NWP piston for 066

[mtngun]

Here's an exaggerated sketch showing my "two-tier" pop-up on one side and a convention pop-up on the other side.

In other news, I let the saw idle through two tanks of gas today, then bucked a little firewood. I didn't have time to re-check compression but it didn't feel like it had improved. Oh, it starts and runs, but doesn't do anything earthshattering.

 

[parrisw]

Will inspired me to take a five-day hiatus in a padded cell.

ha ha. I bet you had a good time. Got some things thought out?

Here's an exaggerated sketch showing my "two-tier" pop-up on one side and a convention pop-up on the other side.

In other news, I let the saw idle through two tanks of gas today, then bucked a little firewood. I didn't have time to re-check compression but it didn't feel like it had improved. Oh, it starts and runs, but doesn't do anything earthshattering.

With the 2 tiered popup, wouldln't that cause a tight spot on the squish band? how far does the squish band come out? and how far till the first step on your popup.

 

[mtngun]

How did you clean the cylinder? You could try to hone it, hopefully the rings will seat a little better.

Cleaning was a problem.

This jug had severe sawdust poisoning in its previous life. You wouldn't think sawdust would do a lot of damage, but it does. It creates a lot of heat and friction and bakes on and doesn't want to come off.

When the sawdust incident happened, the saw never actually broke. It still ran, but it didn't have any guts, and it would bog very easily. A compression check showed lower than normal compression, about 100 psi if I remember correctly.

Unlike a lean seizure, there was no metal transfer, and the piston was still in fair shape. However, everything was coated with baked on sawdust gunk, there was some pitting and scratching on the cylinder wall below the exhaust port, and the rings had obviously stopped sealing.

Because of the pitting, I was reluctant to reuse the cylinder until this project came along. I stated up front that this jug had issues and might not work out, but I was willing to take that chance, because my only out-of-pocket cost was for the piston, and I viewed the project as a learning experience.

Anyway, I tried about every soap, household cleaner, solvent, penetrating oil, etc., with no luck. Then it got tossed in a bucket of diesel for about 6 months. You would think that would at least soften up the sawdust gunk. Nope, not at all.

Finally I stuck it in the blast cabinet and had at it with glass beads. That got 90% of it off, but not 100%.

Then I took some fine emery cloth to the pitting and scratches that the sawdust incident had created. I only tried to knock off any high spots that might damage the piston or rings. I didn't sand the entire cylinder. I was reluctant to sand any more than necessary because I didn't want to wear down the plating.

Unless some miracle happens, I'll probably admit defeat on this jug and order a 2nd generation BB kit when they are available.

 

[parrisw]

It's pretty hard to wear away the plating. If ya got nothing to loose hone the crap out of it. I've done that before with good results. Just keep an eye on the port edges.

 

[mtngun]

With the 2 tiered popup, wouldln't that cause a tight spot on the squish band? how far does the squish band come out? and how far till the first step on your popup.

The machined squish band on the jug is tiny, perhaps 0.080" wide.

I'd have to run out to the shop and grab the CNC program to get the exact numbers, but I can tell you that I erred on the conservative side, so I think there is plenty of clearance, more clearance than I would prefer.

When I checked squish, no tight spots showed up. In fact, it showed that I could have made the outer tier higher. However, the as-cast surface of the dome is pretty rough in that area, so it seemed prudent to leave ample clearance.

Next time I make one of these, I'll get a good chamber impression and make the pop-up follow the contour of the chamber more closely.

 

[mtngun]

If ya got nothing to loose hone the crap out of it.

That's sensible advice. If I can't get the compression up, the P&C will go in the recycle bin, so why not give honing a try ?

 

[parrisw]

The machined squish band on the jug is tiny, perhaps 0.080" wide.

I'd have to run out to the shop and grab the CNC program to get the exact numbers, but I can tell you that I erred on the conservative side, so I think there is plenty of clearance, more clearance than I would prefer.

When I checked squish, no tight spots showed up. In fact, it showed that I could have made the outer tier higher. However, the as-cast surface of the dome is pretty rough in that area, so it seemed prudent to leave ample clearance.

Next time I make one of these, I'll get a good chamber impression and make the pop-up follow the contour of the chamber more closely.

The last one I figured out, I just made it as big as possible, I measured the width of the squish band, gave it a little clearance, and had it made. Ended up with a 200psi 395xp. Not too shabby.

I personally don't see the need for a 2 tiered popup, just make one as big as you can.

 

[parrisw]

That's sensible advice. If I can't get the compression up, the P&C will go in the recycle bin, so why not give honing a try ?

Ya, exactly, why not!! Did you raise the ex at all? That can bring down the compression. I've honed a number of saw cylinders with no problems. some people don't like it though.

 

[riesenbr549]

IMHO . wouldent it be easyer 2 just cut the head off? Maybe you could hone the cylinder as previously stated and cut the head off. then make a new head at least you have cylinder that you know will run to play with.. What about boring the cylinder and having it re plated? make your own bb kit? I have done this with CR500 cylinders with very good results! not sure if there is enough cylinder on a saw thou?

 

[mtngun]

Did you raise the ex at all?

The very top of the exhaust port arch is untouched, complete with casting flaws.

I personally don't see the need for a 2 tiered popup, just make one as big as you can.

Need = cc :rockn:

wouldnt it be easier 2 just cut the head off? Maybe you could hone the cylinder as previously stated and cut the head off. then make a new head at least you have cylinder that you know will run to play with.. What about boring the cylinder and having it re plated? make your own bb kit?

That would be fun to do, but the cost would be as much or more than a used 880.

 

[parrisw]

What about a domed piston? Wonder if that would bring up the comp more then a popup, maybe just cut the top of the piston on a taper?

 

[Justsaws]

Using modeling clay works very well for getting an image of the combustion chamber. Apply the clay to a clean oil free piston and lightly oil the cylinder.

For a permanent image to compare your piston to, make a plaster casting mold of the top of the piston/clay. If you are super picky make a couple so you can cut them up for profile images.

In terms of tracking the cause for the lack luster compression. Coat the cylinder wall with a thin dye(let it dry) or similar substance and mount it back up. Turn the saw through the compression area once slowly by hand once and pull the cylinder take a peak. Not usually worth the effort but has answer a couple of questions for me in the past.

Some times the aftermarket rings are very hard are do not conform to a well used heavy worn cylinder for a long time. Could just be a bad port configuration.

I enjoy projects like the one you are currently working on more than just slapping parts on. The victories are more satisfying and as long as it was enjoyable there are no defeats. Good luck.

 

[oneoldbanjo]

Using modeling clay works very well for getting an image of the combustion chamber. Apply the clay to a clean oil free piston and lightly oil the cylinder. Good luck.

When I was putting together an engine that had been bored and stroked I wanted to check the fit of the piston and head. I put the engine together with the light coat of oil as suggested and had the spark plug hole facing up. I melted candle wax and poured it into the open spark plug hole and let it harden. When I took the engine apart I had a nice wax model of the area between the piston and head. There is probably some shrinkage of the wax as it cools and you may not want to use it to measure tight clearances - but it does give you a good visual representation of how tight the clearances are with the pop up and the cylinder head.

 

[mtngun]

What about a domed piston? Wonder if that would bring up the comp more then a popup, maybe just cut the top of the piston on a taper?

The added cc is a function of how much you can safely shave off the edge of the piston. That in turn is a function of how much metal the piston has above the top ring and how much risk you are willing to take.

Jacob had recommended leaving 0.1" above the ring for a milling saw. I can't argue with his logic, but I took it down to 0.09". That only produced 0.035" pop-up, not a heckuva lot.

I think a rounded dome would be easy to cast and that is probably the reason some OEM pistons have a rounded dome. But in this CNC age, it is easy to CNC any shape you want.

It might be a few days before I can do any more work on the saw. No hurry, it's winter here, no logs to mill, and it'll be several weeks before the 2nd generation BB kits start shipping.

Parrisw, have you made your piston fixture yet, for holding the piston in the lathe ? I was thinking to try one like Timberwolf's. His seems to index on the bottom of the skirts, and it remains to be seen if that is a good system, but it must work or else he wouldn't use it.

 

[Hddnis]

Many of the Huskies use a single ring. I wonder if you could go .1 over the second ring, cutting away the top ring to give yourself a big pop up. You would have to know for sure how much meat the piston has on top to know if that would work. Only other option is to tig on a dome.



Mr. HE

 

[parrisw]

The added cc is a function of how much you can safely shave off the edge of the piston. That in turn is a function of how much metal the piston has above the top ring and how much risk you are willing to take.

Jacob had recommended leaving 0.1" above the ring for a milling saw. I can't argue with his logic, but I took it down to 0.09". That only produced 0.035" pop-up, not a heckuva lot.

I think a rounded dome would be easy to cast and that is probably the reason some OEM pistons have a rounded dome. But in this CNC age, it is easy to CNC any shape you want.

It might be a few days before I can do any more work on the saw. No hurry, it's winter here, no logs to mill, and it'll be several weeks before the 2nd generation BB kits start shipping.

Parrisw, have you made your piston fixture yet, for holding the piston in the lathe ? I was thinking to try one like Timberwolf's. His seems to index on the bottom of the skirts, and it remains to be seen if that is a good system, but it must work or else he wouldn't use it.

Yes, my piston holder is finished, I've only got one groove cut in it so far for 54mm pistons. I've only done 1 test cut with it on a junk piston, didn't work so well since I couldn't index it too well with the 3 jaw, I never marked it when I machined it. So now I have a 4 jaw, I'll just use that with the holder. Haven't tried it yet. Should work just fine, that's all TW uses.

Many of the Huskies use a single ring. I wonder if you could go .1 over the second ring, cutting away the top ring to give yourself a big pop up. You would have to know for sure how much meat the piston has on top to know if that would work. Only other option is to tig on a dome.



Mr. HE

That sounds like a good idea actually.

 

[mtngun]

Yes, my piston holder is finished, I've only got one groove cut in it so far for 54mm pistons.

Gonna post a thread on it ?

 

[parrisw]

Gonna post a thread on it ?

I guess I should of done an update on it. LOL, I did have a post about it a while back. I'll see what I can do. Maybe, I'll chuck it up in the 4 jaw, and take another cut on the piston and see what happens. I've been so busy with working on House reno's and going through all the Stihl parts I just bought, I bought out a Stihl dealership's parts stock, and now organizing it and cataloging all the parts, lots of work. Ive put in at least 6 hours so far and maybe and 1/8 done.:jawdrop:

 

[mtngun]

Finally found a few hours to get back to this project.

Since last time, the saw has idled through a couple tanks of fuel, and then another 1/2 tank bucking firewood at part throttle.

Compression is up ...... to a whopping 136 psi. The math said the pop-up should produce 145+, so obviously the rings are not seating well.

Tear it apart again -- scratches on the exhaust side of the piston. The photo doesn't do it justice, but take my word for it, most of the scratches clock exactly with the scratches on the OEM piston. In other words, I didn't do a nearly good enough job cleaning up the damage from last year's sawdust poisoning incident, which had left a couple of pits and a deep scratch below the exhaust port.

Note the NWP still has about half of its moly coating on the skirt. I'm surprised the moly has lasted this long.

There was new damage, too. A new pit -- a pinhead sized piece of plating had flaked off -- below the exhaust port, that I don't recall seeing before. It's not a good thing to have pieces of plating flaking off and wandering around in the engine. If the plating continues to flake, then this jug is a lost cause.

Also a new scratch in the upper part of the jug, near, but not directly above a transfer port. The location of the scratch doesn't correlate to anything on the piston or cylinder, so I'm guessing it was caused by some random piece of contamination -- like the previously mentioned flaking plating.

I honed the jug very aggressively this time, and gave extra honing to the scratches and pits. I could still hook a fingernail on the scratch above the transfer, just barely.

I also lightly sanded the high spots off the piston, and lightly sanded the rings. Pic after sanding.

I'll let it idle through a couple more tanks of fuel and re-check compression. There is no reason to be optimistic, but I might as well give it every chance. There is nothing to lose.

 

[blsnelling]

Forget letting the saw idle through tanks of fuel like that. Also, part throttle bucking is a no no. WOT always when in the cut like that.