Topic: The Advantages of a Mild Overbore?

[OakBehringer]

In a CB160 that needs new pistons and has never been bored, is there much of a horsepower gain between a S1 .25mm and S3 .75mm overbore?

Thanks!
Adam

[Tim.]

I would imaging nothing will change at all.  The factory spec overbores are all intended to work with factory spec carb jetting etc. so you should expect no change in behavior at all.

Don't go bigger than you need, as it just prevents you from being able to go bigger again in the future.  But at the end of the day you'll be governed by what size pistons/rings are most readily and cheaply available.

[OakBehringer]

That's exactly what I assumed. Thanks for the answer.

[MCRider]

That's exactly what I assumed. Thanks for the answer.

It'll feel a lot better than the overbore itself would justify, simply cause the current set up is worn out. The overbore will increase the compression ratio a very little bit as well, which will feel good.

[mlinder]

Some of the 160 racer guys go to 214, or so I've been told. Any bigger and you just can't get them to breathe correctly.
That being said, I've seen them hit 90+mph.

[OakBehringer]

I have been toying with the idea of going with this S12 kit...

http://rogersmotorcycleparts.com/cb160.htm

...but I think the complications of fitting those pistons and getting it to run properly are plentiful.

[MCRider]

I have been toying with the idea of going with this S12 kit...

http://rogersmotorcycleparts.com/cb160.htm

...but I think the complications of fitting those pistons and getting it to run properly are plentiful.

When I was working in a performance shop in 1972/73, I built an S12 160. Turned out really sweet. Had a handmade 2-1 header on it. The shop sold it to a guy. I went back out in 1980 for a visit and he was still riding it. Never shy about turning it up either.

[OakBehringer]

Could your average machine shop bore out my block for those pistons?

[MCRider]

Could your average machine shop bore out my block for those pistons?

An average machine shop may not be able to go that small. You'll need an average motorcycle machine shop.

[OakBehringer]

An average machine shop may not be able to go that small. You'll need an average motorcycle machine shop.

When I Googled "CB160 big bore kit" I read this on the first result:

"This high compression kit requires careful installation by a qualified race mechanic.  Pistons will need to be clay checked and possibly modified to fit your specific engine configuration."

I have no idea what "clay checked" means. It's also worth noting that it might not be the same big bore kit.

[MCRider]

An average machine shop may not be able to go that small. You'll need an average motorcycle machine shop.

When I Googled "CB160 big bore kit" I read this on the first result:

"This high compression kit requires careful installation by a qualified race mechanic.  Pistons will need to be clay checked and possibly modified to fit your specific engine configuration."

I have no idea what "clay checked" means. It's also worth noting that it might not be the same big bore kit.

High compression and big bore are two different things, though often you get both with a big bore kit. Clay checking is what they used to do (and still do?) to check that the higher dome of the high compression piston doesn't hit the valve. Assemble the motor with a layer of clay on the piston tops, turn it over, disassemble and see if the valves left an imprint in the clay.

YOu wouldn't need to do it with a 836 kit on a 750 for example as they are all made to fit. Now if you put a high lift cam in with the 836 kit, you'd be wise to do something to be assured they wouldn't hit.  Or take the word of someone with the same set up or of your vendor.

[OakBehringer]

High compression and big bore are two different things, though often you get both with a big bore kit. Clay checking is what they used to do (and still do?) to check that the higher dome of the high compression piston doesn't hit the valve. Assemble the motor with a layer of clay on the piston tops, turn it over, disassemble and see if the valves left an imprint in the clay.

YOu wouldn't need to do it with a 836 kit on a 750 for example as they are all made to fit. Now if you put a high lift cam in with the 836 kit, you'd be wise to do something to be assured they wouldn't hit.  Or take the word of someone with the same set up or of your vendor.

INTERESTING. So, in other words, a big bore kit won't necessarily have the higher dome?

[MCRider]

High compression and big bore are two different things, though often you get both with a big bore kit. Clay checking is what they used to do (and still do?) to check that the higher dome of the high compression piston doesn't hit the valve. Assemble the motor with a layer of clay on the piston tops, turn it over, disassemble and see if the valves left an imprint in the clay.

YOu wouldn't need to do it with a 836 kit on a 750 for example as they are all made to fit. Now if you put a high lift cam in with the 836 kit, you'd be wise to do something to be assured they wouldn't hit.  Or take the word of someone with the same set up or of your vendor.

INTERESTING. So, in other words, a big bore kit won't necessarily have the higher dome?

That's right, case by case. You cna have a big bore kit with a relatively flat dome or big dome, just depends on whose making it and what the intended use is.

AS mentioned before, you'll get some bump in compression ratio from a flat dome big bore kit just due to the fact you've got a larger volume of mixture in the cylinder squeezing into the same size dome in the head. So a 10.25 to 1 big bore kit may not have much of a dome on the piston at all, while a 12 to 1 would. Still if you don't have a very secure source of info that you trust, you would need to clay check or the modern equivalent, to be sure.

[OakBehringer]

That's right, case by case. You cna have a big bore kit with a relatively flat dome or big dome, just depends on whose making it and what the intended use is.

AS mentioned before, you'll get some bump in compression ratio from a flat dome big bore kit just due to the fact you've got a larger volume of mixture in the cylinder squeezing into the same size dome in the head. So a 10.25 to 1 big bore kit may not have much of a dome on the piston at all, while a 12 to 1 would. Still if you don't have a very secure source of info that you trust, you would need to clay check or the modern equivalent, to be sure.

Awesome, thanks. So, if someone were to say this...

"Try to avoid the Hap Jones big bore kit for the 160, it lowers the compression to 8:1 for some reason.  I haven't look at the HJ pistons, but they probably have a low crown."

...it would almost have to be user-error as a big bore kit could never lower the compression if installed properly?

[mlinder]

No.

A big bore kit could have dished pistons, or weird, wrong wrist-pin to deck height, etc. This would lower compression.

Bore is the diameter of the cylinder. Has nothing to do with height, shape, or anything BUT diameter of the piston.

[MCRider]

That's right, case by case. You cna have a big bore kit with a relatively flat dome or big dome, just depends on whose making it and what the intended use is.

AS mentioned before, you'll get some bump in compression ratio from a flat dome big bore kit just due to the fact you've got a larger volume of mixture in the cylinder squeezing into the same size dome in the head. So a 10.25 to 1 big bore kit may not have much of a dome on the piston at all, while a 12 to 1 would. Still if you don't have a very secure source of info that you trust, you would need to clay check or the modern equivalent, to be sure.

Awesome, thanks. So, if someone were to say this...

"Try to avoid the Hap Jones big bore kit for the 160, it lowers the compression to 8:1 for some reason.  I haven't look at the HJ pistons, but they probably have a low crown."

...it would almost have to be user-error as a big bore kit could never lower the compression if installed properly?

Well, if the domes were dished, the CR could be lower. I've never heard of a normal aspirated engine running dish domes, usually reserved for turbos, where they do it on purpose.  I would have to ask the person making the statement to explain it to me.

You've about exhausted the limit of my knowledge on the subject so I defer to anyone who knows better.

[OakBehringer]

I think I understand everything, now. Thanks for all the replies.

[MCRider]

No.

A big bore kit could have dished pistons, or weird, wrong wrist-pin to deck height, etc. This would lower compression.

Bore is the diameter of the cylinder. Has nothing to do with height, shape, or anything BUT diameter of the piston.

Granted the dished domes or soemthing weird etc. But HapJones is a mass marketer, not likely to mass market something weird.

Am I wrong in saying that all else being equal, a larger bore increases the compression ratio?

http://www.carcraft.com/techarticles/calculate_compression_ratio/index.html

Scroll down to cylinder swept volume.

[mlinder]

Yep, all other things being equal, compression would increase.

[Sam Green Racing]

In a CB160 that needs new pistons and has never been bored, is there much of a horsepower gain between a S1 .25mm and S3 .75mm overbore?

Thanks!
Adam

Adam, as the guys have said here, you would be hard pushed to feel the difference in performance from a bore of that size.
A stock CB161 is 161.027cc. The size 3 + .75 takes you to 165.894cc. I doubt the extra 4.867cc would add any more than 0.25bhp to the factory 16.5bhp motor.

Sam.

[mlinder]

Sam, the engine makes 1hp per 10cc! He should see a .44 hp increase!

[Sam Green Racing]

Yes my learned friend, but by the time it reaches the back wheel, he won't feel it.

Sam.

[mlinder]

My uber-sensitive butt-mounted-accelerometers could. Oh yes, they could.