NWP piston for 066

While waiting for the 2nd generation BB kits to be released, I thought I'd try to resurrect my OEM cylinder that died of sawdust poisoning last year. The cylinder has damage to the plating below the ports -- more on that later -- so it may be a lost cause, but I figured it's worth a try, and if it fails, I'm not out anything except the cost of a piston.

I took a chance on the Bailey's NWP piston. Why the NWP, you ask, when the much touted Meteor is only $5 more ? Well, because I'm planning to shave a pop-up, and Baileys claims the NWP is 0.5mm taller than a standard 066 piston, 17.5mm pin-to-top compared to 17.0mm on most other 066 pistons. At first glance, a taller piston seems like a better candidate for a pop-up. Plus, I've been pleased with the fit and finish of the Bailey's parts, even though the design details aren't always to my liking.

However, as best I could measure, the pin-to-top measured 17.3 - 17.4 mm, not 17.5mm. Still, that is taller than OEM.


Here they are conjoined by a pin so you can eyeball the height differences. The bad news is that the distance between the top groove and the top of the piston is smaller on the NWP, 0.127" vs. 0.141" OEM. That wouldn't make a bit of difference if I were using the piston "as-is", but since I plan to shave a pop-up, my pop-up will be limited by how much meat remains above the top ring. They say your should leave at least 0.100", but that won't give me the compression I want. I'm tempted to shave it down to 0.080". What say you, experienced popper uppers ?

BTW, the skirts are equally wide on both pistons.


The NWP windows are quite a bit larger than OEM.


I cleaned up the flashing around the windows, but otherwise there was little room for improvement since they are already larger than OEM. As expected, the fit and finish of the NWP piston was just as good as OEM. BTW, I really like my new Astro Pneumatic 218 pencil grinder.


Another view showing the windows after lightly cleaning them up.


Continued on the next post .......

It depends on what you're going to use the saw for. If you're going to mill with it at some point, I'd say go no more than .100". If it's going to be a falling and bucking saw then you can get away with .080". If it's just gonna be a fun saw, then you can go down to .060".

I did an 044 for a guy once and he wanted a real screamer. I used a Tecomec full-circle 266 piston and shaved it down to .060" above the top ring land, which only gave me about .020" over the ring locating pin. That was four years ago and it's still running. He only breaks it out once in a great while though.

Showing the bottom of the pistons.


A 0.002" feeler gage would just barely fit between the NWP skirt and the cylinder wall. Of course, that's with the moly coating, which will quickly wear off.

Rings have the little groove in the middle. One ring had 0.009" gap while the other ring had 0.007" gap.

Both pistons weighed 3.4 oz. If there is a difference in weight, my postal scale could not detect it. However, the NWP piston pin is thicker and heavier than OEM (I plan to reuse the OEM pin).

This project will drag on for a while. The cylinder is getting a newbie woods port. The piston will be shaved for a 0.047" pop-up (leaving 0.080" above the top ring). The cylinder base will be turned down as required for 0.020" squish. All of this stuff will be a first for me so I could easily screw it up at any step. And then there is the questionable OEM cylinder. If things work out, great, if they don't work out, I'll write it off as a learning experience and by that time the 2nd generation BB kits should be available.

Jacob J. said:

It depends on what you're going to use the saw for. If you're going to mill with it at some point, I'd say go no more than .100". If it's going to be a falling and bucking saw then you can get away with .080". If it's just gonna be a fun saw, then you can go down to .060".

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I was afraid you were going to say that, but you've done a lot of these, so you should know.

It's a milling saw and will be used hard.

My problem is that I mill at 5000 feet altitude and the compression ratio on the OEM jug (and the 1st generation BB kit) is anemic. I'd like to get at least 150 psi out of it and 160 psi would be better. As for the 180 - 200 psi that you guys get near sea level, I can only dream about that.

I'll think about it. This whole project is sort of an experiment and learning experience for me so I'm willing to take a little more risk than I usually would.

A late night in the shop making the pop-up.

Several layers of masking tape to protect the piston from the chuck jaws.


Dialing in the side of the piston. I was proud of myself because it only took a couple of minutes to dial in the side.


Then I checked the top for run out. It was way off, darn it. :angry2: It took quite a while to dial in the top. Then when I rechecked the side, the side was way out. Spent an hour going back and forth between the side and the top. Seemed like it was impossible to get them both dialed in at the same time. Finally got both to 0.0025" runout, not up to my usual standards, but it was getting late and I was tired, so I declared a victory.


First pass, cutting the pop-up so that 0.100" remained above the ring, as per Jacob's suggestion. That's only a 0.025" pop-up, pretty gosh darned lame.


Measuring the thickness above the ring -- right on the money ! ! ! I studied it for a while and decided to take off another 0.010", leaving 0.090" above the ring, and giving a 0.035" pop-up.

Looks GOOOOOOOOD !!!

Second pass.


It's a 2-stage dome, attempting to match the contour of the chamber.


But as you can see from this view, a 0.035" pop-up ain't much.


And that's as far as I got tonight.

Looking good so far. Are you shaving the cylinder as well? You could probably squeeze it down a bit more if needed.

Jacob J. said:

Looking good so far. Are you shaving the cylinder as well?

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Presumably, I want to end up with 0.020" squish. I'm working 7 days a week so this project will progress slowly.

You could probably squeeze it down a bit more if needed.

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Well, this is a milling saw, so I trying to be reasonable.

Normally I am very conservative with saw tuning, but the combination of altitude and wimpy compression ratio is getting old. When I acquired the 066 (freshly rebuilt) it had 130 - 135 psi with a standard base gasket.

That P&C died of sawdust poisoning so it got a BB kit. The BB kit would only blow 125 - 130 psi with 0.020" squish, and the BB's freeport issue makes a pop-up unappealing.

So .... now I'm going back to the not-so-great OEM cylinder and this pop up. I'm thinking I'll end up with 150 psi. I'd be in heaven if it blew 160. I consider it an experiment, and an education.

Getting a cam ground piston dialed in is no fun, but the squish band and combustion chamber are no where near as concentric as your piston popup is so it's no big deal. I've got some Dolmar 5100 cylinders on the bench, and the chamber is way offset in the head, so one would have to run an offset popup, or do something else in one of these.

I've got an early 066 cylinder and piston that weren't in the greatest shape so I made a .040 popup on dropped the cylinder and squish is .022. It's fine but only blows 150 psi, my gauge is probably pretty honest. It runs verry strong compared to my others that have more compression, compression isn't everything but it helps. I've made a mandrill and tool to cut the squish band in the cylinder, so my next experment with this is to cut the band a bit and drop the jug some more, or go to a flat top piston. A popup is a good way to gain, but it has it's downfalls also when filling the cylinder with fresh air. I'll have to re cut the ports a little to keep the timeing numbers where I'd like. It's an experment and it's better to ruin junk parts rather than new parts.

I was looking at the specs for the NWP, Meteor pistons for the 066, 460bb, and Husky 288. I've got a few pistons that should be here this week and am going to start working on a 288 build. I'd also like to do a piston swap in a Dolmar 7900bb kit or the 460 bb putting the 066 or 288 piston in one of them when time allows.

wi50 said:

Getting a cam ground piston dialed in is no fun, but the squish band and combustion chamber are no where near as concentric as your piston popup is so it's no big deal.

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Here's fixture that Timberwolf uses. It must work, or else he wouldn't use it, but seems like it assumes that the bottom of the skirts were machined square with the bore ?

I've made a mandrill and tool to cut the squish band in the cylinder, so my next experment with this is to cut the band a bit and drop the jug some more, or go to a flat top piston.

Click to expand...

Well, don't be shy about posting pictures of your projects.

This evening's task was to measure the chamber volume with the pop-up and compare it to the OEM chamber volume.

The piston, with an old pair of rings, was pushed up into the cylinder until it came to rest against the squish band. A 10cc syringe was used to dribble water through the spark plug hole until the bottom plug thead was wetted. The measurement was repeated 3 times and averaged. We'll call this the "zero squish" chamber volume.


zero squish volume with OEM piston = 9.5 cc

zero squish volume with pop-up = 8.75 cc

That's a 7.8% improvement. I'd like more, but ....... say, Grande Dog, if you are reading this, have you ever considered offering a NWP pop-up for the 066 ? If you made the dome say, 0.100" high, we could always cut it down as needed. Taking metal off is easier than putting metal on.

Getting back to chamber volume -- and this requires some math, so avert your eyes if you are not into math -- we have to figure how much the squish contributes to the chamber volume.

OEM squish is typically what ? 0.030" - 0.035" ?

Of course, I'll be aiming for 0.020" - 0.025" squish with the pop-up.

For this 54mm jug, each 0.01" of squish contributes 0.582 cc.

Total chamber volume = zero squish volume + squish volume.

OEM chamber volume @ 0.030" squish = 9.5 cc + 3 x 0.582 cc = 11.25 cc
or
OEM chamber volume @ 0.035" squish = 9.5 cc + 3.5 x 0.582cc = 11.54 cc

pop-up chamber volume @ 0.025" squish = 8.75 cc + 2.5 x 0.582 cc = 10.21 cc
or
pop-up chamber volume @ 0.020" squish = 8.75 cc + 2 x 0.582 cc = 9.91 cc

Depending on how the squish falls out, compression should be up 9% to 14%. It would have to gain 15% just to compensate for altitude losses, but I knew I wasn't going to get that just by shaving the piston.

Since the OEM compression ran around 130 - 135 psi, the pop-up should produce 145 - 150 psi. Not exactly earthshattering, but I'll take it.

My other project this evening was to make a grinder mandrel to hold emory cloth. I used a scrap piece of 300 series stainless, just because it was handy.


And it worked just great ....... for about 5 minutes.......


...... and then it fatigued and snapped. I'll have to try again, with a better design and with heat treated drill rod.

Similar to TW's holders.

mtngun said:

My other project this evening was to make a grinder mandrel to hold emory cloth. I used a scrap piece of 300 series stainless, just because it was handy.

And it worked just great ....... for about 5 minutes.......

...... and then it fatigued and snapped. I'll have to try again, with a better design and with heat treated drill rod.

Click to expand...

A little tip. I've got one, just put a rubber tip on it, no slot, and wind the paper on in the direction of the turn, as it turns it holds the paper tight, sticky back paper works even better.

Thanks for the pic, parrisw.

mtngun said:

Thanks for the pic, parrisw.

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Sure no problem!

I ordered some dremel-style sanding bands and mandrels, but in the meantime I've finished the porting with emory cloth and files.

Here are two damaged spots left over from a previous case of severe sawdust poisoning. The plating flaked away. It's below the exhaust port, so it won't affect compression. However, the flaked spots are swept by the rings, so there's a chance the plating will continue to flake, which could lead to serious damage.


I originally did not plan to put this damaged cylinder back into service, but then talked myself into using it as a guinea pig for this project. I wouldn't want to use a good 066 cylinder until I gain more confidence at porting.

Next step is to check the squish, and then I'll probably have to shave the cylinder base.

Guys, your knowledge in mechanics is light years in front of mine. Hope one day I can apply the knowledge you teach me here to improve my saws.
Thank you very much for the excellent threads you post here.

mtngun, you're a GENIUS!

caotropheus said:

mtngun, you're a GENIUS!

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Thanks for the compliment, but I'm a newbie, not a genius. Note I'm not showing any pictures of my port work because I need to practice on another cylinder or two before my porting will be good enough to show off. :newbie:

AS does have some chainsaw geniuses like timberwolf, Snelling, Lakeside, Jacob, Aussie, and a few others. I would not have attempted this project without the tidbits of knowledge I've gleaned from them.

This evening I removed the BB kit that was on the saw and checked the new pop-up piston for squish.

This is why I NEED a pop-up. The BB kit maxed out at about 130 psi early in its life, when I installed it last summer. Tonight it blew about 123 psi, and that's with squish set to 0.018" - 0.020". The uncorrected compression ratio is a pitiful 8.2:1, about the same as OEM.


The BB already is freeporting. It would freeport more if I attempted a pop-up.


When I removed the muffler, a little sliver of metal fell out. Oops !!!! It's not a good thing to have little pieces of metal floating around in the muffler. They could find their way back into the engine. This muffler and its DP cover have been welded many times because it keeps cracking. There are welds on top of welds and new cracks appear every time I look at it. Ya think it's time for a new muffler ?


The inside of the engine was in nearly perfect shape. A 0.0015" feeler gage would fit between the piston skirt and cylinder wall, but a 0.002" gage would not.

The top ring had 0.011" gap while the bottom ring was 0.014".

Looking at the exhaust side of the piston. They don't show up in this photo because of the glare, but the machining marks were still visible.


Some little itty bitty piece of metal had bounced around inside the engine, though. There was no damage to the rings or ports, but some small dings in the cylinder wall below the intake port, and a couple of tiny dings in the top of the piston. There are no missing parts, so I have no explanation, unless it was a sliver from the perpetually cracking muffler.