just how flat can you make a port roof?

[thompsoncustom]

I was wondering what the limits were on how flat you could make the intake and exhaust roofs?

I know the worry is snagging a ring but couldn't you get around that by really beveling the roof? and with the roof really beveled would there really be an increase in ring wear?

just wondering what you guys thought and/or found out regarding port flatness.

I know there is a big difference in stock saws from pictures I've seen with some being almost flat and other being very curved.

 

[huskihl]

Depends on the port width percentage vs bore size. Usually the intake doesn’t expose the ring/s but it’s worth a look. The exhaust roof and floor are more important

 

[thompsoncustom]

Depends on the port width percentage vs bore size. Usually the intake doesn’t expose the ring/s but it’s worth a look. The exhaust roof and floor are more important

hmmm so I assume you can't have it as flat on one ported to 70% vs 60% why is that? Does the ring actually move more into the port the wider you go? I would have just assumed the cylinder would still keep the ring on the same path.

 

[drf256]

Best to speak of the exhaust and intake as 2 separate ports.

For the exhaust, the majority of the port flow is when the Roof is opened by the piston crown.

So if the inside has a cheater bevel that approximates a rounder roof, it makes little difference if the roof looks flat. it’s what the ring feels. It actually bulges into the port and needs to be slowly pushed back in.

Like Kev said, how flat you go safely depends on the port width. If you catch a ring, game over. Have to have more of an arc if the port is wider. Stihl seems to have pushed the limit on the 034S/044-10mm/064. They have a super flat roof that I personally raise in the center if I widen the port.

The intake port totally different, as most flow occurs at the floor and no rings pass that area, only skirt. The roof can be raised for performance, but only to the point where the skirt is above it at TDC, otherwise it’s useless. If there is a ring pin near the intake roof, it should stay fully covered by plating at BDC (or at least 50% if you have a wider ring and racing). You have to be careful widening the roof if the ring pins are on the sides of the intake port (like most older Stihl). The intake side is the thrust side, so the wider and squarer the intake port is, the more piston skirt wear. Older Stihls had the ”uvula” hanging from the intake roof. It kept the lower ring tucked, but the newer Stihls eliminated it.

So there is no definite answer on how perfectly flat a port opening can be with a ring passing it.

 

[thompsoncustom]

Best to speak of the exhaust and intake as 2 separate ports.

For the exhaust, the majority of the port flow is when the Roof is opened by the piston crown.

So if the inside has a cheater bevel that approximates a rounder roof, it makes little difference if the roof looks flat. it’s what the ring feels. It actually bulges into the port and needs to be slowly pushed back in.

Like Kev said, how flat you go safely depends on the port width. If you catch a ring, game over. Have to have more of an arc if the port is wider. Stihl seems to have pushed the limit on the 034S/044-10mm/064. They have a super flat roof that I personally raise in the center if I widen the port.

The intake port totally different, as most flow occurs at the floor and no rings pass that area, only skirt. The roof can be raised for performance, but only to the point where the skirt is above it at TDC, otherwise it’s useless. If there is a ring pin near the intake roof, it should stay fully covered by plating at TDC (or at least 50% if you have a wider ring and racing). You have to be careful widening the roof if the ring pins are on the sides of the intake port (like most older Stihl). The intake side is the thrust side, so the wider and squarer the intake port is, the more piston skirt wear. Older Stihls had the ”uvula” hanging from the intake roof. It kept the lower ring tucked, but the newer Stihls eliminated it.

So there is no definite answer on how perfectly flat a port opening can be with a ring passing it.

Thanks if was actually a video I seen that showed just how flat the 064 exhaust port is that made me think of this.

 

[cus_deluxe]

hmmm so I assume you can't have it as flat on one ported to 70% vs 60% why is that? Does the ring actually move more into the port the wider you go? I would have just assumed the cylinder would still keep the ring on the same path.

one thing to also be aware of, is sides of ex port. anything beyond just taking the bur off is unnecessary and will lead to the ring getting farther into the ex port and then being more likely to hang on the roof/floor.
this one was too flat btw…

 

[Paul Fithian]

I returned a Cross cylinder for a G660, both roof and floor looked too flat to me. I did not want to take the risk that this would catch a ring and wreck a new saw.


I ported the cylinder the saw came with, runs perfect. I could have gone a bit flatter on this, but I wanted long term reliability out of this one.

 

[srcarr52]

I returned a Cross cylinder for a G660, both roof and floor looked too flat to me. I did not want to take the risk that this would catch a ring and wreck a new saw.
View attachment 375527

I ported the cylinder the saw came with, runs perfect. I could have gone a bit flatter on this, but I wanted long term reliability out of this one.
View attachment 375528

It probably would have been fine, I've seen worse run.

 

[huskihl]

hmmm so I assume you can't have it as flat on one ported to 70% vs 60% why is that? Does the ring actually move more into the port the wider you go? I would have just assumed the cylinder would still keep the ring on the same path.

What would happen if you opened the exhaust to 100% the width of the bore? The same thing happens in a small opening, but it’s exponential the wider you go. Up to maybe 30, 40, 50?% of the bore width you probably don’t even need much of a bevel on a flat roof since there is so much force holding the ring in. The wider you go, the more bevel and arch you need

 

[Ketchup]

The exhaust floor makes me the most nervous. I don’t see any harm in adding some extra bevel there unless freeport is a concern.

There is a point where really flat, wide ports just flow funny too. Intakes get lazy and exhausts can short circuit scavenging.

I’m not a fan of square port corners anymore either. I used to spend hours trying to make perfect corners on transfer uppers but now I think they flow better with a wider radius to the corner. Same on the intake.

 

[huskihl]

The exhaust floor makes me the most nervous. I don’t see any harm in adding some extra bevel there unless freeport is a concern.

There is a point where really flat, wide ports just flow funny too. Intakes get lazy and exhausts can short circuit scavenging.

I’m not a fan of square port corners anymore either. I used to spend hours trying to make perfect corners on transfer uppers but now I think they flow better with a wider radius to the corner. Same on the intake.

Yes and no on the transfer ports. A big radius on a small narrow port might take 6-8° to open completely.

 

[thompsoncustom]

I returned a Cross cylinder for a G660, both roof and floor looked too flat to me. I did not want to take the risk that this would catch a ring and wreck a new saw.
View attachment 375527

I ported the cylinder the saw came with, runs perfect. I could have gone a bit flatter on this, but I wanted long term reliability out of this one.
View attachment 375528

See I like the roof on the first picture more seems like it would make more power than the second picture (if only comparing the roofs) but maybe i'm wrong.

 

[huskihl]

See I like the roof on the first picture more seems like it would make more power than the second picture (if only comparing the roofs) but maybe i'm wrong.

Wide flat roof makes better power up high. A more rounded roof won’t be so peaky, lower hp and more forgiving