Welded pop-up

Anybody running them? Anybody had any failures? Seems to me its a far better alternative to cutting the jug down, as you can replace it with a standard piston if need be.

Here is the one for a 55 Husky that a friend of mine welded for me, and I machined. The pop up is still .080 tall, so im sure its gonna have to come down quite a bit.

Sorry for the crappy photography.

I've run a few with no problems. The best one I had was in an 056 Magnum II.

The only real issue I've run into them is distorting the shape of the piston by getting it too hot, and then having to re-size it by hand sanding.

I've wondered if the trapped air -- since it would only be tigged around the edges, would adversely affect things.

Do you drill through the top of the piston?

i wondered about doing this but figured there was a reason i'd not heard about it. very interested to see how it turns out.

Jacob J. said:

The only real issue I've run into them is distorting the shape of the piston by getting it too hot, and then having to re-size it by hand sanding.

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I can see where that would throw a damper in things.

The slug seemed to fit the jug just as well as it did before. I will have to do a bit more measuring, I suppose. Hopefully I can get this saw put back together in a week or two and give her a whirl.

I cant help but have visions of a shiney little aluminum disc being hurled thru the exhaust port

The guy that welded it for me, assured me it wasnt going anywhere. He has been tig welding for years. I was still kinda leery.

Winn R said:

I've wondered if the trapped air -- since it would only be tigged around the edges, would adversely affect things.

Do you drill through the top of the piston?

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No drilling. He started from the center, and worked his way around. I guess time will tell.

There have been several threads on welded pop-ups.

You gotta have Tig, otherwise I'd be doing them.

Not sure that I understand the questions on "trapped air" and drilling into the piston ? There shouldn't be any trapped air, and no need to drill the piston.

Wiggs, why is your pop-up diameter so small ? Is the chamber squish band really that wide ?

mtngun said:

Wiggs, why is your pop-up diameter so small ? Is the chamber squish band really that wide ?

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Yep. The combustion chamber is very small, and the squish band is huge. Ill see if I can get a pic. Crappy camera phone might have other plans though.

here is the jug. Dont mind the bearing dents, as that happened in a previous life. That is one reason I didnt mind experimenting with this Jug and slug, as it has had a pretty hard life before I got it.

wigglesworth said:

here is the jug. Dont mind the bearing dents, as that happened in a previous life. That is one reason I didnt mind experimenting with this Jug and slug, as it has had a pretty hard life before I got it.

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Is this a Sachs-Dolmar?

Jacob J. said:

Is this a Sachs-Dolmar?

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no. 55 husky, open port. You sound worried.

I nearly tried this 028 super piston I have, as dimesnionly, it is very close, and could be made to work, but the skirt width is what stopped me, and I wasnt sure about the windowed piston.

Should be able to do it with MIG and a spool gun, but never tried that though. I don't have a spool gun and any push feeding of Aluminium wire I tried ended in a rats nest in the feeder.

So far I have not had any troubles with welded crowns, not the easiest thing to weld and keep from warping, good to practice on a bucket of scrap pistons before going at $100 oem pistons.

There are down sides to getting compression by welding a pop up. One is added weight to the piston, the other is that when chamber volume gets smaller without reducing squish volume the end gas ratio gets worse and efficiency goes down the crapper, polution from unburnt hydrocarbons also likely rises if it matters to you.

Can't imagine it would be good to weld a slug on, better to build up the crown and keep it monolythic hopefully free of any inclusions or stress risers.

timberwolf said:

Should be able to do it with MIG and a spool gun, but never tried that though. I don't have a spool gun and any push feeding of Aluminium wire I tried ended in a rats nest in the feeder.

So far I have not had any troubles with welded crowns, not the easiest thing to weld and keep from warping, good to practice on a bucket of scrap pistons before going at $100 oem pistons.

There are down sides to getting compression by welding a pop up. One is added weight to the piston, the other is that when chamber volume gets smaller without reducing squish volume the end gas ratio gets worse and efficiency goes down the crapper, polution from unburnt hydrocarbons also likely rises if it matters to you.

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I have been the rats nest route, ended in a not so pretty mess. I firmly believe that aluminum wire is not meant to be fed thru a mig welder. I have never had any luck with it.

This piston has some damage already, so it was a good test bed. I do plan on checking the runout before I put it together, just to see if there was any warping. The guy who welded it for me is a professional (aint we all?). I plan on setting the squish in the .020 neighborhood. Would you suggest tighter?

Hard to go much tighter than .020 without running into problems, you might get away a couple thou tighter but that won't make much difference. That is really where having a cut off head with more control over squish dimentions comes in.

If the piston warps a couple thou likely no big deal, look how much piston skirts wear. Long as it does not feel sticky in the bore cold. Also good to check the top ring land gives free movment to the ring. Get the crown too hot during welding and it is easy to mess the top ring land up.

Dumb question

When you set squish with these do you go from the top of the pop up or do you set it from the rest of the piston top?

Squish is measured out at the edge (edit where it should be the closest), the pup up is all under the combuston chamber.

I just got a very expensive aluminum pulse mig welder at our shop for doing production work. It's considerable faster then using a tig and makes welds that could pass for tig welds.

It has a push-pull 20' whip that allows you to run a big spool in the welder. Once we got the tip setup correct we haven't had anymore problems with rats nest. The really cool feature is the arc control that allows you to set the distance of the arc but get to closer to the work then the length and you'll weld a tip.

I can't decide if I use the pulse mig or the tig for my first investigation into pop ups. Anyone ever made one for a 272xp?

To minimize distortion remember to preheat the whole piston. It's not the heat that distorts the metal it is un-even heat distribution that causes areas to shrink when cooling. Then there is the question of heat treatment and annealing or making it too brittle. When I was younger welding cylinder heads we would always let them cool with a fan on them. Water quenching was way too fast (brittle often cracking) and free air cooling was too slow (annealing).

You can always weight the piston before and grind some material from the bottom after you finished the pop up to get the piston back to the original weight. But single cylinder engines are always under balanced because the fore/aft accelerations caused by the counterweights would be worse then the vertical by the piston. I don't think a little extra weight is going to make a big difference in the vibrations at your hands. But if you have a race saw cranking 1700 RPM it could cause the forces on the piston pin to get out of hand.

With a sweet MIG set up like that I would think you could build up very well with minimal heat into the piston. Just as long as you get good fusion to the base metal.

Heat treatment from what I looked into it is not going to be practical, with these alloys the temperatures involved would be right up into the plastic range and the piston would never hold dimention.

Your bang on the with the pre heat, getting the piston good and hot keeps the weld and base metal close to the same temp limiting warpage and cracking as they cool together.

I don't know if you really need to weld a piston up for a 272 easy to pop them up over 200 psi without welding.

Balance in a two stroke single is really not that critical, without a counter shaft they can only be partially balanced anyway. An extra gram or two will make no difference. Point in fact is aftermarket piston pins that are not tapered and weigh an extra couple grams. I would keep the extra metal under the crown as the aluminium in the HAZ will not be as strong as the original aluminium was so a little thinker section will help with running higher cylinder pressures that come hand in hand with compression.

timberwolf said:

I don't know if you really need to weld a piston up for a 272 easy to pop them up over 200 psi without welding.

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That's it! I'm going to buy a new gauge. I removed the base gasket out of all of mine and I haven't seen over 165 psi. I add more exhaust duration in them so I'm sure I'm taking some out that way, but still I would like to see 190ish.

Have you cut a pop up onto the existing crown leaving only about .100 above the top ring and turned the base down to lower the whole jug 30 or 40 thou?