In another thread I had mentioned that my Jonsered 920 was a little low on compression and that I would be putting new rings in it and that I would likely do some very mild porting. Well, I did just that but I have concerns about the plating. Now that the nikasil has been breached on the top of the exhaust port isn't there a risk of the plating being chipped away over time by the rings? As soon as I touched the carbide bit to the port and saw the plating peel off of the beveled edge of the port I immediately fealt as though I shouldn't have done this and that I should of just put the rings in and left the ports alone. Some of the pictures I have seen on AS show some pretty radical porting and additional booster ports etc. I am just concerned that the plating may not hold up. As far as the porting I did I only took a couple of mm off the top of the exhaust and the bottom of the intake.
Nikasil Plating And Porting Concerns
- Thread starter amdburner
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blis
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the thing with nicasil plating and porting is that you need high-rpm tools for it, something like dremel to enlarge ports without fear of chipping the plating..
One way to prevent the plating being damaged further is to take really fine sandpaper (1500-2000) and gently sand off the rough edges from where plating has been broken, and same thing with all port edges, give them slight sanding... will help to increase the life span of piston rings...
One way to prevent the plating being damaged further is to take really fine sandpaper (1500-2000) and gently sand off the rough edges from where plating has been broken, and same thing with all port edges, give them slight sanding... will help to increase the life span of piston rings...
timberwolf
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Carbide burs are a bit coarse for cutting the niksil realy they break it rather than cut it, diamond is better, but the aluminum plugs it up. I forget who posted it, but hydrolic fluid helps keep the diamond burs from pluging up. Rather than rounding off the corners, chanfering them is a better way. I have used a small diamond file followed with aluminum oxided paper to smooth it out after, never bothered to go beyond 600 grit, have not had any problems so far.
but raising the exhaust without other work may not have gained you much, maybe even lost torque?
Might now need to drop the jug to get some compression back as well as effective power stroke length.
but raising the exhaust without other work may not have gained you much, maybe even lost torque?
Might now need to drop the jug to get some compression back as well as effective power stroke length.
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Thanks for the replies. The carbide I used was a straight bit with spiral cutters rather than "teeth" so I think that it did a pretty decent job. I think that I will take the cylinder back off though and hit the port edges with some wet and dry sand paper. I did chamfer the edges with a ball ended bit so I believe that the rings should be OK. I was just envisioning the rings catching a little bit of plating every time they slide by the top of the port. Thanks again.
sawinredneck
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If you need to file anything amd, file away from the plating, this will break off what is loose, not loosen it more, just another tip.
Andy
Andy
timberwolf
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Good point, even when working the burr it is importaint to work the cutting side of the burr pushing into the cut rather than away. That is where having a proper revesible tool is nice. Bit like the tip of the chainsaw bar you dont want to touch anything with the top side of the tip.
Problem with carbide burrs is the tungsten carbide is typically softer than the nickel silicon carbide your trying to cut. It wont cut it but ends up breaking it away from the aluminum instead.
Problem with carbide burrs is the tungsten carbide is typically softer than the nickel silicon carbide your trying to cut. It wont cut it but ends up breaking it away from the aluminum instead.
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Timberwolf,
This saw has plenty of compression. It had 130lbs with the old rings (about a 2mm+ ring gap) and though I haven't put a guage on it yet it is very difficult to pull over now without really leaning on it. The saw had ample torque prior to the new rings so I do not believe that I will have lost any. I would have reassembled it without a base gasket but I do not have a means of porting the transfer ports back to spec with the tools that I have. Most of my stuff is for bigger engines. This is my first attempt at tweaking an engine this small. The exhaust port height on these engines is pretty short where 2-stroke performance is concerned. If you compare an 80cc chainsaw engine to an 80cc dirt bike engine there is a world of difference. At any rate I will let you know how it runs once I am satisfied that I have smoothed the edge of the plating enough.
Dave
This saw has plenty of compression. It had 130lbs with the old rings (about a 2mm+ ring gap) and though I haven't put a guage on it yet it is very difficult to pull over now without really leaning on it. The saw had ample torque prior to the new rings so I do not believe that I will have lost any. I would have reassembled it without a base gasket but I do not have a means of porting the transfer ports back to spec with the tools that I have. Most of my stuff is for bigger engines. This is my first attempt at tweaking an engine this small. The exhaust port height on these engines is pretty short where 2-stroke performance is concerned. If you compare an 80cc chainsaw engine to an 80cc dirt bike engine there is a world of difference. At any rate I will let you know how it runs once I am satisfied that I have smoothed the edge of the plating enough.
Dave
blis
Addicted to ArboristSite
amdburner said:
dremel with 90 degree angle piece does wonders (if it fits)... not much of choice otherwise...
btw, when comparing chainsaw engines and motorcycle engines you should note that motorcycles have proper exhaust which increases the perfomance ALOT and also allows usage of bigger ports (and carbs)...
blis,
You are 100% correct. If it was sensible I would love to put a pipe on my saw but it is cumbersome enough with my milling attchment on it. HMMM... a Mikuni slide bore carb and a performance pipe!!! Believe me I am just sick enough to try and graft a cylinder off of a dirt bike to a chainsaw!!!
Dave
You are 100% correct. If it was sensible I would love to put a pipe on my saw but it is cumbersome enough with my milling attchment on it. HMMM... a Mikuni slide bore carb and a performance pipe!!! Believe me I am just sick enough to try and graft a cylinder off of a dirt bike to a chainsaw!!!
Dave
timberwolf
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No issue,
If you have raised the exhaust port 2 mm you waill have reduced the trapped volume and compression will be lowered. I am guessing that that saw has about a 37 mm stroke and a 2 mm change to the exhaust port will add about 15 deg to the exhaust duration. that is a big change.
If your milling, I am worried that the higher exhaust port will hurt more than help, seems to some extent hp can be found with the higher RPM permitted by a longer exhaust duration, but at the expense of torque.
If you have raised the exhaust port 2 mm you waill have reduced the trapped volume and compression will be lowered. I am guessing that that saw has about a 37 mm stroke and a 2 mm change to the exhaust port will add about 15 deg to the exhaust duration. that is a big change.
If your milling, I am worried that the higher exhaust port will hurt more than help, seems to some extent hp can be found with the higher RPM permitted by a longer exhaust duration, but at the expense of torque.
Timberwolf,
15 degrees is quite a bit. Is it possible that with the new rings I will have a net effect of zero? Not optimal of coarse but I can survive with it until I can get the proper attachments for my dremel in order to raise the transfers a little. I have about .050 squish with the gasket and about .020 without it. So I would have to raise them .030. In posts that I have read that disclosed any port changes 2mm seemed to be a conservative amount to remove and individuals that did so claimed to have improved power even in instances where they retained the base gasket. Hopefully, I will see an improvement also. If not then I will persue lowering the cylinder.
Dave
15 degrees is quite a bit. Is it possible that with the new rings I will have a net effect of zero? Not optimal of coarse but I can survive with it until I can get the proper attachments for my dremel in order to raise the transfers a little. I have about .050 squish with the gasket and about .020 without it. So I would have to raise them .030. In posts that I have read that disclosed any port changes 2mm seemed to be a conservative amount to remove and individuals that did so claimed to have improved power even in instances where they retained the base gasket. Hopefully, I will see an improvement also. If not then I will persue lowering the cylinder.
Dave
timberwolf
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If you have taken the .030 base gasket out, that will more than off set the raised exhaust port as far as compression. Did not know you had done that. thatwill also lower the exhaust port about .5 mm.
blis
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just a tought about porting those transfers, keep the angles of tranfers at exactly same as they are in stock cylinder, incase you havent messed with transfer angles and patterns before, otherwise you will propably loose alot of power since the angle and pattern of flush means alot in 2-strokers...
I would definately keep the angles stock. I would like to avoid them altogether if I can. Once I can get a chance to fire her up and slab a log I will know whether or not I will need to do anything. It ran well before but since the rings were well used I figured what the heck. Hopefully this weekend I will get a chance to give her a rip.
timberwolf
Addicted to ArboristSite
Another option, not ideal, but easier to do.
Remove some material from the piston crown where the transfers come in, this effectivly raises the transfers. You don't want to leave much less than .100 of piston crown above the ring. Doing it this way if you dont like it, it is just a new piston not a new jug.
It is a work arround, call it hack, but it will work. What it does structurally to the piston and dynamically to the balance and squish velocity is another question.
Remove some material from the piston crown where the transfers come in, this effectivly raises the transfers. You don't want to leave much less than .100 of piston crown above the ring. Doing it this way if you dont like it, it is just a new piston not a new jug.
It is a work arround, call it hack, but it will work. What it does structurally to the piston and dynamically to the balance and squish velocity is another question.
