Teflon Buttons on Pistons?

[HawgRyder]

We used to put Teflon buttons on the skirts of pistons for drag racing purposes.
I have also used them on long haul (high mileage) engines as a test.
Has anyone any info on Porsche engines with them?
They stabilize the skirts to reduce or eliminate "rocking" of the piston on the pin, and so, reduce the "barreling" of rings.
The other advantage is be able to use a standard piston with over-size rings in a bored out cylinder (1st over).
Just a thought...it's late at night...and my brain won't slow down....(joke).
Bob

[Flieger]

Why not use dry film lubricant and thermal barrier coatings?

[911pcars]

What's the service life of the teflon buttons after a few quarter miles?

Drag engines operate under different conditions than a street engine.

I'd pass on these unless you need a quick fix for worn pistons and cylinders.

Sherwood

[HawgRyder]

Good points all...although I have them in a Harley engine that is about 10 yrs old...and they seem to still be doing the job.
The longest running engine I have done has about 400K miles on it...big block chev....632 cu in.
The heads get changed (2 sets) about every 150K and from the look of the bore, the buttons are still doing their job.
The benefit that I see is that the rings do not "barrel" and provide a better seal with a sharp leading edge.
The type of teflon is important, I used Red, Hard type.
It came from the aircraft industry as a 3/8" diameter stick.
I do not know the composition or numbers on it.
I started with 9 feet and have about 18" left now.
Bob

[lindy 911]

After thinking about your post for a while I don't think there would be any advantage to buttons in a Porsche motor. The piston must fit the bore properly in a Porsche because of expansion rates of the two components. In a drag motor, using and over bored cylinder with +1 rings and stock bore pistons, the idea is to make more room for the piston as it grows from the extreme heat caused by blowers and such on short runs ( .25 mile). Here the buttons can control the walk of the piston somewhat when it is at it's smallest diameter or cool. The piston in this application expands at a much faster rate the the iron block so there needs to be more room.

A Porsche motor doesn't operate under those conditions. The fit of the piston in the boar won't allow much rocking in a Porsche motor like you see in a drag motor.

Lindy

[Eagledriver]

Your "buttons" would have to be .004 inches thick on a normal set of 911 pistons. Plenty of used sets out there with this clearance or less.

-Andy

[HawgRyder]

I know that if I use std pistons it would be tight, but in that case I would use 1st over (.010") piston rings on a .010" over-bore and then use .007" (approx) thick buttons to compensate.
The idea is to jam the buttons into the bore so that the expansion would relieve the tension slightly and still keep the piston centered with the rings being held at 90deg to the wall.
Teflon comes in various degrees of hardness and "bounce-back" and the red stuff I have is fairly hard but does have some "spongyness" to it.
A big plus on this is the reduction of friction.
Oil and teflon make a very slippery pair.
If you want a really difficult engine to try this on, do it to a 2 stroke with ports in the walls.
You have to line up the buttons with the wall dividers so that the buttons don't fall into the transfer or exhaust holes....LOL.
Bob

[Flieger]

I thought in a normal engine, the piston does not actually touch the cyliner bore, just the rings. If so, then why add something to the piston to touch on the cylinder wall, even if it is slippery it seems nothing touching is more slippery than anything touching.

[HawgRyder]

The skirt of the piston touches the cylinder wall!
If you look at a piston that has been in service for any time, you will see the vertical lines and scrapes on the skirt at 90 deg to the pin.
The clearance of the piston to wall gives the piston room to "rock" on the wrist pin.
This rocking motion is what "barrels" the outside edge of the rings.
When the rings get worn...so that they no longer have a sharp edge on the top and bottom, they begin to allow either compression to "blow by" or oil to be forced up into the combustion chamber.
Keeping the rings sharp at the edges is what this is all about.
Bob

[AlfonsoR]

I think that in drag racing, with extreme piston pressures/temperatures you install these to preserve the sealing efficiency of the rings. A few HP loss to friction of oil on teflon is worth keeping the piston and rings as parallel as possible to the cylinder walls.

I am not sure what HP these things are running now-a-days, 2500??

But even in a more pedestrian application, it might be worth doing. By design, a piston in a recip engine will have some degree of rock, even if the piston skirt does not touch the wall.

[Flieger]

Well, if the rings were equidistant from the piston ring and there were no offset, in theory the forces/moments would cancel and all the rocking force would be taken by the rings and the piston would not rock.

[911pcars]

Are proponents of teflon buttons implying this is a worthwhile service procedure for worn pistons?

If in-spec pistons have the correct skirt clearance (e.g. 0.015"):

Would buttons be installed flush with the skirt to maintain this clearance?

Would "worn" pistons be reused with buttons installed to maintain an in-spec skirt clearance?

Do I have to buy a blown fuel dragster to use piston buttons?

Sherwood

[HawgRyder]

Sherwood: The quick answer is....you would be able to use the original pistons with an over-bored cylinder (and rings) .
We used to over-bore by .005" (instead of .010") and go with the buttons.
The advantage is obvious...no new pistons...and less money spent at the machinist.
The only con part that I can think of is the amount of ring exposed is more..and the "twist" factor may increase if the ring lands are sloppy.
BTW....piston "clearance" in this case is not an issue....the buttons are actually forced into the bore and exert pressure on the wall at all times...the skirt of the piston never touches the wall.
This whole system needs to have a smoother surface to the wall so that the buttons are not worn away as quickly as they would be if the normal cross-hatch is present.
The rings can then be selected for "fast seating" properties to match the wall.
Bob