be a different story, however.

The engine is a LOT taller; apart from the fuel tank "cut'n'shut', you have to remove the top engine mount (unless you can somehow get it to fit, by modifying the frame). The bottom end looks the same (as far as shape, mounts, etc.), but the kickstart doesn't clear the footpeg, so you have to fold it up when starting. Also, there's not a lot of room for the carb and filter (usually use a pod-filter) between the engine and frame rear downtube.

Anyhoo, it's somewhat academic now, but it's one of those things you'd want to do properly or not bother, so certification while costing money and opening you up to the bureaucrats would save a lot of hassles in the long run (like when/if you wanted to sell it).

[xwhatsit]

Ah theres my problem, mine doesn't seem to have the connector from the cam chain to the tacho cable, so im never entirely sure how many revs its doing (only guessing from the scream and vibrations). Plus it only manages 130 if the conditions are right, downhill + tailwind + 4wd/large car to slipstream behind

So what was involved in the engine rebuild? (just in case i have to do one)

Ah, my tacho drive blew up recently too (I think the cable). Not fussed, replacing the smashed square speedo with a nice round XL250 one as soon as I can fab a bracket. Will leave the tacho off, I know the revs by now.

I can get 140kph on the flat if I flatten myself on the tank; lb99 reckons I should be able to get 150kph but I think my motor is a bit down on compression and the timing isn't quite right either I think. Our bikes are pretty elderly these days so 120kph should be good enough

The engine rebuild... there's a thread somewhere... full of noise and questions and not very straightforward. Wasn't really a proper rebuild, engine had a cracked head (only found out when the exhaust valve seat dropped out, pinning the valve open), which I tried to replace and fucked up, causing the valves to hit the piston a few miles down the road. Then I repaired that damage .

Hey xerxesdaphat,
I am in no way shape or form a mechanic but I have an old magazine article I was given by a friend about a CB250 that had been converted to a 500. If you'd like I can dig it out, photocopy it & post it to you. Just PM me with your postal address - I'm not 100% sure if the article has any mechanical advice b/c I haven't laid my hands on it for a while.

Cheers CSL

Hey CSL, nice to see you've graced us with your presence again It's alright, this is an old old thread from November last year when I hadn't fixed my own engine up yet. Not sure if I'd undertake such a process now. Appreciate the thought though. Good luck with uni! Congrats on the impending graduation!

The engine is a LOT taller; apart from the fuel tank "cut'n'shut', you have to remove the top engine mount (unless you can somehow get it to fit, by modifying the frame). The bottom end looks the same (as far as shape, mounts, etc.), but the kickstart doesn't clear the footpeg, so you have to fold it up when starting. Also, there's not a lot of room for the carb and filter (usually use a pod-filter) between the engine and frame rear downtube.

Anyhoo, it's somewhat academic now, but it's one of those things you'd want to do properly or not bother, so certification while costing money and opening you up to the bureaucrats would save a lot of hassles in the long run (like when/if you wanted to sell it).

The kickstart doesn't clear the footpeg in the standard CB250RS either. The whole footpeg and foot control arrangement is actually really bizarrely put together, seems like they ran out of space.

Yeah, you're right, if you're going to go that far you might as well certify it. I'm wondering now, if I ever do half of the things I want to do to this bike (new headlight, clipons, tank, seat, tail light etc) whether it's a good idea to certify or stick with my original plan (just bolt on the old parts every 6 months).

Not sure if I'll ever sell this bike while I can help it.

[Ms Piggy]

Hey CSL, nice to see you've graced us with your presence again It's alright, this is an old old thread from November last year when I hadn't fixed my own engine up yet. Not sure if I'd undertake such a process now. Appreciate the thought though. Good luck with uni! Congrats on the impending graduation!

I must still have essay brain b/c I didn't even pick up on that! Derrrrr!!

Thanks for the congrats

[FROSTY:Me]

Mate a bit of advice here -unless you WANT to do this kinda project its never gonna be an ecconomic alternative to just selling ya bike and buying a bigger bike

[tommorth]

ther was a lot of aftermarket stuff for these motors back in the day mostly for the 500 though honda could supply you with ported blueprinted heat with oversize valves a bigger cam higher comp pistons bigger mikuni carbs and differnt headers end cans even a oil cooler
spare barrels arnt to hard to come by weisco piston was bout 200bucks when i did it
six speed came81 xr250r much closer ratios and some xl250s but 5speeds were made in both models at same time xr runs higher comp than xl and suposedly a lighter flywheel