Preservation of a Piston....

[DKWRACER]

Equally divided in their parts of our beloved "soldiers from yester-year", I wish to offer to the GODS of the 2-stroke motor:

The bore is straight, the piston is NOT
You might actually notice this....

I have taken carefull measurements of various pistons (new)
What is discovered and well known is that pistons are not symetrical, they are tapered....Why?

The mass on top expands at a greater rates.....
I measured consistent .3mm difference from top to bottom regardless of bore!! that equates to about .001 in.

Is the piston the "sacrificial lamb"?
I see no reason why modern age coatings cannot come to our aid.

SwainTech and Boretech might be a combination to deal with the problem.

Gone for the W/E.... BFN

[john durrill]

DKW,
 No God on this end but i can pass along what i have learned over the years on piston design.
The differences you see are due to the loads , both heat and thrust, that the piston is subjected to . The measurements we work with are while the metal is cold.
 Pistons are both tapered and egg shaped. More metal at the top and down to the wrist pin boss for strength under load. Lighter toward the bottom to reduce mass and improve performace at higher RPM. At the top the rings keep the piston centered , so the area with the ring groove's should never touch the cylinder.
 The load put on the piston while its rotating is from front to back.
 At the bottom ,90 deg from the wrist pin holes , the piston is taperd out and larger in diameter than the rest of the piston. It expands less . It has less mass. It stablizes the piston in the bore. Thats why and where we check the pistons for clearance.
 When up to operating temp the piston looks round to the cylinder.
We have had some success with (knerling sp is wrong} Pistons.
 It is a process where the piston is fitted to a lathe in a jig and  the front an back surfaces below the ring lands are knerled to raises metal and closes up the clearance. Its a plus in another way also. The knerls hold more oil. If every thing is working right the piston rides on a thin film of oil and does not contact the cylinder so the fact that the raise metal is very small in area does not seem to matter much.
 Knerling was a process used many years ago in car engines. Perfect Circle had the jigs for knerling. They were at that time a big manufactuer of rings.
 Its hard to find a machine shop anymore that will do it though.
I had to refit the piston after getting them back but they held up well.
 Its when things are not exactly right that the skirt wears. On a piston port 2 stroke its also due to dirt coming in the intake.
The area below the intake port is where i see the most wear on cylinders.
We have found when refiting a 2 stroke piston  , the ring lands ,ring retaining pins and the wrist pin bore should be looked at for excessive wear. If they are ok then the piston is good to go. The cylinder should not be tapered too much also. The rings wear on the top section and the dirt and skirt of the piston work the bottom part over. Its hard to check the taper with out a good bore gauge but the long feeler gauge strips that Snap On and the MAC tool guys carry on their trucks is a fair way to gauge taper.
Was this any help?
John D.

[DKWRACER]

Ah, well said....Also interesting is Swaintech's technical site for questions and answers. Sorry about the knerling typo...I have used this process to expand older pistons and it does work, we have several shops in the Denver area who still do this process, last one was $8.00, not bad.....Thanks for the followup......

[dirtbike]

The process is to turn or grind the piston heated to it's working temp. When it cools down it will be tapered and oval but hopefully return to a pretty round shape when reaching it's temp again!