ADVrider

Go Back   ADVrider > Gear > The Garage
User Name
Password
Register Inmates Photos Site Rules Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
Old 04-09-2012, 06:34 PM   #1
svejkovat OP
Studly Adventurer
 
svejkovat's Avatar
 
Joined: Jun 2009
Oddometer: 856
Floating piston wrist pin

I'm replacing the bronze bushing in an old two stroke. Somethings got me curious. I've pressed the bushing in the rod and honed it so that the wrist pin is just right in it. I install it and notice that it floats/rotates as freely in the aluminum piston as it does in the bronze bushing. Seems odd. Is this right? It must be since there are evidently oil access holes under the piston skirt to the wrist pin where you'd expect them to be if this was a bearing surface. You'd think that it would have been engineered so that the wrist pin presses into the aluminum snugly so that it was forced to rotate within the replaceable bronze connecting rod bushing.

What's up?
svejkovat is offline   Reply With Quote
Old 04-09-2012, 06:56 PM   #2
Tosh Togo
Beastly Adventurer
 
Joined: Nov 2010
Oddometer: 1,594
Quote:
Originally Posted by svejkovat View Post
I'm replacing the bronze bushing in an old two stroke. Somethings got me curious. I've pressed the bushing in the rod and honed it so that the wrist pin is just right in it. I install it and notice that it floats/rotates as freely in the aluminum piston as it does in the bronze bushing. Seems odd. Is this right? It must be since there are evidently oil access holes under the piston skirt to the wrist pin where you'd expect them to be if this was a bearing surface. You'd think that it would have been engineered so that the wrist pin presses into the aluminum snugly so that it was forced to rotate within the replaceable bronze connecting rod bushing.

What's up?
What bike/engine is this?.
Tosh Togo is offline   Reply With Quote
Old 04-09-2012, 07:31 PM   #3
Benesesso
Beastly Adventurer
 
Benesesso's Avatar
 
Joined: Jul 2008
Location: West of Phoenix, Arizona
Oddometer: 9,898
Quote:
Originally Posted by svejkovat View Post
I'm replacing the bronze bushing in an old two stroke. Somethings got me curious. I've pressed the bushing in the rod and honed it so that the wrist pin is just right in it. I install it and notice that it floats/rotates as freely in the aluminum piston as it does in the bronze bushing. Seems odd. Is this right? It must be since there are evidently oil access holes under the piston skirt to the wrist pin where you'd expect them to be if this was a bearing surface. You'd think that it would have been engineered so that the wrist pin presses into the aluminum snugly so that it was forced to rotate within the replaceable bronze connecting rod bushing.

What's up?
The pin stays nearly stationary in the piston because of its inertia. The rod has no choice but to swing back and forth and rotate on the pin, but the pin resists rotating back and forth so fast.
__________________
US out of the UN, UN out of the US.
Benesesso is offline   Reply With Quote
Old 04-09-2012, 11:18 PM   #4
mudgepondexpress
Beastly Adventurer
 
Joined: Jan 2007
Location: Spokane, WA
Oddometer: 1,148
You have a full floating pin...not uncommon.

You can float either or both...but full floating has the least friction (but possibly the most wear).

Kenny
mudgepondexpress is offline   Reply With Quote
Old 04-10-2012, 03:02 AM   #5
Beezer
Beastly Adventurer
 
Joined: Oct 2004
Location: Anchorage, formerly Spenard (hub of the universe)
Oddometer: 5,544
Every aircraft piston engine I have worked on (hundreds) has them. Even a light press fit into the aluminum piston becomes a loose fit at operating temps. On aircraft the pin is prevented from hitting the cylinder walls by aluminum buttons... some are pressed in with an interference fit and some are just a plug.
Beezer is offline   Reply With Quote
Old 04-10-2012, 05:00 AM   #6
ragtoplvr
Beastly Adventurer
 
Joined: Oct 2007
Location: S. W. Mssouri
Oddometer: 5,848
that is the way they are supposed to be. Wherever the lowest combination of friction and inertia occurs determines who and how much, in practice the pin moves more on the wrist pin bearing but there is movement in the piston. This combination is lowest in friction loss, and under poor lubrication conditions which ever bearing has the most lubrication moves. Having seen several 2 strokes run without oil, I have never seen the wrist pin bearings fail. Usually the rod or the piston galls up.

Rod
ragtoplvr is offline   Reply With Quote
Old 04-10-2012, 07:08 AM   #7
Wolfgang55
Beastly Adventurer
 
Wolfgang55's Avatar
 
Joined: Dec 2006
Location: Next to Rio Bravo
Oddometer: 3,371
This info is good stuff.

Why did you go to a different material for the bushing?
Wolfgang55 is offline   Reply With Quote
Old 04-10-2012, 12:41 PM   #8
svejkovat OP
Studly Adventurer
 
svejkovat's Avatar
 
Joined: Jun 2009
Oddometer: 856


The innards of a 2 speed Puch 50cc moped engine. Wonderfully simple. Two centrifugal clutches inside the bell and a sprag clutch inside the first gear ring. First gear drives the second gear clutch until it expands, takes over duty on the bell, sprag releases the first gear ring. The thinner bell is driven by the starter cone clutch when you pull the handlebar lever. Wish I'd added a quarter or a coke can or something for perspective. That piston is about the size of a small aspirin bottle.

I didn't go to a different material on the piston wrist. This has bronze sleeve on top and needles on the con rod bottom end. At 2HP the bronze was adequate for 11,000 miles and takes, literally, 20 minutes to replace. All the work was in lathing one to fit. 12mm ID 14mm OD is not stock anywhere I could find.

Ordered replacement bearings for it via this site....http://www.locateballbearings.com/Search
...which (so far) I highly recommend. If you have part number, they've got your bearing and at good prices. Phone help was quick and good. They even stocked the STEYR (might as well keep it OEM) originals. The STEYR's were actually less than the no-name chinese bearings stocked by the moped vendors.

Here's to another 11,000 miles of fun! I ride this thing as much as any of my other bikes during the summer. I'm blessed with almost no need to ever look "cool". But ironically, I get more admiring questions and thumbs up when beating around on this thing than anything else.

svejkovat screwed with this post 04-10-2012 at 12:49 PM
svejkovat is offline   Reply With Quote
Old 04-12-2012, 12:41 PM   #9
ericrat
Beastly Adventurer
 
Joined: May 2007
Location: Cincinnati, OH
Oddometer: 1,150
Back in yesteryear

Nice to see that old piston with a proper, long skirt for modesty. Those new fangled pistons, why the only thing modest about them is the amount of material in the skirts. They go around flashing their pin bosses to anybody who cares to glance.
ericrat is offline   Reply With Quote
Reply

Share

Thread Tools Search this Thread
Search this Thread:

.
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump


Times are GMT -7.   It's 05:21 PM.


Powered by vBulletin® Version 3.8.5
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright ADVrider 2011-2014