DIY porting and valve reseating

[fastdruid]

I played around with Mini cyclinder heads in my younger years, and I have to say that without a flowbench there's little you can do to improve the standard set-up. Changing exhaust manifolds, air filters and camshafts on the other hand does a lot for extra power, as does replacing standard valves/seats for three angle ones (as mentioned earlier).

Looking into the ports of NC35#3 and there are quite a few flaws to the casting, I'm pretty sure I could make a difference by tidying up in there so to speak. I'm not going to bother though, going to leave this one pretty much standard.

I may have a play with the original NC35#1 engine as I need to strip it to fix the gearbox.

Druid

[alexibrow]

Looking into the ports of NC35#3 and there are quite a few flaws to the casting, I'm pretty sure I could make a difference by tidying up in there so to speak.

You may be right, but is the difference going to be beneficial? The only way to know is to get the heads on a flow bench, to see what the current flow rate is for each cylinder, ie the current baseline. If all cylinders are flowing at equal rates, then machining only one of the cylinder's ports may give an uneven result across all four cylinders, if that makes sense.

Please don't think I'm having a dig, it's just that I think it's important to know where you're starting from, especially when it comes to machining chunks off the inside of the engine, as it's difficult to put the metal back if you change your mind!

[mutly]

Ive got an engine that I did this on.It just needs the valve clearances done now I didnt finish it as ended up racing in proddie class and head work wasnt allowed.Its had new stem seals, head gasket etc.If any body is interested make me an offer I aint wanting silly money for it.cheers.

[fastdruid]

Looking into the ports of NC35#3 and there are quite a few flaws to the casting, I'm pretty sure I could make a difference by tidying up in there so to speak.

You may be right, but is the difference going to be beneficial? The only way to know is to get the heads on a flow bench, to see what the current flow rate is for each cylinder, ie the current baseline. If all cylinders are flowing at equal rates, then machining only one of the cylinder's ports may give an uneven result across all four cylinders, if that makes sense.

Please don't think I'm having a dig, it's just that I think it's important to know where you're starting from, especially when it comes to machining chunks off the inside of the engine, as it's difficult to put the metal back if you change your mind!

Oh I agree, I wouldn't be doing a lot and the engine I'd be playing with has been on the dyno so I know what it gave before as well so I'd be able to actually see what (if any) difference the changes had made.

At the same time a flow bench doesn't really even give an accurate assesment of if your changes make a positive difference as in reality the flow isn't constant but it does as you say allow you to make sure all are flowing the same.

It wouldn't be that hard to make a flow bench either, not obviously anything like as good as a pro setup but I would have thought enough to give a reasonable indication.

Not that I'm planning on doing anything soon (if at all).

Druid

[Simon_Ed]

these are before and after, one of the heads anyway, the heads were skimmed after this.

Si

[fastdruid]

Er I hope you replaced those valve guides afterwards.

Druid

[Simon_Ed]

why do you say that?

[fastdruid]

why do you say that?

Because (and I could be wrong in this) I was under the impression that it wasn't a good idea to cut down the valve guides as you get both the valve rocking and it's not actually that good for flow as instead of a nice boss you have an abrupt shaft just sticking out of the wall.

The 4 stroke tuning book I posted a link to before says that "bifurcated ports and valve guide bosses should be made as long as possible in the direction of the gas flow".

Druid

[Simon_Ed]

to be honest the guy who tuned the engine has been tuning engines for 10+ years and i trust him 100%, the engine work was done nearly 2 years ago and it's never missed a beat, and uncorrected it was producing 68@ the back wheel. He did tell me how and why but can't remember just at the mo. For whats been done to the engine its producing good power and is very reliable...


Si

[NC35!]

Hi,

The ground down inlet valve guides are not so much a problem, and is actually recommended as part of the "back-filling" procedure. I know Tony "Slick" Bass does it, pretty sure Phil Seton does it, and know for sure the Motoman chap here http://mototuneusa.com/the_2007_superbike.htm" onclick="window.open(this.href);return false; (approx 1/3rd of the way down) does it too.

Because there is not so much heat in the inlet as opposed to the exhaust ports, it is okay.

Thanks.