What causes a piston head to mushroom when you reset the piston rod?

What does the cup of your piston holder look like? Is it perfectly flat or does it have a raised portion in the center? Proper ones have a little raised portion that is supposed to support the piston right under the ball and socket joint. I never really used the piston holder, I have a little anvil that has a nice flat surface on it that I use when resetting pistons. I rotate the tool while tapping with my little hammer, NO HARD BLOWS. It takes about 5 or so minutes.

Also I'm sure you know this but its good to bring up, WATCH FOR 3 PIECE PISTONS, you'll destroy it if you try to reset it with a reset tool. You may even ruin your tool.

Use a piece of 3/8" K&S tubing about 4-6" long. Wrap some tape around the top of the tube about 1-2 wraps. With the taped end up, drop the rod down the center of the tubing. The tape will create a snug fit where the skirt goes onto the tubing. Push it until it seats firmly. Insert the piston upside down in the cylinder from the bottom up. Don't pull the piston up to TDC in the cylinder. Pull the piston skirt up until it closes off the exhaust ports. I take a Q-tip and soak the swab end in Brasso. Place the soaked Q-tip onto the piston skirt through the exhaust. begin twirling the piston using the brass tube working it up and down. Spend more time at the bottom of the cylinder. Don't lap near TDC even with the skirt. You may feel a bit of resistance during this process. The Brasso starts to solidify and thus may require an additional application. Brasso is a very mild abrasive. Even Mother's aluminum polish can be aggressive so go lightly. I try not to get any of the abrasive in the ball socket area. This can't be removed and it will act as a lapping compound if it does get in there so use caution. I keep a small baby food jar of lacquer thinner on hand with a small wire hook which I hook the rod into . This usually takes off any residue followed by a soft cloth. At this stage, I use a .049 bore brush and proceed to scuff up the cylinder walls . I clean thoroughly with hot soapy water. Reassemble and check for any binds. Sometimes I will do a final lap with the piston assembled in the cylinder in it's proper orientation. If material is removed from the skirt of the piston and or the bottom of the cylinder, it usually assists in rpm's. Ken

I had a reset piston mushroom and drag exactly as you describe. I decided to mix up some high castor content fuel and "run it in". It turned into one of my fastest piston cylinder sets and has over three hours tun time on it since reset.
Just my personal experience.

Jim

Thanks for all the advice. Yes, I did use the reset tool as you described with a raised piece in the center bottom of the tool piece for the head.  sorry no pics today.
   and yes, Ron, I did check the piston in several different cylinders. at least the 3 cylinders that were with the pistons i reset and 2 or 3 more cylinders and the piston binds about halfway up the cylinder.
   I thought about just swapping pistons and moving on, but i agree with the last 2 posts, (sorry I forgot the names), and I MAY try to lap the piston and cylinder to each other. That's what my dad always did in the old days, (1950's).

again thanks for the advice. I will keep you posted.

happydad

   

JPvelo,

Yes, I had one mushroom piston. I use toothpaste on the piston with its cylinder and lap it carefully, and clean it in ultrasonic cleaner before re-assemble. after careful run-in , I noticed that this is my most powerful piston/cylinder I had ever owned.

HD,

To answer your question: What you've INADVERTENTLY done is peened the top of the piston. Imagine if you will turning the piston upright, holding it somehow & hammering on the top of the piston. As expected the top of the piston crown expands out (by 50 millionths of an inch give or take). That's why you aren't able to measure it, although you can sure feel it! Imagine that. Your hands can feel half a ten-thousandth of precision! Who needs tools when your hands can tell the tale!

SOoo..why was your piston crown expanded? It may be a combination of things, but what I feel is happening is that re-set tool force is being TRANSFERRED all the way across the piston. You DON'T want this. You ONLY want the hitting force directed to the socket area ( ~1/8" dia.) of the piston. A straight milled piston holding fixture will cause mushrooming. The fixture ( if used) needs to have the center portion RAISED.

What I've personally found though (after ruining a few pistons) is to dispense with the holding fixture entirely & use a thick heavy FLAT surface like the flat on my steel vice or a 1/2" or thicker glass plate. Both work well for me. DO NOT reset a piston on a less than solid bench. I use my hobby vice on the concrete floor & have not had a problem since. Too heavy a hit ( I use a 16 oz hammer), just causes the ball-socket to be too tight (requiring re-breakin), but doesn't necessarily ruin the piston. As said by others, lapping can restore an expanded piston crown if necessary.

BTW, now that you can "feel" micro precision with your hands, you may find that finger prints alone can be felt to cause a perceptable difference to the cylinder to piston fit if everything is perfectly cleaned & dry... ( SO if you're ever into swapping pistons, you may want to keep that in mind)

Cheers,

Paul (aka Mr. Mouse)

My question is, how do I do it on purpose?

The folks that have had the problem had good runners once lapped. Seems to me that it would breath life into a tired PC.

Paulgibeault wrote:HD,

To answer your question: What you've INADVERTENTLY done is peened the top of the piston. Imagine if you will turning the piston upright, holding it somehow & hammering on the top of the piston. As expected the top of the piston crown expands out (by 50 millionths of an inch give or take). That's why you aren't able to measure it, although you can sure feel it! Imagine that. Your hands can feel half a ten-thousandth of precision! Who needs tools when your hands can tell the tale!

SOoo..why was your piston crown expanded?  It may be a combination of things, but what I feel is happening is that re-set tool force is being TRANSFERRED all the way across the piston. You DON'T  want this. You ONLY  want the hitting force directed to the socket area ( ~1/8" dia.) of the piston. A straight milled piston holding fixture will cause mushrooming. The fixture ( if used) needs to have the center portion RAISED.

What I've personally found though (after ruining a few pistons) is to dispense with the holding fixture entirely & use a thick heavy FLAT surface like the flat on my steel vice or a 1/2" or thicker glass plate. Both work well for me. DO NOT reset a piston on a less than solid bench. I use my hobby vice on the concrete floor & have not had a problem since. Too heavy a hit ( I use a 16 oz hammer), just causes the ball-socket to be too tight (requiring re-breakin), but doesn't necessarily ruin the piston. As said by others, lapping can restore an expanded piston crown if necessary.

BTW, now that you can "feel" micro precision with your hands, you may find that finger prints alone can be felt to cause a perceptable difference to the cylinder to piston fit if everything is perfectly cleaned & dry... ( SO if you're ever into swapping pistons, you may want to keep that in mind)

Cheers,

Paul  (aka Mr. Mouse)

Paul: thanks for the input. I did use the reset tool with the raised portion in the center of the tool as you mention. and i used a small ball peen hammer. maybe 4 to 6 ounces.  I'll try to post a picture latter today.

edited 6-30-2014 at 8:35p.m. PST to add a picture

here is a picture of the head part of the reset tool


notice the raised part inside the center of the tool. Bye the way the ball peen hammer was 10ounce.

happydad

p.s. what happened to the emoticons?? they wern't here at 1p.m. PST when I first answered this post, but are available now at 8:40p.m. PST

   

Cribbs74 wrote:My question is, how do I do it on purpose?

The folks that have had the problem had good runners once lapped. Seems to me that it would breath life into a tired PC.

Really!

I wonder if using a hard steel pipe (brake line?) inside the skirt around the socket will cause the desired spreading. Might allow saving of clapped out .051 engines.

Phil

Phil,

I was thinking maybe leave the piston in the cylinder and setting it on some 3/8" sheet steel that had a hole drilled for the con rod. Them find a suitably sized drift to whack the piston crown ever so gently.

Might be a trial and error thing, but I bet it could be done. I have plenty of fragged cylinders to try it out on... 

Heck, it was standard procedure on McCoy's back in the day.

Ron

I had a cylinder wrench slip on exhaust port,my first thoughts was a small brake cylinder type hone to clean it back up,maybe that's not a good idea after reading thread,any thoughts-Ken what is K&S tubing.rat

rat9000 wrote:I had a cylinder wrench slip on exhaust port,my first thoughts was a small brake cylinder type hone to clean it back up,maybe that's not a good idea after reading thread,any thoughts-Ken what is K&S tubing.rat

If it is a burr, a legitimate burr, and the piston will not pass, then using a hone or very fine sandpaper ON THE BURR AND ONLY ON THE BURR is an option. Be careful as you do not want to open the exhaust up and change the engine timing. When the piston passes, switch to compound.

If the piston will pass then lapping with some type of compound including toothpaste (not gel) or rubbing compound (for paint) will get the job done better.

K&S is a brand of tubing that most hobby shops and craft stores sell.

Phil

Cribbs74 :

I tried, quite hard to mushroom it by purpose. I don't have old piston to try out right now. I'll look for one to "Cherry bomb" it, by heat it up and drop into oil to cool it. Just don't know if the copper coating on the piston head will withstand the heat.

I's worth a shot, let me know how it turns out.

I'd read about expanding the piston to recover compression.  This method is generally only used on domed-top pistons.  I tried it once on a Gilbert 7.  Placed the piston upright on a suitably sized steel pipe fitting in the vise with the rod hanging through the hole in the centre.  Then slid the cylinder over the piston.  Wrapped tape around a pin punch to protect the bore and gave (I thought) a light tap.  Well the Gilbert pistons are obviously very soft.  I no longer had a domed piston but a flat top which was now firmly jammed in the cylinder.  I heap of lapping got it moving smoothly again, but because the rod is attached to the centre of the crown via the socket joint, the outer edge of the pistin now comes up higher and hits the head.  The lapping was obviously far too agressive as the piston fit is now terrible.

I had tried this type of method before though.  (Should have learned my lesson   ).  Before I knew about any of this stuff, years ago I had a Taipan diesel 1.5cc with a badly worn piston.  I came up with a theory about expanding the crown of the flat-top piston.  I removed the rod, placed the piston inverted on a hard steel surface and placed a large ball bearing inside the skirt resting on the inside of the crown.  I was too heavy handed in that instance too.  It certainly worked but worked too well.  Took a lot of work to lap in the piston but the final result was poor.

I have a couple of rebuild kits with instructions about resizing the pistons for a couple of McCoy Red Heads I have.  I'm now not game to try.

Rod.

Oldenginerod,

Thanks for your info. Cribbs74, hours ago, I heated up one of my old piston to cherry red and drop it into oil several times, but no luck. The piston remains the same old size. I'm thinking of building a special tool to swell the piston , but no idea yet.