Honda CB 250G into 360cc

I wanted to know if any has converted the Honda CB250G cc into 360cc by just replacing the cylinder jugs and what problems i might come into.

All best

Gisli

You're going to have a hard time with compression unless you replace the pistons, too.

Are anyone out there who has done this?

All best

Gisli

Heads are different (different squish area.....The jugs, Head, pistons are all needed. The bottom end is the same.

Talk to CrazyPJ, he took a 250G to 378CC .... check his build blog on it....You can overbore and make it go faster...You don;t necessarily need to get 360 parts.

Yep, just bore it..
I forget stuff but I think gudgeon pin is smaller on 250 than 360? (15mm vs 16mm)
If you want to go to a decent oversize you will need 'new' liners, Kawasaki 1000 or Suzuki GS1000 fit OK (about 0.004" interference)
I got a GS1000 block that had 'eaten number 4 cyl (3 good ones though 8) ) 70mm gives you a 391cc. If you come across a ready bored block let me know, my 390 was stolen 2 yrs ago
Kawasaki pistons may be a better bet than Suzuki ones as pin is smaller diameter

Thanks guys!!!

I will check this out. When you say bore the 250cc do you mean go to 270 or approximately. I can't go from 250 to 360 right

I will find the blog you mentioned!

More information welcomed!!

All best

Gisli

I went to 390cc with GS1000 pistons and liners. It was a LOT of work, particularly as GS has bigger gudgeon pins.
If anyone offers you a 'kit' or motor, it's mine, bike was stolen couple of years ago

gassa said:

When you say bore the 250cc do you mean go to 270 or approximately. I can't go from 250 to 360 right

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no you cant safely go to 360

Cant find the blog where 250 to 360 is done


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barnett468 said:

no you cant safely go to 360

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How come?

Sonreir said:

How come?

Click to expand...

I believe barnett is referencing the question of boring the stock sleeves to 390. The o.p. had asked because PJ was a bit unclear in his first post, but then added that he used different sleeves to achieve the 390 motor.

I hope somebody finds that bike for ya someday PJ.

Now we need to get it clear if we can bore 250 to 360...anyone?


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for the 3rd time no

gassa said:

Now we need to get it clear if we can bore 250 to 360...anyone?

Click to expand...

You can't bore the cylinders for 56mm all the way to 64 or 66 or whatever because the liners are not thick enough. You could probably bore the 250 liners to take bigger liners but as someone already mentioned that is a lot of work. PJ mentioned that the piston (gudgeon) pin may be a different diameter. I would check that first. The easy/cheap way is to check the clips and see if they are the same or different.

If the pin diameter is different, a 360 top end will not just drop on. You would need to source different (non 360) pistons to make it work and that means going even larger in size pistons and liners and so on. If the pins are different it might be cheaper to buy a complete 360 motor and drop that in.

Another thing to think about is that if you bore the 250 barrels to take larger liners they may break through between the fins. Honda typically cast a slightly different cylinder for the 250 and 350 versions of their motors and larger diameter liners tend to show. It isn't a real problem but better to start with a thicker 360 block.

One more thing to look at is the crankcase mouth. Honda typically machines them to clear teh liners fitted to that motor. a 250 crankcase may need to be opened up to take a 360 top end. That is the case on a Db160 going up to 200cc or a CB72 going to 305cc but you would have to measure the mouth and the liners to determine if that would be an issue.

Honda machines the "squish band" differently n a 250 versus a 350/360 to allow for the larger piston. The 250 head may be a better starting point to machine whatever squish band shape/width/angle you want.

I'm not 100% certain (pistons are 4500 miles away) but I think Kawasaki 900 or 1000 pistons will fit 250 rods (I haven't looked at parts since late 1970's~early 80's)
250 block will need re-linering to go more than 3mm oversize ( max 59mm before liner gets too thin)
360 block will drop onto 250 bottom end but I'm pretty sure small end is smaller on 250 (hell, I haven't modified a 250 since the mid 80's)
You may be able to find pistons from a modern 4 valve 600 or 750 that will fit 250 rods. They will need less machining than early 2 valve pistons and you shouldn't have to fly-cut combustion chamber with a 'flat top' piston.
Personally, if I was doing 250 mods today, I would get a GS850 block, shrink out liners then fit 850 (69mm piston) liners into 250 block. Bore for 1mm OS GS850 pistons and you have a 391cc motor.
It could do with 1mm oversize intake valves but works fine with stock parts (120mph)
The crankcase will need machining, I just bolted mine to lathe saddle with spacers and used boring bar in chuck to open 0.020" larger than liner.
It's a non-critical dimension so set-up didn't take too long
I'm probably the first person to do a real big bore conversion on 250 motor, most people started with 360's. If you want to go 'huge' you need to fit GS1100 connecting rods and modified 72~74mm pistons

And i thougth it would be enough to just replace 250 cylinder jugs and pistons with 360 ....not to forget one jet in carburator and top gasket!


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