Jump to content
Sign in to follow this  

question on piston design

Recommended Posts

just curious if anyone has seen or knows theory behind a piston with...

"additional piston top land clearance"

any idea as to what reason was behind say 1mm piston to wall clearance on top land area (from top ring to dome has 1mm clearance)

i bought an old wiseco forged that seller claims wasn't modified and actually a design incorporated feature

Share this post


Link to post
Share on other sites

The top land clearance used to be referred to as "carbon clearance". As yoiu look at a piston, you'll notice the skirt (which, BTW, is not really cylindrical) is a larger diameter than the ring lands are. This is intended to eliminate the possibility of the ring lands wearing against the bore walls, which could lead to the outer edges of the ring grooves becoming deformed, which would interfere with the ring's ability to move freely to create and maintain a good seal. The rings also actually seal more effectively as a system when there is a small volume of space between them to serve as a gas trap.

But the top ring land is a special situation, because of the direct exposure to combustion gases and by products. There are two main considerations at work here. One is that a wider top land will protect the top ring to a greater degree from becoming fouled by carbon deposits. In the real world, combustion rarely will continue all the way to the corners of the chamber, so what the top land is usually exposed to is hot gas and partially burned goo, which will eventually become carbon as they are repeatedly partially reburned, heated/cooled, so on.

The second consideration then, is that there needs to be enough room for this deposit to form without dragging on the bore, and tilting and/or restricting the natural motion of the piston.

These two considerations are at odds with each other to an extent, since more top land clearance means less protection for the top ring. If a piston manufacturer determines it needs more top land clearance, it was likely that it has observed problems with carbon impacting in the crowns of the previous release.

Share this post


Link to post
Share on other sites

kudos there grayracer,

i'd think this also effects port timing too as crown/cylinder wall intersection point is changed to the top ring/cylinder wall as intersection point of port openings.

i should of included that this was a piston for a 2 stroke.

bites this guy won't return this piston...blah-blah-blah aye

thanks though sincerely for the reply and input

Share this post


Link to post
Share on other sites

i'd think this also effects port timing too as crown/cylinder wall intersection point is changed to the top ring/cylinder wall as intersection point of port openings.

Because of the speeds and time intervals involved, I really doubt that the ring is the effective point that opens or closes the port. I believe that apart from an extremely brief bit of leakage, the piston crown is what really does that job.

Share this post


Link to post
Share on other sites

"I really doubt that the ring is the effective point that opens or closes the port"

for whatever reason "wiseco or someone" modded this piston the ring now controls it

i'm not even sure what 1mm side clearance does to squish figuires or flows

it would be if the ring is the last diameter against the wall...

usually there's like 8mm of top piston land...not with this one...

it has been like turned down on a lathe so it dropped that intersection point to rest on the top surface of the top ring...

leaving the top of the top ring exposed to 1 mm of burn surface

all i know is this one's not going in my engine as i don't know enough about all the effects of this geometry.

i'd think this would be a carbon trap or build up point probably going to cause top ring failure

yuck...more $$$ scratch lost on this rebuild of course

Share this post


Link to post
Share on other sites

Is yours a Dykes-ring piston, like this one?

CIMG2814.jpg

If so, it will probably work quite well, all else being equal... you might want to check for the availability of replacement rings, though.

Ray

Share this post


Link to post
Share on other sites

EVERY piston that I have ever purchased from wiseco or was a wiseco has about .5mm extra clearance at the top of the piston. Above the rings.

It confused me at first because I couldn't figure out HOW to measure a previously installed piston to determine replacement size. The number kept coming up short. I almost bought too small a piston.

So, I consider it to be faily normal. Also, as GR reponded earlier, the gap is important for how the piston fairs the the cylinder with added deposits that form after a lot of use. It is to keep the life of the piston and not start smacking one corner or the other when deposits are deposited after lots of hours. -BIG DAN:thumbsup:

Share this post


Link to post
Share on other sites

daled,

what are you working on? I haven't seen a dykes ring since the old Bultaco !

That's just a random picture I pulled off the Web - I think it may be a kart engine of some kind.

Come to think of it, Dykes rings are pretty rare. I had a Wiseco Dykes piston for the MR a while back and sold it, that's the only one I've seen for many years.

Ray

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...