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longhorn717 11-19-2010 05:30 PM

Thoughts on boring out my cylinders
Hey guys, got my CB400t and I was looking into boring out my cylinders. My question is does anyone know what the bore sizes are that I can go out to or how to find them? Also, where do I find oversized pistons to match my increased bore size? I purchased an extra cylinder head on ebay that looks decent I was going to bore it out and keep the original stock. Any thoughts on this mod would be appreciated.

longhorn717 11-20-2010 05:59 PM

Anyone have two cents for me?

YamaCat 11-20-2010 06:27 PM

Are you boring them out because of wear issues? or are you wanting to take a project bike and get it "hotted up" just a bit?

longhorn717 11-20-2010 06:32 PM

want to hop it up a bit. Wondering how big I can go before I don't have any "meat" left in the walls. I mean how many CC's could I gain? 20? 50? 75? I'm going to put pods on my carbs and scrap the airbox and rebuild/rejet the carbs as well. I've got a 34 tooth rear sprocket, as well, which is three teeth smaller than stock.

YamaCat 11-20-2010 09:29 PM

This is the Bore and Stroke Ratio that Honda probably used to arrive at 4 cylinders totaling 399 cc's of displacement:

Bore 55.0, Stroke 42.0, Stroke Ratio 1.3095

Figuring out how much cylinder wall you have is not the issue because, If for instance, a piston ring popped loose and gouged the cylinder wall too badly, you wouldn't be able to fix that without putting a sleeve in to pass the leak test. So obviously there is a known limit or people would just bore the cylinders out to fix that sort of damage instead of re-sleeving.

A bigger issue is boring out the cylinder without changing the stroke.

Here is a java based calculator which will help you see your predicament:

Stan Weiss' - Automotive Performance Software / Interactive JavaScript to Calculate Engine Size in Cubic Inches and CC's

The_AirHawk 11-20-2010 10:52 PM

The major issue isn't "how far?", but rather "are there oversize pistons available?"

Typical standard overbore is 1mm or 2mm.

If you're serious about it, any competent machine-shop can handle the chore - but I would acquire the pistons first.

An overbore itself will give you little or no performance gain. Cams, porting, larger valves, and carb-tuning will net more Oompa-Loompas. But then you open a larger can of worms - the engine and driveline components are engineered to handle only so much torque and HP. Failure is their complaint-mode. Some bikes are "overbuilt" more than others. Finding the weakness in your own can prove expensive.

Good Luck; you're gonna need it.

longhorn717 11-21-2010 01:41 PM

YamaCat to clarify I have a Twin not a Four.

All the information I've found says the stock bore is 70.6 x 50.5 I was looking at something like 73.5 x 53.5 or something similar. I'm just getting into this and don't know what is "reasonable" and "out there".

I've already got plans for the suspension and final drive. I realize pushing almost double the ponies (my goal) is gonna stress her quite a bit. I got an extra cylinder on ebay as stated and I have found a few companies online that make semi-custom or full custom pistons. I've done some reading on overboring older bikes and have read the same thing you said Airhawk. Get the pistons first and then worry about the actual boring. I'm wondering while I'm at it if I should get new connecting rods as well.

New thought....would it be possible to bore out the actual cylinder and put in larger sleeves?

Some items on my to do list:

porting and polishing. (not sure what this does exactly but I know it helps)
Cleaning and rejetting carbs
custom two into one shorty exhaust
K&N pod filters
Replace rear wheel with slightly wider rim with disc brake setup, to accept slightly bigger tires and a retrofitted caliper.
replace standard chain with either o-ring or x-ring chain

YamaCat 11-21-2010 02:05 PM


Originally Posted by longhorn717 (Post 252342)
YamaCat to clarify I have a Twin not a Four.

Whups. Why did I think you had an inline four?

Anyway, here's a link to help stir the grey matter:

Automotive Performance Software / Interactive JavaScripts Calculators

All of the java script calculators are about 2/3's of the way down the page.

Good luck with the project ...

MOKE1K 11-21-2010 11:59 PM

Wiesco should have a wide array of pistons to choose from. Older supersport rules used to allow a 1-3mm overbore and with the basic exhaust mod jetkit, and some suspension. You'll be surprised, it made quite a difference on a 91 cbr600 a friend of mine used to race.

MOKE1K 11-22-2010 12:00 AM

Basically made it feel like a very light 750.

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