Another 260 port job.

[Onan18]

Great info and pics Doc, thanks for sharing.

 

[Alaskanrocket]

Is that an oem piston and cylinder?

Never seen a piston like that. There seems to be an outer relief edge on it for the band. Your pic shows machining marks right though the step on it (Clearly at 0600 in pic). I’m no expert, but that don’t look like it happened from the band slapping the piston. The machine marks could just be from the photo I guess.

Is it a 44 or 44.7 mm jug?

I’ve used an 044 carb and boot on a few. They are race more than Work saws, throttle response and idling better with a 194 carb.

If it’s a 44.7, no gasket squish is usually .015-017. The 44mm usually comes in too tight, around .007-009.

You can scrape the band out with a wood chisel and sand the entire band with a piston with adhesive backed sandpaper on it. You can also cut the entire band and piston flat, which is what I usually do.

You’ll need to cut the base to tighten squish again when you do that.

The ring on the piston is from some flashing left on the inner part of the squish band, I think the photo does make it look different. It wasn't like that when I originally put it in.
It is a 44mm and with no base gasket and after I sanded the squish band back I'm at about .012 squish. I sanded the squish ring with an arbor I ground to cylinder diameter and those 1/4 turn sanding discs, I figured it wasn't terribly flat so I just went real slow and looked at the sanded widths of the squish band while doing it.

 

[Alaskanrocket]

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@drf256 I can't thank you enough for taking the time to post all of those pictures. That's great.

One question I do have is when machining a piston are you using a jig chuck it up or just chucking the piston alone? I actually took the old piston and turned it flat this weekend to use a sanding block, then realized it was a 44.7mm piston when I was done. I just put it straight in the chuck and tapped the runout flat, I wasn't worried about hurting it because it was shot anyways. I am a little afraid of messing up a good piston by chucking it up and either scoring it or warping the skirt.

Other than that, I'm going to go to town to my steel supply to get some stock so I can make a couple of arbors, one for cutting squish, and one for turning the deck. I'll keep you guys posted. Thanks for the help so far.

 

[drf256]

Best to make a sleeve for piston.

.045 off of a meteor and .043 off of an OEM is about right.

 

[Deets066]

@drf256 I can't thank you enough for taking the time to post all of those pictures. That's great.

One question I do have is when machining a piston are you using a jig chuck it up or just chucking the piston alone? I actually took the old piston and turned it flat this weekend to use a sanding block, then realized it was a 44.7mm piston when I was done. I just put it straight in the chuck and tapped the runout flat, I wasn't worried about hurting it because it was shot anyways. I am a little afraid of messing up a good piston by chucking it up and either scoring it or warping the skirt.

Other than that, I'm going to go to town to my steel supply to get some stock so I can make a couple of arbors, one for cutting squish, and one for turning the deck. I'll keep you guys posted. Thanks for the help so far.

I cut em in the mill.

 

[drf256]

@drf256 I can't thank you enough for taking the time to post all of those pictures. That's great.

One question I do have is when machining a piston are you using a jig chuck it up or just chucking the piston alone? I actually took the old piston and turned it flat this weekend to use a sanding block, then realized it was a 44.7mm piston when I was done. I just put it straight in the chuck and tapped the runout flat, I wasn't worried about hurting it because it was shot anyways. I am a little afraid of messing up a good piston by chucking it up and either scoring it or warping the skirt.

Other than that, I'm going to go to town to my steel supply to get some stock so I can make a couple of arbors, one for cutting squish, and one for turning the deck. I'll keep you guys posted. Thanks for the help so far.

I cut em in the mill.

I’ve met lots of good guys here. Being I go through quite a few, Deets has milled the top off for me. I keep them cut already in stock.

I’ve also cut them myself on the lathe. Once you get the sleeve setup, it’s fairly easy to loosen the chuck and pop another in. Check runout, and cut.

Think Deets just finished my third order with him. All in, I’ve used 25 that Deets has done already and around 15 I’ve done myself. Hard to believe.

 

[Alaskanrocket]

I’ve met lots of good guys here. Being I go through quite a few, Deets has milled the top off for me. I keep them cut already in stock.

I’ve also cut them myself on the lathe. Once you get the sleeve setup, it’s fairly easy to loosen the chuck and pop another in. Check runout, and cut.

Think Deets just finished my third order with him. All in, I’ve used 25 that Deets has done already and around 15 I’ve done myself. Hard to believe.

Yeah, Its funny how you spend twice the amount of time on building tooling and setup then you do actually machining the part in the first place. The good part is, once the sleeve is built its done and you can use it over and over, until you decide to work on a different saw, then its back to the steel bin.

 

[Alaskanrocket]

I'm still having trouble finding the time to get the machine work done for the mandrels. We had a big maintenance shutdown at the mill I work at this week, between that and the family it's hard to find time in the shop. I have to travel to the states next week and then I'll be gone for another week after that, needless to say this thread may slow down for a couple of weeks.

One thing that I'm figuring out is, the porting should come after the machine work to the cylinder, Depending on what you want your timings to be, you're going to have to either raise or lower ports a fair amount just to get your timings where they need to be which will take from your over all port opening area, so where I'm at now I may have already screwed myself on this cylinder by going too wide and not leaving myself any room to raising the roof if need be as it seems there is an ideal total opening area that needs to be achieved.

It's crazy that I thought I knew anything about 2 strokes before I started reading all of the info in this forum, I'm still trying to wrap my head around all of this theory. Im pretty sure I've read the Part 1,2,3....series like 3 or 4 times over.

 

[Deets066]

I'm still having trouble finding the time to get the machine work done for the mandrels. We had a big maintenance shutdown at the mill I work at this week, between that and the family it's hard to find time in the shop. I have to travel to the states next week and then I'll be gone for another week after that, needless to say this thread may slow down for a couple of weeks.

One thing that I'm figuring out is, the porting should come after the machine work to the cylinder, Depending on what you want your timings to be, you're going to have to either raise or lower ports a fair amount just to get your timings where they need to be which will take from your over all port opening area, so where I'm at now I may have already screwed myself on this cylinder by going too wide and not leaving myself any room to raising the roof if need be as it seems there is an ideal total opening area that needs to be achieved.

It's crazy that I thought I knew anything about 2 strokes before I started reading all of the info in this forum, I'm still trying to wrap my head around all of this theory. Im pretty sure I've read the Part 1,2,3....series like 3 or 4 times over.

On a 260 you want the ex pretty low. Drop the cylinder and you won't need to raise the port at all

 

[Alaskanrocket]

On a 260 you want the ex pretty low. Drop the cylinder and you won't need to raise the port at all

Ok,
I'm not totally sure of the numbers I'm looking for yet so that helps a lot. I haven't raised it any yet, I haven't changed the timing but I did flatten it out and open it up some on the roof. My plan is to play with blowdown numbers until I go too far, then back up and use those numbers on a new cylinder. I'm wondering if I went with a 44.7mm on the next cylinder, if those "Ideal" timing numbers will be close or if that extra displacement changes the behavior.

 

[Alaskanrocket]

This is my goal, I know I'm literally lightyears away from the knowledge and skills of Timberwolf, but its good to have goals right?