china cylinder took death like a champ.

[thompsoncustom]

well was playing around this putting stuff in the combustion chamber on my ms260 and ended up casting some aluminum and holding it in with a bolt and using jb weld as a gap filler.



compression came in at 245psi before breakin it in. after about 5 mins of run time the insert failed from to much pressure getting behind it.




there was aluminum everywhere exhaust and case were full I even found some in the air filter. but after cleaning everything up and flushing everything out I'm going to continue testing with all the same parts.

used some red scotch brite by hand in the cylinder.



just thought I would share since I didn't ever image it would survive.

 

[Ketchup]

That’s pretty cool. Great photos.
I wonder if a steel insert or some other material would work better?
Maybe a ceramic epoxy?

 

[thompsoncustom]

That’s pretty cool. Great photos.
I wonder if a steel insert or some other material would work better?
Maybe a ceramic epoxy?

I think the aluminum would be fine it was the way I cut the chamber in it that caused it to blow up.

if it was steel I'm sure it would have survived either way.

I looked into some good ceramic adhesives and i settled on resbond 907gf-7 but I think a tube was 70 bucks so a little spendy since it may or may not work but i might still try it. I thought about just trying straight jb weld but I think the flame would end up burning it slowly over time.

 

[huskihl]

The JB Weld probably blocked the heat transfer through to the head and it melted it

 

[Powerstroke Cowboy]

The JB Weld probably blocked the heat transfer through to the head and it melted it

Yes, JB weld is not a conductor but an insulator.

 

[Ketchup]

Some thoughts, take them or leave them:

The location of the spark and the shape of the chamber really matter. If you make the chamber too flat combustion doesn’t happen efficiently and can damage the walls of the chamber, like in a detonation event. In this case, it looks like you reduced the chamber down to diesel ratios and you plug looks detonated. Your plug was so close to the spark (nearly centered in the combustion sphere), and insulated by epoxy. If you try again I would give the plug more space and maybe skip the epoxy all together.

This is a really interesting experiment. You should get more tools. Having a lathe is a game changer. In this case, if you could machine the shape of the chamber to fit your plug then epoxy filler wouldn’t be necessary.

Though if you can keep the gap between your plug and the original chamber under 0.010” the epoxy probably isn’t needed anyway.

I think @huskihl is right. The epoxy is acting as an insulator. Ceramic would be even worse. Mixing aluminum dust into the epoxy might help, or maybe there are thermally conductive epoxies.

Oh yeah, and your bearings are now suspect regardless of how much you flushed them. That’s probably okay for an experiment but I wouldn’t leave those in a saw you rely on.

 

[thompsoncustom]

The JB Weld probably blocked the heat transfer through to the head and it melted it

no signs of anything melting and I'm sure it blew apart. my fault I should angled it better just didn't have enough room to work in such a small chamber with a bolt in the center.

good to know jb weld is a insulator I'll have to look into the ceramic adhesives and see if they do the same.

 

[thompsoncustom]

Some thoughts, take them or leave them:

The location of the spark and the shape of the chamber really matter. If you make the chamber too flat combustion doesn’t happen efficiently and can damage the walls of the chamber, like in a detonation event. In this case, it looks like you reduced the chamber down to diesel ratios and you plug looks detonated. Your plug was so close to the spark (nearly centered in the combustion sphere), and insulated by epoxy. If you try again I would give the plug more space and maybe skip the epoxy all together.

This is a really interesting experiment. You should get more tools. Having a lathe is a game changer. In this case, if you could machine the shape of the chamber to fit your plug then epoxy filler wouldn’t be necessary.

Though if you can keep the gap between your plug and the original chamber under 0.010” the epoxy probably isn’t needed anyway.

I think @huskihl is right. The epoxy is acting as an insulator. Ceramic would be even worse. Mixing aluminum dust into the epoxy might help, or maybe there are thermally conductive epoxies.

Oh yeah, and your bearings are now suspect regardless of how much you flushed them. That’s probably okay for an experiment but I wouldn’t leave those in a saw you rely on.

I think the company I was talking with had a epoxy with aluminum in it that was good up past 1000 degrees but I still think any epoxy is gonna burn close to the plug. the ceramic ones are listed as flame proof.

I agree chamber shape does make a difference I know in the one I brazed and made a smaller chamber even running really rich it burned very complete/clean plug looked like it came out of a 4 stroke.

I did flush the bearing like crazy so hopefully most of the aluminum is out of there.

if I had the room I would love to get a lathe.

I have more saws and I just cut firewood for the most part this one is more of a play saw. I think it looks pretty good for being a china saw tho

 

[Ketchup]

I think the company I was talking with had a epoxy with aluminum in it that was good up past 1000 degrees but I still think any epoxy is gonna burn close to the plug. the ceramic ones are listed as flame proof.

I agree chamber shape does make a difference I know in the one I brazed and made a smaller chamber even running really rich it burned very complete/clean plug looked like it came out of a 4 stroke.

I did flush the bearing like crazy so hopefully most of the aluminum is out of there.

if I had the room I would love to get a lathe.

I have more saws and I just cut firewood for the most part this one is more of a play saw. I think it looks pretty good for being a china saw tho

View attachment 383929

Looks badass. The best Storm Troopers were clones.


My thinking on the chamber size and shape is mostly from looking at piston wash patterns. Usually the squish band circle on the piston (and the jug) is cleaner. Sometimes there’s a sort of honeycomb pattern around the inside edge. It seems like if you reduce the squish too far it doesn’t burn. But like you said, the brazed chamber looked fine.

I feel ya. Lathes take up a lot of room. I had to sell my band saw to cram a lathe in but it was worth it. Finding space for a mill is even worse. Also worth it.

 

[thompsoncustom]

Looks badass. The best Storm Troopers were clones.


My thinking on the chamber size and shape is mostly from looking at piston wash patterns. Usually the squish band circle on the piston (and the jug) is cleaner. Sometimes there’s a sort of honeycomb pattern around the inside edge. It seems like if you reduce the squish too far it doesn’t burn. But like you said, the brazed chamber looked fine.

I feel ya. Lathes take up a lot of room. I had to sell my band saw to cram a lathe in but it was worth it. Finding space for a mill is even worse. Also worth it.

Well If I need some lathe work I'll just have to pay you to do it

I wonder if it seem like the squish band isnt burning as much is just because there is less fuel/air in the band to burn the tighter you get?

Ya that cylinder I brazed is why I'm playing with this, I ran that saw really rich breaking it in and the plug was super clean making me think that the pressure and temp are higher but it's burning much more of the fuel and oil (32:1).

I'll probably just end up ordering some of that ceramic adhesive to give that a try as it should be easy to keep roughly the same shape but just shrink it down, I like your idea of mixing in some aluminum dust to help transfer heat tho they probably make a thermally conductive one I just need to look into it.

 

[thompsoncustom]

@drf256
@Ketchup

Well I went ahead and ordered the resbond 709gf as the make one that does a good job with heat transfer but it needs to be cured at 600 degrees and I worry about it warping.

I will fill in the holes in the head and bring it back to around stock combustion chamber shape and I will start with adding resbond to the exhaust port to see if I can lower it to 110 degrees and work my way back up. Pretty sure my transfers were at 121 so it will only have 11 degrees of blowdown with this long exhaust any thoughts on this? The exhaust floor is above the piston at BDC so I'll be able to gain some area there atleast.

Depending on where compression ends up by lower the exhaust if it can even be done I will go back in and reshape the combustion chamber.

 

[huskihl]

@drf256
@Ketchup

Well I went ahead and ordered the resbond 709gf as the make one that does a good job with heat transfer but it needs to be cured at 600 degrees and I worry about it warping.

I will fill in the holes in the head and bring it back to around stock combustion chamber shape and I will start with adding resbond to the exhaust port to see if I can lower it to 110 degrees and work my way back up. Pretty sure my transfers were at 121 so it will only have 11 degrees of blowdown with this long exhaust any thoughts on this? The exhaust floor is above the piston at BDC so I'll be able to gain some area there atleast.

Depending on where compression ends up by lower the exhaust if it can even be done I will go back in and reshape the combustion chamber.

You’ve spent more on glue to hold the chylinder together than you did on the chylinder

 

[Ketchup]

@drf256
@Ketchup

Well I went ahead and ordered the resbond 709gf as the make one that does a good job with heat transfer but it needs to be cured at 600 degrees and I worry about it warping.

I will fill in the holes in the head and bring it back to around stock combustion chamber shape and I will start with adding resbond to the exhaust port to see if I can lower it to 110 degrees and work my way back up. Pretty sure my transfers were at 121 so it will only have 11 degrees of blowdown with this long exhaust any thoughts on this? The exhaust floor is above the piston at BDC so I'll be able to gain some area there atleast.

Depending on where compression ends up by lower the exhaust if it can even be done I will go back in and reshape the combustion chamber.

I would start at 107. That gets you 14 degrees of blowdown. If people see good results around 105 then 110 is just going to be slow.

You’ve spent more on glue to hold the chylinder together than you did on the chylinder

That’s a special R&D Chylinder.

 

[thompsoncustom]

You’ve spent more on glue to hold the chylinder together than you did on the chylinder

over double but like ketchup said it's more for R&D than anything else which is the same with the numbers. 106 exhaust and 15 degrees blow down would run good for sure that's why I wanted to start higher as it's unexplored territory.

 

[Ketchup]

I can’t think of any 50cc saws that run better above 107.

 

[thompsoncustom]

I can’t think of any 50cc saws that run better above 107.

oh I didn't say it was gonna run better I just want to see how it runs. I use this saw more for little stuff so a higher exhaust would serve me better for sure but I've never heard of anyone running a 026/260 with a 110 exhaust before so it should be interesting.

 

[thompsoncustom]

Got a couple of questions for anyone with some experience.

Can't lower the exhaust floor much without free porting is there a certain amount of free porting that can be had before it causes issues or am I better off not free porting it at all. What would be the bare minimum of side skirt overlap on the exhaust. Don't plan on going more than .100 on the intake.

 

[cus_deluxe]

a little bit of freeport on bottom wont kill ya, probably…
i dont do saws every day like some on here, but anything less than a couple millimeters on sides is too easy for me to grind past, and freeporting on sides will make a turd for sure.

 

[Ketchup]

I’ve heard you can do 0.5mm (0.020”) on the sides, but it’s high consequence so I measure several times and sneak close to my lines with a diamond burr.

As far as freeporting the bottom of the exhaust, a couple degrees on either side of TDC won’t matter. Too much will hurt intake. I would say 12 degrees duration is pushing your luck but you might get more in a saw that fills quickly.

 

[thompsoncustom]

Thanks guys, lowering the exhaust to 110 I'm going to be lacking time and area on the exhaust port so gonna have to get a little wild to give it a chance to perform. Resbond should show up this week so hopefully I can get it in there and see what I end up with for exhaust port size.