Porting 101

[matt9923]

I got a cylinder i can practice on thanks to super!!! :yourock::yourock:

I will reread this this week and take pictures and get involved.
What should i use a carbide burr? or start with a slower stone.

 

[TraditionalTool]

I got a cylinder i can practice on thanks to super!!! :yourock::yourock:

Excellent! Keep us posted on your progress.

I will reread this this week and take pictures and get involved. What should i use a carbide burr? or start with a slower stone.

There is some good info in this thread, at least there was for me.

I have my saw back together with the modified muffler on it, the cylinder ported, and all seems ok. I will fire it up tomorrow.

Modified muffler reinstalled.

Left side of reassembled saw.

 

[Terry Syd]

Ring edge

I also think it is prudent to look at the ring/s. If the ring has a sharp edge you may want to take the edge off with a bit of emery cloth or very fine carborundum sandpaper.

If you do leave a small spot of unbeveled port window, the ring can still get past it without causing an impact between two horizonal surfaces.

I don't bevel my ports. I use a bit of fine sandpaper on the end of my finger and pull/push it across the newly ported area. You can tell with your naked finger if the edge is still sharp.

If I am trying to get the transfers to direct the flow, then I don't want a bevel on the nozzle end of the transfer, I want the flow to be tight so there will be less mixing and shortcircuiting. I just use the sandpaper to take the edge off. On the exhaust a bevel is ok as it can help direct the flow into the port.

 

[edisto]

I also think it is prudent to look at the ring/s. If the ring has a sharp edge you may want to take the edge off with a bit of emery cloth or very fine carborundum sandpaper.

Best to leave the beveling for the cylinder and let the rings do their job.

I don't bevel my ports. I use a bit of fine sandpaper on the end of my finger and pull/push it across the newly ported area. You can tell with your naked finger if the edge is still sharp.

From what I've read, the bevel is as important or more important than the curve of the port in easing the rings back into the cylinder.

If I am trying to get the transfers to direct the flow, then I don't want a bevel on the nozzle end of the transfer, I want the flow to be tight so there will be less mixing and shortcircuiting. I just use the sandpaper to take the edge off. On the exhaust a bevel is ok as it can help direct the flow into the port.

I fail to see how a bevel on the top or bottom would lead to short-circuiting, or increased mixing. It would tend to slow the flow slightly, but that's not a bad thing, as it has been found that increasing velocity by decreasing crankcase volume can lead to turbulence that will increase mixing. Of course, transfer ports generally are the narrowest of the ports, so the inportance of the bevel is diminished somewhat.

 

[timberwolf]

Originally Posted by Terry Syd
I also think it is prudent to look at the ring/s. If the ring has a sharp edge you may want to take the edge off with a bit of emery cloth or very fine carborundum sandpaper.

Best to leave the beveling for the cylinder and let the rings do their job.

+100% sanding the edge of the rings is a bad idea, to work rings must have pressure on the back side to make a seal, if there is a bevel on the front side it will allow cylinder pressure to move down the front side of the ring and will work against the pressure on the backside to unseat the ring and increase blow-by causing loss of charge purity and power, also at higher RPM unseating rings can cause problems with ring flutter.

Bevel is importaint, esp the wider the ports get. However the plating is only a few thou thick so it is surprising how little bevel is really needed. A shallow angle bevel is a lot beter at coaxing the rings into place than rounding the edge off.

Also Having absolute square edges on the discharge edge of a outlet actually causes more resistance to flow and turbulence than having a slight radius on the edges of the discharge orifice.

 

[TraditionalTool]

+100% sanding the edge of the rings is a bad idea, to work rings must have pressure on the back side to make a seal, if there is a bevel on the front side it will allow cylinder pressure to move down the front side of the ring and will work against the pressure on the backside to unseat the ring and increase blow-by causing loss of charge purity and power, also at higher RPM unseating rings can cause problems with ring flutter.

Bevel is importaint, esp the wider the ports get. However the plating is only a few thou thick so it is surprising how little bevel is really needed. A shallow angle bevel is a lot beter at coaxing the rings into place than rounding the edge off.

Also Having absolute square edges on the discharge edge of a outlet actually causes more resistance to flow and turbulence than having a slight radius on the edges of the discharge orifice.

Thanks Ed and Timberwolf, I didn't touch the ring at all, only put a slight bevel on the roof, and just a touch on the floor, but I only used a grinding stone, moving it by hand across the edge. So it is very slight in that regard.

Good news is that it fires up and runs. I still need to tune it, it doesn't stay idling and I suspect just an adjustment on the idle screw will do it. Unfortunately my wife is teaching a cooking class right now and I don't want to bother them, so waiting until she is finished with them.

It sounds healthy, although I know that the cylinder wasn't perfect, it seemed a very slight area could have had the nikisil wore through, it was however smooth to the touch. I'll find out more after I get it running smoothly and let the rings set.

 

[SWE#Kipp]

+100% sanding the edge of the rings is a bad idea, to work rings must have pressure on the back side to make a seal, if there is a bevel on the front side it will allow cylinder pressure to move down the front side of the ring and will work against the pressure on the backside to unseat the ring and increase blow-by causing loss of charge purity and power, also at higher RPM unseating rings can cause problems with ring flutter.

Bevel is importaint, esp the wider the ports get. However the plating is only a few thou thick so it is surprising how little bevel is really needed. A shallow angle bevel is a lot beter at coaxing the rings into place than rounding the edge off.

Also Having absolute square edges on the discharge edge of a outlet actually causes more resistance to flow and turbulence than having a slight radius on the edges of the discharge orifice.

What is the best way to bevel ?
i'm thinking of the tools, burr, mandrel or is there something else that does the job better or easier ??

 

[blsnelling]

I use a round stone and then finish off by hand with fine grit paper.

 

[Terry Syd]

Chamfer

I should remind myself not to be too concise when discussing technical issues. OK, here is the longer version.

I think we are on the same page when I see TW write:

“Bevel is importaint, esp the wider the ports get. However the plating is only a few thou thick so it is surprising how little bevel is really needed. A shallow angle bevel is a lot beter at coaxing the rings into place than rounding the edge off.”

Also, Brad writes:

“I use a round stone and then finish off by hand with fine grit paper.”

What we are trying to do is to protect the engine from damage. We want to protect the ring from peeling the very thin coating from the cylinder. The problem I see with the discussion is the term ‘bevel’. For a layman that has never done any porting he views the term ‘bevel’ as being a bevel.

I prefer the term chamfer and more precisely the profile on the chamfer. As TW said, we want a shallow angle to coax the rings back into the piston groove.

We are not trying to put a chamfer on the port that we think is best, we are trying to put a chamfer on the port that the RING thinks best.

The best model for this profile is an old worn cylinder. Take a look at how the rings wear the port windows. There is more wear in the center of the port window than at the edges. That is because the ring is extending further into the port at the center. Further, the wear is not a single angle of say 45 degrees, it is curving profile with the wear blending into the surface of the cylinder.

You cannot get that profile with a porting tool. You can get it, or close to it, by using a bit of fine sandpaper on your finger tip and touching up the edge of the port. The center of the port will have more profile than the area closer to the edges of the port. You can stick you naked finger in there and feel the difference as you smooth it out.

If you are beveling past the coating and into the aluminum, then you are porting. This is especially true for rear boost ports where the profile can make significant changes in the shape of the power curve at and after peak power.

As far as the ring, you are not putting a ‘bevel’ on it. Think of a knife edge and the difference between a sharp knife and a dull knife. You want to just dull the sharp edge (if it has one). If you take off any more than the thickness of the cylinder coating then you have gone too far. I consider it a prudent check to do after porting. If the ring does catch on the cylinder coating when you fire it up, then you are going to have a whole lot more problems on your hands. Whether you want to check it is a matter for you.

Many rings do come with bevels on the rings. This is especially true for chrome plated top rings. The chrome plating is always smaller than the thickness of the ring and there is a bevel to the full thickness of the ring. Also, some rings come with a taper surface to help promote a rapid break-in period. The taper puts a small surface against the cylinder that quickly wears down and allows the ring to seat early.

Ring flutter is only a problem when the inertia of the ring floats the ring off the bottom ring land and the gas seal is broken. When the ring hits the top of the ring land the gas pressure behind the ring collapses and the ring can be slammed back into the groove by the cylinder pressure.

I do not suggest that anyone ever put a grinding tool into the cylinder to put a ‘bevel’ on the ports. I have seen far too many botched up cylinders with jackasses taking a perfecting good port job (that has taken many hours) and stuffing up the final result. The usual screw-up is that they put the bit on the edge of the port and then move it across to the side of the port, they STOP there and then go back to the other side, where they again STOP, then go back again. The may carry this horrendous ritual out several times. The result is a ‘bevel’ where more material is taken away from the side of the ports than in the center. This is exactly the OPPOSITE of what the ring wants.

Whenever I clean up the edges of the ports, I think in terms of the ring’s requirements. I’ve been doing it this way for over 40 years and I have never had any problems.

 

[timberwolf]

Yeh, looks like we are saying much the same thing just different way of saying it.

You cannot get that profile with a porting tool. You can get it, or close to it, by using a bit of fine sandpaper on your finger tip and touching up the edge of the port. The center of the port will have more profile than the area closer to the edges of the port. You can stick you naked finger in there and feel the difference as you smooth it out.

Of all the tools to put an edge on a port I do find a round stone the best, with care it is possible to control the point of contact on the round stone to grind an angle on the port edge. It is without doubt a less is more thing and the sides get almost zero, that is a beef I have with stihls latest 460/660 cylinders where the chamfer is close to 1/8th of an inch wide all the way around. Cheep machine work is all that is. You bet I finish the edge too with 600 grit.

 

[timberwolf]

Here is the lower side of an exhaust port I was working on. It's not perfect, but I think a decent job with a round stone. The angle of the cut is fairly constant across the port, but tapers to near nothing at the port edges.

Terry, would this be in line with what you shoot for? Could likely use a little more 600 grit.

Application here is a alky racer motor with port width out as wide as possible with meen piston speed up over 4000 ft/min.

 

[nmurph]

so TW, is the the way all ports should be beveled or is this specific to racing saws with a different taper of the radius for work-ported saws? is the intake beveled differently?

 

[Terry Syd]

Picture

TW, a picture is worth a thousand words. We could keep waffling on about what it should look like, but the picture says it all.

I saw the pictures of the Stihls - that was ugly. It was like they wanted to use a shotgun approach to getting the mixture into the cylinder rather than a directed rifle shot. Actually, those pictures would make a good 'what not to do' addition to this thread.

Another thing about the beveling on those ports - timing. A large radius bevel on these over-square, short-stroke engines is going to make a lot bigger timing change than a longer stroke motorcycle engine. I wonder how much timing change occurred from the factory designed specifications.

By the way, I'm a big fan of yours for all the documentation that you do of your emperical testing - good stuff.

 

[edisto]

Here is the lower side of an exhaust port I was working on. It's not perfect, but I think a decent job with a round stone. The angle of the cut is fairly constant across the port, but tapers to near nothing at the port edges.

If that's what Terry was describing, the we are all talking about the same thing (it tells us what the ring wants preciousssssss...).

The old motorcycle books recommend a 10 degree angle, 0.5mm deep and 2 mm in height. Probably a little excessive for chainsaws? The only rounding, however is right where the bevel meets the port.

There's a reason they use ramps instead of rounded curbs for wheel chairs. Run some wheels over a square curb, and it will eventually wear to be rounded, but that doesn't mean that is the best design for rolling up a curb.

 

[TraditionalTool]

Here is the lower side of an exhaust port I was working on. It's not perfect, but I think a decent job with a round stone.

timberwolf,

Looking at this make me comfortable with what I did to mine. I used a rounded tip grinding stone, it was easier to get in, but these were designed for running in a die grinder and not a dremel so I just rubbed it by hand along the edge. I didn't take any pics of it though...

 

[TraditionalTool]

Another thing about the beveling on those ports - timing.

This is something I've been wondering about. How does one go about setting the timing on these saws. I have seen reference to marking the flywheel somehow and using a timing light, but not sure what to do...or even if that can be set.

I know that I really need a tach, and been eying the TechTach 20k. Just not flush with cash right now...it might not be avoidable for this type of work.

 

[edisto]

The reference was to port timing...when the ports are open and closed, not ignition timing, which should be fixed unless you take the key out of the flywheel or use an offset key.

Port timing is altered whenever you use a different-sized gasket, or cut a port to change height.

 

[TraditionalTool]

The reference was to port timing...when the ports are open and closed, not ignition timing, which should be fixed unless you take the key out of the flywheel or use an offset key.

Port timing is altered whenever you use a different-sized gasket, or cut a port to change height.

So, how does one calculate the port timing, that must be based on the openings and/or angles of the ports, but somehow the transfers seem to have an effect on all of it.

How does one compensate for changes that are made to the height of the port?

 

[tdi-rick]

[snip]

The old motorcycle books recommend a 10 degree angle, 0.5mm deep and 2 mm in height. Probably a little excessive for chainsaws? The only rounding, however is right where the bevel meets the port.

[snip]

Bell does say 1mm/0.3 deep on all the ports other than the exhaust as the ports are smaller.

 

[timberwolf]

so TW, is the the way all ports should be beveled or is this specific to racing saws with a different taper of the radius for work-ported saws? is the intake beveled differently?

On a work engine with lower piston speeds, and less exposed ring the port edges can use even less softening.

Here are some of those ugly stihl ports.... Flame me for saying that.

Especially love the intake, they have chamfers cut where the rings never cross. Just production short cutting I think, likely the port edges are done by some robot at 1000 cylinders an hour.