Honda CB 250 G5 to 360

Hi, I have a '75 honda CB 250 G5.
the bike is exactly the same as the CB360. The only diference is the bore size.
Back in the day, I know that was a normal upgrade on the bike.
My question is, can I bore the 250 cylinder to 360, or I need a 360 cylinder?

thank's for the help

CHances are youll need a 360 head, plus its probably cheaper to get a used jugs and head off ebay than to have a 250 bored to 360 anyway.

Keep in mind I'm taking your word for it as far as if it will work

I think that's a pretty large amount to bore to get a 250 to a 360... Here's the math:

CB250 bore and stroke (56mm x 50.6mm) gotten from here: http://autobikegallery.com/1972-honda-cb250/

displacement = bore2 * stroke * # of cylinders * pi/4
pi/4 is about 0.7854

displacementCB250= 249257.05728 = 562 * 50.6 * 2 * 0.7854

Then to get it to a 360, you set displacement = 360000 and solve for the bore:
displacementCB250 to 360 = 360000 = bore2 * 50.6 * 2 * 0.7854
bore = 67.3mm

tl;dr You'd have to bore each cylinder by about 9mm...

What about doing a straight engine swap?

Edit:

SONICJK said:

CHances are youll need a 360 head, plus its probably cheaper to get a used jugs and head off ebay than to have a 250 bored to 360 anyway.

Keep in mind I'm taking your word for it as far as if it will work

Click to expand...

This is also an option.

The 250g and 360 are the same except for the cylinders and bore. Both have the same stroke.

Easiest path is to just pick up the cylinders and pistons from a CB360. Basically the whole top end.

They never sold the CB250g in the US, AFAIK, and the dealers in the US don't list parts for them. The Shop manual covers both, and they only difference mentioned is the bore.

The CJ360 also shares the same parts.

The headgasket is probably different, but the rest should be the same.

Your Profile shows you in the US, is the bike Gray Market?

mydlyfkryzis said:

The 250g and 360 are the same except for the cylinders and bore. Both have the same stroke.

Easiest path is to just pick up the cylinders and pistons from a CB360. Basically the whole top end.

They never sold the CB250g in the US, AFAIK, and the dealers in the US don't list parts for them. The Shop manual covers both, and they only difference mentioned is the bore.

The CJ360 also shares the same parts.

The headgasket is probably different, but the rest should be the same.

Your Profile shows you in the US, is the bike Gray Market?

Click to expand...

Wouldnt you need the head too?
How would the 360 and 250 head be the same if the cylinders are significantly different sizes?

I think not. I checked the part numbers for the cam and valves, identical.

If the Valves are the same size, the head clearance should be about the same....

The parts for everything but the pistons, pins, and sleeves are the only real differences.

The difference on the older CB250/350 were similar. Honda only changed the bore....

I am not 100% sure, but if I was going to do this, I'd check part numbers on a few more parts.

The exhaust ports are same size. It is the same engine, all the other parts interchange.

mydlyfkryzis is correct. The heads are the same.

Can someone explain that to me?
I dont get it haha, seems the head (or cyl pockets in the head rather) would be too small for the 360 piston if they work with the 250 piston.

I believe you guys, I just cant say I understand how that works out, maybe im just dumb :-[

Hi, mydlyfkryzis is on the spot.
CB 250 and 360 are exactly the same, except in the cylinders and pistons. The other different part is the secondary main jet in the carbs.
everything else is equal.
I believe that the 250 was created for some markets to be more adequate to laws in different countries.
In the back of my head I knew that rebore was far from the best choice/option.
I just finished restoring the bike. It was my first project and it took me 2 years :-\
I had to put one new piston and new rings on both, didn't do anything to the cylinders, because they looked fine.
But when hot, the engine breather makes a lot of smoke symptom of a weak engine.
If I have to strip the engine again, I want to go BIG ;D

Thank's everybody for the quick responses.

PS: I'm from Portugal

Did you change your 250 to 360?