How wide is the piston skirt on the intake side, is it still completely closing the intake port off when the piston is down, BDC.?
I'm a porting idiot.....
- Thread starter Trent Curtis
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Chris-PA
Where the Wild Things Are
How can that work? I'm sure the top of that intake port is still well below the point where the exhaust and transfers open.
Trent Curtis
ArboristSite Guru
What?!?! That's not possible- compression isn't built until after the piston passes the exhaust port.
I put a bevel on the ports using a round file- but it sounds like I better put a better bevel one there.
I'll tear it down here shortly and include some more pics. I'll measure squish as well to see if I can get rid of that gasket.
Trent Curtis
ArboristSite Guru
Here's a pic of the piston at BDC- completely closed off. The ring not dropping into the intake-
Trent Curtis
ArboristSite Guru
After further examining that piston and it's fit to the cylinder I am ordering a new piston and ring.I'd like tonsee if I can't find a piston w fuel rings rather than the single. See if I can't get that compression up closer to 175.
Not possible. You don't begin making compression until the top of the exhaust port.
Yes, that piston has a lot of wear. Go Meteor or OEM only on the piston.
Gugi47
ArboristSite Hit Man
You right...I'm bad.
Looks ok from the pict. How loose is the piston in the bore?
Trent Curtis
ArboristSite Guru
Yessir- can you get new OEM pistons for these saws?
Trent Curtis
ArboristSite Guru
It's way to loose- don't have a feeler gauge or I'd give you a number. Def needs a new piston!
jmssaws
Addicted to ArboristSite
Look at the skirt thickness.
Is the intake skirt thinner than the exhaust skirt?
If so you need a piston and if they look the same then you need a set of rings.
The grinding didn't hurt it but I doubt it will help it either.
What's compression on the 281?
A 281 has a better timed stock cylinder than a 288 and if your 288 is 140lb I have no doubt that the 281 is stronger.
Is the intake skirt thinner than the exhaust skirt?
If so you need a piston and if they look the same then you need a set of rings.
The grinding didn't hurt it but I doubt it will help it either.
What's compression on the 281?
A 281 has a better timed stock cylinder than a 288 and if your 288 is 140lb I have no doubt that the 281 is stronger.
Trent Curtis
ArboristSite Guru
281 is blowing 175.
Hmm interesting thought on the piston skirt. That's a nifty little tell tale!
Trent Curtis
ArboristSite Guru
I can't seem to find a piston with duel rings.... is the single ring the only option for the 288?
jmssaws
Addicted to ArboristSite
No they have dual ring 288 pistons but there not needed.
Trent Curtis
ArboristSite Guru
I know they aren't needed, but do you think a duel ring can offer better longevity? Double barrier if you will-
jmssaws
Addicted to ArboristSite
However many up and downs it takes to wear out the top ring will surely wear out the one below it.
The only benefit I see in 2 rings is you have one more to displace heat.
Also 2 rings create more drag so does that wear parts out?
It takes a lot of run time to wear one out.
A lifetime for most
drf255
BAD CAD
You won't know a thing till you slap a degree wheel on it.
Think of a 4 stroke car engine. Hog out the ports and put a huge cam in. What happens? You shift the powerband upwards. That's what you did in a way. Except you ground big ports and left the intake valves the stock size.
You increased case volume. You didn't touch the intake floor, thank god, because that's where you build case compression. So you made less velocity/inertia in the intake port. Make sure your skirt edges and bottom aren't freeporting in the intake and exhaust.
It looks like you ground on the exhaust roof to me. It looks flat. Could be an illusion.
Most of what you did can be fixed if you're not beyond the skirts with your ports.
Intended use plays a role here. If you're making a race saw it will be different than a worksaw.
You don't need super high compression to make a good running saw. Compression simply preheats the charge in the combustion chamber. It robs power to do so. So when the power gained is exceeded by the power used, you get a weaker saw that wants to overheat and won't keep tune after cutting.
Smaller bores like more compression because they can cool better. Larger bores can't cool as well, retain heat, and need less compression for optimal power/burn.
You need to show pics inside and out of your jug, both ports with piston at bdc/tdc/mid stroke including edges. Post your squish and time your ports.
You're not gonna get any helpful answers unless you do this. Way too many variables.
Think of a 4 stroke car engine. Hog out the ports and put a huge cam in. What happens? You shift the powerband upwards. That's what you did in a way. Except you ground big ports and left the intake valves the stock size.
You increased case volume. You didn't touch the intake floor, thank god, because that's where you build case compression. So you made less velocity/inertia in the intake port. Make sure your skirt edges and bottom aren't freeporting in the intake and exhaust.
It looks like you ground on the exhaust roof to me. It looks flat. Could be an illusion.
Most of what you did can be fixed if you're not beyond the skirts with your ports.
Intended use plays a role here. If you're making a race saw it will be different than a worksaw.
You don't need super high compression to make a good running saw. Compression simply preheats the charge in the combustion chamber. It robs power to do so. So when the power gained is exceeded by the power used, you get a weaker saw that wants to overheat and won't keep tune after cutting.
Smaller bores like more compression because they can cool better. Larger bores can't cool as well, retain heat, and need less compression for optimal power/burn.
You need to show pics inside and out of your jug, both ports with piston at bdc/tdc/mid stroke including edges. Post your squish and time your ports.
You're not gonna get any helpful answers unless you do this. Way too many variables.
drf255
BAD CAD
If it makes you feel any better, I've been there myself. I believe I can say most of us have.
Big ports dont always make big power. Always need to remember, the engineers that designed these saws weren't idiots. Think of how much they spent on design alone.
What they did was make a nearly perfect saw in terms of endurance, performance, emissions, noise and economy to list a few.
All you do when you port a saw is trade one of the above balanced factors for another.
It's a good thing to keep in mind before you touch a burr to any jug.
Big ports dont always make big power. Always need to remember, the engineers that designed these saws weren't idiots. Think of how much they spent on design alone.
What they did was make a nearly perfect saw in terms of endurance, performance, emissions, noise and economy to list a few.
All you do when you port a saw is trade one of the above balanced factors for another.
It's a good thing to keep in mind before you touch a burr to any jug.
Go for the single ring piston. You will get better performance and see no lost in compression. The rings create a lot of drag.
It's a known fact that the top ring wears faster than the bottom ring. It's common practice to swap top and bottom rings to pick up some compression in an old saw. The only thing the second ring is good for is being a bandaid when that top ring is worn out. If that top ring is worn out, both it and possibly the piston should be replaced, not relying on a second ring to do the job. You're never going to know the difference in the real world.
As far as heat displacement...I've never seen evidence to suggest that. Most of the saws I build run only a single ring and have never had issues with heat.
It's a known fact that the top ring wears faster than the bottom ring. It's common practice to swap top and bottom rings to pick up some compression in an old saw. The only thing the second ring is good for is being a bandaid when that top ring is worn out. If that top ring is worn out, both it and possibly the piston should be replaced, not relying on a second ring to do the job. You're never going to know the difference in the real world.
As far as heat displacement...I've never seen evidence to suggest that. Most of the saws I build run only a single ring and have never had issues with heat.
