Can I Dome A Piston to Increase Compression?

[RedGas]

Hey, all,

I'm getting ready to put together a Farmertec 361 and wondered -- could I use my TIG welder to add some aluminum to the center of the piston top, to make a small domed protrusion in the middle of the piston, to increase compression?

I don't have a lathe to get it all smooth and symmetrical, but I figured I could use a file and sandpaper to get it "good enough." Or maybe I could convince my machinist buddy to chuck it in his lathe and clean it up some.

If this is a workable plan, I guess I'd need to remove an equal mass of aluminum elsewhere on the piston to prevent imbalance/vibration? Where/how would I do this?

Thanks for any clues...

 

[Dub11]

I'm pretty I've seen @drf256 do something like this.

 

[MustangMike]

I know adding a dome is possible, but if your squish is good it is likely not necessary.

It is also very difficult to do this w/o ruining the piston.

Unless you are just looking to experiment, I would not recommend it.

 

[jacob j.]

Several of our builders have done it successfully, but as Mike points out - it is very easy to ruin the piston.

I had a piston done years ago for an 044, before the current trend of machining and lowering the cylinder to increase compression and it worked fine.

 

[RedGas]

Thanks guys. I'm curious how the pistons are getting ruined. The tendency of the weld shrinkage/distortion should be to try to make the piston crown concave, but the added aluminum should more than make up for any concavity. And the web/skirt etc should hold everything lower on the piston in place, I would think...unless the concavity is increasing the distance between the crown edge and the wrist pin bore and/or making those bores no longer co-axial...

Worse comes to worse, I could just buy another FT piston (or go with a Meteor p+c which is probably what I should do from the start)...

 

[blsnelling]

Yes it can be done. However you'll have to do something to control the heat or you'll warp the piston and ruin it.

 

[beaglebriar]

I did one in my 353. As Brad said you'll want to heat sync it to control warpage.

The pin went in kinda hard for me but it worked fine.

 

[Al Smith]

Domed pistons have been around for decades but they were specially made aftermarket or factory installed for like MC or auto engines .Keep in mind a piston is alloy not pure aluminum and thus the weldability would be in question .IMO the dome,either raised or hemi has more to do with flow than compression .The flow portion is a study and subject all within itself and like everything is debatable .

 

[RedGas]

Thanks, guys. I'll let you know if I try it.

...you'll have to do something to control the heat or you'll warp the piston and ruin it.

When in doubt, give it another 50A!

 

[Larry B]

I just built an Hutzl 361. I did a base gasket delete and squish came in at .015 and compression is 194psi. What psi number you looking for? At 194 I had to drill out the hole in the side of the decompression valve to take stress off the starter and my fingers. Might be easier to sand on the bottom of the cylinder to get squish as small as you dare to get more compression. Easier than trying to weld up the dome.

 

[drf256]

I've done it.

You need 4047 filler (I have plenty), but the weight of the piston may counteract the added performance increase from the compression gain.

 

[wcorey]

I haven't had any problems with cast pistons or cylinders distorting, even without using any heat sink.
Where I have seen issues is filling in a dished piston, you get so close to the ring groove that if you're not careful with the heat it can collapse a bit.
Usually easy to file it back to fit unless it ends up near the ring pin, that's a pain...

Balance doesn't seem to be an issue on these, though obviously a heavier piston puts more of a load on things.
A common reference to this is where the same saw chassis came come with multiple bore sizes from the factory with no differences to the crank and seemingly no ill effects.

 

[Chainsaw Jim]

I'm pretty I've seen @drf256 do something like this.

Yeah, but what's being pretty have to do with it?

 

[Dub11]

Yeah, but what's being pretty have to do with it?

Every thing

 

[RedGas]

Thanks, fellas. I appreciate the help.

 

[Lee H]

I can be done with great success but must be done correctly. I've seen some weld tops
by putting the piston in a bowl of water with the dome sticking out to control the heat
at the skirts. I saw an article in an old karting book where a guy was welding one for a
west bend motor.

 

[wcorey]

I think the forged race stuff is much more prone to distortion than the typical cast pistons we mostly deal with.

 

[drf256]

Here’s an 039 piston I did before Hyway offered one for sale.

 

[RedGas]

That looks good, Drf256. How did it perform in the saw? Did you remove any aluminum from elsewhere on the piston to keep the weight the same, and if so, where/how did you remove it?

Also, will 4043 work? I don't have any 4047.

 

[drf256]

IDK on filler. I bought the 4047 because I researched it and it’s the closest alloy to what’s used in pistons. Apparently the highest silicon content of the filler rod alloys.

Ive got a few pounds of it if you need.

No idea on performance. I agreed to do it for nothing for a member on AS at the time. Guy had a build thread going.

The member flaked out on me, posted 1 post with the piston in the saw, and disappeared. I even had to eat the shipping costs he had agreed to pay.