I did try to build up a scrap piston crown a while back, but at that time was still having a hard time with aluminium welding in general. Quite a few hours practice and a couple hundred bucks of argon later and I think I am ready to give it another try.

I used 4043 filler here in the fist atempt a couple months back and got too much heat. it did hold up to distructive testing of the cross section pretty well but fractured along an inclusion.

Will do a better job controling heat next time.

 

Can't you just mill the head?

David

 

The heads are attached, one piece castings. I have made heads and converted the jugs to two piec heads, but depending on race rules that's not always permitted.

Another option would be to weld up in the top of the head, but would not be easy to work (weld and machiene) down in a 4 inch deep hole only 2 inches across.

Here are a couple pics, First is a standard jug. The second is a jug with custonm 2 piece head. If I make a two piece head than no need to weld on the piston crown as the combustion chamber can be resized as requred. However in some cases it would still be an advantage to make the piston skirt a couple mm wider.

 

Couple more, custom with two piece head and a standard stock jug with attached head.

 

If you're looking for silicon, go for 4047 instead of 4043. 12% silicon. Most LWD's should have some in stock, or can get in for you in a day or two. 550 F isn't a problem since the eutectic in the weld wont even go plastic until around 1000 F.

Regards, Kbnit

 

I understand welding the piston dome for compression.

What is the purpose of welding the piston skirt?

 

Craig, if the skirt is shorter, there is less frictional loss. Buuuttt...with a 2 stroke, the skirts are often grooved, to carry some lubricating oils on them. Too short a skirt can lead to a lack of oiling on the cylinder walls.
Also, if the skirt is too short, you can get some nasty piston slap, especially in a single cylinder motor. It already vibrates, regardless of the amount of counterweight on the crankshaft. The single firing pulse shakes the motor, and at certain rpm, the shaking plus piston slap will be fatal.

OP- use 4043 for the build up on the piston, or 4047. The 5356 is not a good filler choice for very high service temps. You will see too much embrittlement on the piston crown in an oil cooled motor. A water pumper, it would be more acceptable, but probably not recommended.

 

Craig,

I think it stops the compression from bleeding down the sides of the piston. Wider skirt prevents the piston rings from flexing as much.

 

Will look into some 4047 filler, thanks. That is likely closer levels of silicon to what is used in pistons and jugs.

With going to extremes in getting these things to go fast it helps to have very wide ports, in some cases wider than what a stock piston will cover and seal.

A custom CNC billet piston is worth several hundred bucks plus the design work, custom forgings more yet. So if there is a way to build up a little here and there it could be worth while.

Also do some welding on the jugs themselves to allow extra ports and larger intake and exhaust flanges, so that 4047 might be the ticket for that too. Jugs also use quite a bit of silicon to keep thermal expansion in check and make them a little stiffer and heat reistant.

 

??? The skirt doesn't seal things, the rings seal things.

The top of the piston does the compression and reacts to the combustion pressure.

The rings seal the piston in the cylinder.

The skirt helps hold the piston together and makes an area to connect the connecting rod to.

5356 is NOT the right material. It is not recommended/rated for service temps over 150F.

4047 as stated is about the highest silicon level you can get in a filler, at 11-13% Si . 4043 is 4.5-6.0% Si. 5356 is 0.25% max Si, but has about the most Mg in a filler alloy.

Lincoln lists 4043 and 5356 as cut length fillers, but has 4043, 5356, and 4047 listed as spooled filler (aka MIG wire).

As to what is the 'right' filler to attempt what you want to do, 5356 is NOT a good choice, but the correct choice really depends on -what- alloy the piston is made of. It could be a wrought heat-treated alloy, it could be a wrought non-heat-treated alloy, it could be a cast alloy again in heat-treated or non-heat-treated alloys.

In the pics of the pistons, the right piston is beefier and the arrow points to a bump-out in the skirt area that looks like it is there as 'beef' for the con-rod connection to make it stronger.

Crazy Canucks and their chainsaws. :drinkup:

 

Timberwolf, Side question. What are your home made heads made from (you have my wheels turning)? Mondo job.
Is anybody trying to put a water jacket on head or barrel or both with a small thermo siphon (heat circulation of the water - no pump) radiator somewhere on the rig? Too big?
How long to these engines have to run after start up and warm up?
Stew

 

??? The skirt doesn't seal things, the rings seal things.

Well.... No.

The skirt of the piston seals both the intake and exhaust ports from the base of the engine. This alows the base of the engine to compress the incomming charge and pump it up into the cylinder to be burnt. Look up piston ported two stroke engines. Without the piston skirt sealing the ports base compression would be short circited out the exhaust and back into the intake as the piston moved towards BDC.

I realise that 5356 has issues with higher temperatures, thats specifically why I asked and posted the temperature requirements. 4043 seams very soft. 4047 sounds like a much better match, but can't seam to find 4047 as tig rods, but if I found a spool of mig wire that would be fine too. Guess I can always cut up scrap pistons into strips and use them as filler (PITA though)

There are different alloys being used, and not much disclosure as to what specific alloy any given manufacturere uses. 4032, 2618, MS75, AFC12, 390 as well as propriatary alloys, but most range from about 9 to 12% silicon. There are both cast and forged pistons in these motors, but most I see are cast.

Heads, mostly I have been using 6061, easy to get, easy to machiene and I have had no problems with this.

For this application water jackets are a problem. with a water jacket the jug tends not to expand much as heat is controled, yet the piston swells to 300 C on the crown so too much issue for siezing the engine. Max run time for these saws would be about 10 seconds or less burning a mix of Nitromethane, methanol and some other nasties.

 

Ding, the light goes on. They're two stroke motors, with holes cut in the cylinder walls for intake, tranfer and exhaust ports. The piston skirt controls the opening and closing of these ports.

That's a heck of a wide port if you need to weld up that corner of the skirt.

Looks like you're going to get a lot of BANG fer yer buck.

 

Bear in mind that pistons are not round/cylinderical to take thermal expansion into account. The piston crown has to deal with far more heat than the skirt AND the gudgeon pin bosses mean that the wall thickness of the skirt varies lots so they tend to taper from top to bottom and be a few thou out of round.

I don't know the fine details of 2 stroke tuning but i do a fair amount of barrel/crankcase welding for one of my gearbox reconditioning clients- his hobby is building Lambrettas that can out drag most things! Port welding (to change timing), adding an extra transfer port, he's even had me weld around transfer ports (build it up to the OD of the fins!) in his continual search for more power

He takes perverse pleasure from getting me to weld in the most awkward places (torch hand stuffed IN the barrel, filler bent and added through one of the ports, welding half blind trying to peer through a different port, sometimes via a mirror!). Steers clear of welding pistons though. Nothing to with cost as he's always having barrels re-nikasiled and these bikes are pretty much no expense spared.

Might be a different story with such short running times though, he expects at least one evenings worth of 'baiting' before a rebuild!

You should be able to find 4047 pretty easily in straight lengths, most of the big players produce it

 

his hobby is building Lambrettas that can out drag most things!

Bl00dy hairdriers

 

Welding piston domes? No problems there, esepcially for that little toy engine, 4047 should fit the bill as stated above. Might be wise to stress relieve the casting however, or else it will relieve itself the first time you run the engine and may warp to a shape that invites mr. stick. Now as far as welding skirts and machining them, forget it. I dont know you, but I know you dont have the capability to measure, develop or machine a skirt profile. Think its round, or a simple oval? Think again, and it only gets worse with power density.

 

Ahh, the skirt seals off the intake and exhaust ports in a 2-stroke.

The rings still do seal off the combustion chamber and enable the piston to compress the air-fuel mix and to hold the pressure in the cylinder so that the piston can be forced downward by said combustion pressure. Poor ring seal means poor compression and/or ring blow-by.

Bloody wide port if the entire skirt 'width' is the port width.

To me, it still looks like the smooth (casting) material increase in the beefy aftermarket piston leads to more strength. Your ports in the other heads look more round or even in the rectangular ones, the height/width ratio is no-where near excessive enough for those (relatively) small piston skirt extensions to matter.

Besides, even if the port -was- of a dimension that the shown skirt 'bump' matched the port dimension, what would happen during the rest of the piston travel when the 'straight' part of the skirt wasn't blocking the port anymore?

See attached pic with a really wide port (in pink) and the straight part of the skirt that no longer seals off the port (in orange) once the piston moves.

 

Besides, even if the port -was- of a dimension that the shown skirt 'bump' matched the port dimension, what would happen during the rest of the piston travel when the 'straight' part of the skirt wasn't blocking the port anymore?

See attached pic with a really wide port (in pink) and the straight part of the skirt that no longer seals off the port (in orange) once the piston moves.

You guys and your 4 stroke thinking, the poor guy knows what hes talking about here, give him the benefit of the doubt with his idea here for changing the skirt.

To answer your question, your nice fresh new intake mixture that is stored in the crankcase under the the bottom of the piston gets sucked right out around the edges of the piston skirt into the exhaust. Its the exact same thing as making a tube go from your intake directly to the exhaust bypassing the entire cylinder. Great for horsepower :laugh:


This is a relatively low tech and low load engine (relatively speaking, of course) and I think its very easily possible to make it work with the modified skirt area. I'd consider hand filing the areas myself. If they are lower then the rest of the skirt, so what? It'll be an oil holding area then. Just don't let it be higher then the rest around it.:blob4:

I've put some single cylinder 2 stroke bike motors together with "night before the race cobb jobs" that everyone SWORE wouldn't last as long as it took to start up and ran entire seasons. They are very resilient and you'd be extremely surprised and just how much you CAN get away with as long as you aren't expecting it to do a miracle.

 

In some cases you dont not need to make the whole skirt top to bottom full width, as the ports are trapazoids or else there are small boost ports added beside the main ports. But take the piston on the right, it would be nice to add 2mm a side right above the bump to keep the port sealed and permit a wider port or auxilary ports.

The picture of the piston is just an example, some pistons are nearly full circle and have plenty of room for ports, however there are other motors with narrow skirts maybe only 65% of the bore diamiter, and these are the one that need a little widening.

As far a piston shape, yes many automotive and larger pistons are oval, however not one of these small two stroke pistons is oval. They are all round and just have a slight taper cut in the top 1/2 and inch or so. I can mangage that machiening. And I don't mind doing a little hand lapping to bring things down to the 4th decimal place if need be.

I know this chainsaw racing is a bit hard for most to wrap their head around, guess i'm a bit of a hightech *******. But it's not just a passing whim, this is something I am fairly serious about and have invested some time and coin into.

Here are a couple wider ports from the inside, they are getting out close to the limit structually, but that where they need to be.

I will have to look into stress relieving and heat treatments some more.

 

Put some weld on the outside of that cylinder to build up the thin places and open up the ports even further

 

Been thinking on doing just that also. So far I mostly cut the existing flanges off and bolt on a block, but welding would be quicker. There is concern for warping the bore, but quite often the jugs get bored out and replated anyway so that takes care of things getting a smidge out of round.

With the intake and transfer ports though there is a right size, going bigger is not better. Sure they can flow more, but if velocity goes down too much in the port then low speed opperation suffers. Most races require starting the saw on the ground and then making 3 cuts so the saw needs to go from 0 to say 18,000 RPM in less than half a second and have no flat spots in acceleration.

Couple videos for anyone who is interested, first is a 59cc muffler saw, second is 80cc class piped saw.

[ame="http://www.youtube.com/watch?v=sIbwQK_jr-I"]http://www.youtube.com/watch?v=sIbwQK_jr-I[/ame]

[ame="http://www.youtube.com/watch?v=F-iKroEAU14"]http://www.youtube.com/watch?v=F-iKroEAU14[/ame]

 

Damn I need you to build me a motor for an RC airboat project I got... I want a small motor to stay lite "I got a 21cc" , I know the TQ wont be high but I'd like it to spin the prop a little faster....

I've taking a Weed Eater FeatherLite 21cc and trimmed off all the wacker stuff to end up with a big RC motor....

Just didnt know about the extremes some people went to on these types of motors.

Dude you rock......:waving:

 

Can you triple port the exhaust? You can get some more area and that way your not getting past that 80% threshold where the rings start bowing out of the bore into the port. Transfers are really tricky, right size, right height, right cross section angle so they point at each other, right up and down angles, etc. etc. You know all this already though more then likely.

I didn't pay close enough attention to the cylinder pics, I see now they look to be nikasiled and not the iron bore setups I'm use to dealing with. I'd guess if you were concerned about the bore surface warping and then the plating flaking off you could make up a fixture in your mill to bolt it down to snuggly while you weld it to do the build up work. From the bottom where it goes on the cases and the head area too would probably be the best bet. Make it extra heavy. It would have to help the situation.


What kind of fuels are you guys limited to?

 

Depends on the race, some places or classes limit you to pump gas, other times nitromethane and methanol is ok but no pressurized fuel.

Sometimes it's anything goes.:dizzy::help:

 

HAHAHAHAHAHAHAHAHA

A chainsaw with nitrous on it. Holy **** that is hilarious.

 

x2

Hotrodder:
Thanks for the side by side photo, makes is quite easy to understand.

 

That's some bad chain saws. I've done some crazy mods to 2 stroke bikes but never the small chain saw or weed eater engines

 

Timberwolf, Too freakin' much! You are an inspiration to we 2 popper fans. Man, you are Livin' the dream - I am in awe!
When I was building air cooled motorcycle motors in the 60's, I used to assemble, set up a big furnace fan/blower towards the cylinder, set the bike up on a big work stand and start the thing up. I'd run it gently for a few and then bring the r's up some towards the end of about a 5 minute run. Shut down, pull the head and cyl, piston off the rod (after it was cool). I took 600 grit sandpaper and went around the skirt over any area that showed a flat color - meaning where that piston had a high spot, raised from the heat of running. Since we also had steel cylinders, I did the same on those high spots.
Reassermble and repeat, running a little harder, and higher. To work the engine a little harder, get it up to temp more I would drag the rear brake a little. I can't remember how many times I did this on-off routine on average but it seems like about 4 -6 would hand lap all the expandable areas off. Those rocket motors would really hang in there in desert races for the guys and in my case, Flat Track or Short Track.
Are you still doing that kind of hand lapping today on these little motors?
Also, I remember Puch in Austria were the first to add a transfer port window that blew some intake mix under the piston crown. It's only purpose was to cool the top a bit. I think those were what were called 7 port motors. Anyway, they noted that adding the cooling got them a few more horsepower.
The attached pic (this from early 60's Go Kart motor) on the right shows a view of openings on the cyl side for extra transfer ports. I wonder if you couldn't machine out a small area and weld in a small block of aluminum right up against the cyl with the inside hogged out for an additional transfer port path, or are you alreadybeyond that?
You go guy.
Stew

 

Timberwolf, small add on. You may already know this. I talked to Al Pizzano at WISECO for some info on a rig I am working on. They are pretty approachable. Additionally, in my case I inquired about some STEEL replacement cyl liners, just FYI. He directed me to LA Sleeve.
I don't know if any of these contact names/numbers would be of value but I figured I'd pass the info. These price quotes are as of Sept 2007.
Stew

RE: Wiseco Help‏
Pizzino, Al (APizzino@wiseco.com)

LA Sleeve in California can be reached at 800-822-6005.
We can build pistons as a custom but the minimum is four pistons and they would cost about $225.00 apiece. Regards, Al