Question on Corsa exhaust port and head gasket

[johnny248]

So I got my Corsa kit today. Looking at the exhaust port, if isn't ported all the way to the gasket lip. There is a picture below. It seems like it would be a good idea to open that up more (lower right side of port) so the exhaust port is the same size as the lip for the gasket like it is in the upper left side.



One other thing I noticed is that I can't see any beveling on the side of the cyliner where all of the transfer ports and exhaust ports are. Isn't this supposed to be filed or beveled to prevent the rings from catching?

Also, does this thing really just use this rubber o-Ring inside of the cylinder head without any sort of head gasket? This seems strange to me.

[johnny248]

..... Alright man. I'll take your word for it. That exhaust port transition into the gasket area just looks funky to me. Have you paid attention to any other cylinders to see if the ports are rounded to prevent ring catching?

[DirtyE]

Im gonna butt in here just for a suggestion. Id never put cylinder made by anyone without a thorough inspection. You may very well not have to touch it, but everyone makes mistakes. Cutting bits in equipment goes dull at some point whether its machine or hand tools.

[Red Eft Performance]

Hello the Polini bore don't come chamfered.
You most definatly should chamfer your ports. When done right the motor/rings will last so much longer. This isn't hard to do just take your time and be careful.

As for the exhaust port. Yes you should match the exhaust port outlet to the exhaust inlet. If your in search of max power you don't want any steps that will create turbulence with the exhaust floing in and out of the cylinder. Shoot me a PM if you need more help.

Keith

[curtis966]

^ I agree I believe every cylinder is do for a few touch ups when brand new even a Polini which I'm sure would run great right out of the box but im sure it can be greater too! There are lots of guys in Hawaii who do full port jobs on the Corsa right out of the box so I'm sure a little clean up is due as well. Same reason we match the boost ports and transfers, so it's all perfect the first time! One down fall of my MHR is I can't do any touch ups d*** nakisal coating lol

[johnny248]

Nothing is ever easy lol. That exhaust port bothers me. I looked at the Malossi ports and my CMR port and they both are matched. It's not a big deal for me to match that with a carbide and not have to worry about it.

The chamfering however after doing some searchs here goes both ways. Lots of people said they never touched theirs and never had a problem. Others say they must be chamfered. I just known on other 2 cycle engines that I have owned, anytime I had the cylinder bored the machinest also chamfered the ports before he gave it back to me.

Just looking at my factory bore and cmr, they dont appear to be chamfered. *maybe* a little bit of chamfering on the exhaust port only. If it is chamfered, it is only slightly.

[johnny248]

Here is the exhaust on all of them.

First is stock bore. All pics are first without packing, then with.




These next two are of my CMR Taiwan 70cc BBK. If you notice, the port is essentially the same size and even with the ID of the seal.




Even though this is a Taiwan kit, to me the exhaust port is better because it is the same size as the I'd of the seal, which means to is a nice even smooth flow of exhaust through the port, through the seal, and into the exhaust pipe.

Now, here is the Corsa. The left side of port looks pretty good because the port is even with the seal ID, however as you get around to the right side, the even turns into a step which gets largest at the lower right side. It almost looks like the port is off center. So no instead of the exhaust floeowing smoothly through the seal and into the exhaust, it has a step to go down before the seal and pipe. All of this disturbs the smooth flow and causes turbulance or less flow.




If you look closely at the first Corsa pic, you can actually see the "line" where the port *should* be bored out to so it is the same size as the seal and pipe.

[paulpauly7]

I cleaned that up on my corsa also polished the inside of the exhaust port to a mirror finish also the top of the piston and inside the head.Ive done over 1800 km on my bore i diddnt chamfer the ports i have a spare piston and rings but havent needed them yet .My jetting is lean for my area but it runs so sweet at temps of 375┬░f up to 400┬░f.

[johnny248]

Did you chamfer the other ports?

[johnny248]

I appreciate everyone's input and thoughts. Like I said, this is all learning. It just threw me off to see the port like that.

When inspecting my other cylinders, none of them seem to have any chamfering on the ports. Other 2 cycles I've owned where chamfered and it was also a "requirement" to chamfer them after boring. So... Was just looking for feedback.

I just want to get this bad boy going badly!

[johnny248]

Thanks G9. As far as matching the exhaust port, might be something I do just for piece of mind. Anyone could do this part. Its just basically coloring in (actually removing) between the lines.

[johnny248]

Looks good now!

[Red Eft Performance]

Look at it this way.
Do you have to chamfer? No you can slap it together and probably won't know the difference.
Chamfering won't yield any power.
What it will do is drastically increase ring life and keep your engine healthier longer.

Have fun! Take your time and do it right and that cylinder will last a long time.

Keith

[kauaianman]

fabrizi has factory chamfer on transfer ports very nice.

maybe they just had extra money to throw away. idk. I still learning. more questions than answers still.

[johnny248]

Any pics?