Shortened 250 barrels for 125 bottom end

[lingeringstin...]

So here we are with the next step in my experimental Chinese engine. The 250 barrels have been shortened by about 15mm to fit to a 125 bottom end. Here's the modified 250 next to some standard 125 barrels.

We did try to see if the 250 liners could be pressed out last week but they wouldn't shift so I'm hoping they're either cast in or at least such a tight fit they won't move now that the upper lip of the liner has gone.



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I'll be getting some slightly bigger pistons for it but only around the 54-55mm size for now. If this experiment works I'd like to keep enough liner for rebores.

I've left a 180 crank at the machine shop and asked the guy to split it and see if it can be put together as a 360 crank. It might be possible but then again who knows what a Chinese crank is made like?

I still haven't decided which head to use but I'll probably go for a 125 head with a 250 cam and needle roller cam bearing modifications.

[Robby]

Are you planning on changing from a plain bearing cam to a needle roller? You may need to have a think about the oil supply side.

Plain bearings usually need a fair amount of oil pressure, but not so much oil flow. Pressure to make sure the oil gets everywhere it needs to, but not a great volume because the surface area is fairly small.

Needle rollers have a lot more surface area - it's the whole point - so need more oil. Pressure is less important, they're already full of holes and lots of movement keeps the oil moving.

This is assuming it has a proper oiling system in the first place. Being an old Honda design, it may just rely on squirting oil up the camchain tunnel and hoping enough of it splashes in the right places. If that is the situation, I would be tempted by a higher capacity oil pump and opening up the oil spray quill a little. It sits between the crankcase and barrels.

[jjdugen]

Needle bearings will be fine, enough oil and mist gets blown around to keep them perfectly happy.
Bit concerned abut the liners. The lip serves two purposes. One, obviously, to keep it all in place and the other to stop oil migration around the liner / cylinder casting. I would be very inclined to make up some form of liner retention. A drilled and tapped hole in the cylinder casting to allow a bolt to nip into the liner, plenty of Locktight in there too.....
I know the liners 'seem' immovable, but heat and vibration do funny things. (Had some of this on the CB72 /77's I re-built).
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[lingeringstin...]

The geezer at the machine shop was able to split the 180 crank and turn it about and press it back together as a 360 degree crank, so that's good for my single carb setup. It was cheaper than me sourcing a good 360 crank from somewhere.

I'm going to go ahead and try the needle roller bearings and see what happens. It's easy to whip the rocker cover off for a look so I can keep an eye on it until I find out if it works or not.

The liners do bother me a bit. But what's the worst that can happen? I don't think they would slip down far enough to expose the rings so the most likely thing would be slipping enough to start grounding out on the crank flywheels and that would probably make a bit of noise before it did much damage so it might be possible to stop before the engine got too buggered up.

I'm still thinking about making some kind of locating pin or screw just to be safe but the geezer at the machine shop says he could make up some new liners with a lip and press them in if I absolutely have to do that, but of course things like that start to get more expensive than it's worth. I can just get a new 150 top end for not too much money.

I'm not overly concerned about it as I do have a bog standard 250 engine that I just rebuilt, just waiting for me to make some motor mounts and do the exhaust and wiring so at least I'll have a some kind of motor no matter what.