Art Vs Modern Wiseco Piston

[Al Harpster]

I tried to attach a photo to this post.

Piston on the left is Art, Wiseco reproduction piston on the right side.

The space in the piston casting for the connecting rod is about 1/4" wider on the Wiseco than on the Art.

This lets the top end bearing move left or right where ever it wants.


The Art piston has less space, keeping the top end bearing centered at all times.


I had the top end of my Ace re-assembled with the wiseco piston when I noticed this. I saw it through the intake port & through the piston windows.

I really wanted to use this piston because it's +.020 and matches my cylinder.

The Art piston is +.010 and it's not going to work for me.

[ossa95d]

Any machinist could make you a couple of aluminum spacers to center the bearing in the piston. Then you could use the piston you bored your cylinder to match. I know I have come across pistons with spacers in my lifetime but can't remember when or on what engine.

[Al Harpster]

I forgot to ask..... has anyone else noticed this in the Strictly Hodaka modern wiseco pistons?

I expect there are quite a few of these modern wiseco's out there.

Perhaps there's no concern about a "floating" top end bearing?

[admin]

Yes this is Strictly Hodaka and we are aware of this. When these were first made a few years back we presented Harry Taylor with this issue and he did not feel this was a problem at all. Could the Wiseco be tighter yes but not without changing the toolings for a huge amount. We have not ever had one single reported failure with the hundred's ( 1000?) 100cc pistons we have made. "floating" is the key word.
If you would like two spacers please email me and I will send them to you.
Paul

[hodakamax]

I had all three on the bench and measured them. I've never had a problem.

Max

[hodakamax]

Since there are no documented failures caused by this excessive clearance, I've noticed but never worried or had any concern about this issue. Curious how the bearing self-centers in this situation but obviously it does. Something to think about in a scientific way sez Dr. Science. These problems or concerns are fun to think about. A good friend of mine is a mechanical engineer and maybe he can give us some insight on what's happening here. I'll be talking to him soon. Beyond Max.
Another great mystery for sure.

Max

[viclioce]

Well, when you think about it, the rod is not flexible, and, the piston always centers itself in the cylinder. So even though it may look concerning, there really isn't any way for the piston or the rod to "move" without there being som other reason than the space between the top of the rod and the wrist pin holes. Maybe minor (OK minute) tolerances, but not enough to be of a significant concern. ; D Victor

[Al Harpster]

Thanks for all the information. I was not sure which way to go with this.

I'm going to put the top end back together and go with it.

Best Regards to all.

Al

[hodakamax]

A little more measuring here. First of all the scenario shown on the right will not happen. As Victor said when assembled the upper rod boss will be centered in the piston. If the new Wiseco piston has a 21.5mm opening and the upper rod boss is 15.0mm wide the most space you can have is 3.25mm not 6.5mm as shown in the picture. Most of the upper rod bearing will be supporting the forces no matter where it floats. Due to the dynamics of the spinning assembly the bearing probably prefers the center (or maybe it does float back and forth.) The bottom line is that it's not as scary as it looks.

Max

[ossa95d]

Max and Vic,
I am a mechanical engineer (in a long ago world) and I can think of no reason why the bearing would center itself, but I am the first to admit that I may be wrong. Vic is right that the piston would stay centered in the cylinder, and if the rod isn't bent, it too would stay centered in the piston. This would mean that the gap increase from the stock piston to the new Wiseco would be a constant 2.25mm on each side. Assuming the bearing does not stay centered, it would still leave the needles in the bearing within the rod race area. While intuitively I don't think it is ideal to allow the needle bearing to float, the lack of any evidence of failure in practical application would lead me to believe that it is OK. Besides, who am I to question Harry Taylor?

[ossa95d]

Ha ha Max it looks like we were both answering at the same time!

[hodakamax]

Great minds think alike! Just kidding, Harry did have quite the edge. Speaking for myself of course!

Maxie

[viclioce]

After thinking about it some more, I think two things should be considered here. One is, even though the rod and piston won't move, I could consider that the bearing might. This wouldn't cause wear to the wrist pin, but, it could cause wear to the top of the rod. And as the top of the rod wears, it could then cause wear to the bottom of the rod, which would lead to a crank rebuild.

So if I had one of these Pistons, I would get the spacers from Paul. It may not happen ever, or it may just not happen right away. So if the spacers are available and he's offering them for free, this would pretty much indicate to me that there is a need to fill that space. Better safe than sorry! ; D Victor

[dirty_rat]

I believe the old CZ's used to use spacers on the sides of the wrist pin. Not positive as it's been a while since I rebuilt one, but that's what I remember. I do remember it was a little more difficult to get it all together (had to reach up under the piston to put the last one in.)

[Darrell]

Is this bearing available in a wider dimension? If so, then the side gap(s) could be minimized -- and the extra surface area of the exposed rollers/needles might pick up more lubrication as a side benefit.

Just thinking out loud...

[hodakamax]

Darrell, nothing wrong with thinking out loud and that might be a real possibility but I really don't see a problem at all. There's no place the bearing can go without supporting the full load. Spacers only add more complications and more rotating mass. The first Hodakas had a bronze bushing that worked well but was difficult to replace. I think all is well. And thanks for your input! (Thinking out loud is how problems are solved.)

Max