Pinning Piston Rings

[Ricks]

I know Bob Kerr used to do this on some of his pistons.  Has anyone else tried this.  I would like to do it and if anyone has, lets hear how you did it, and would you do it again.  KartJockey, Mike Sal?

[Mike Sal]

I had planned to do this but haven't gotten around to pulling the head off yet.  I know Bob spent a lot of time talking about this potential hazard our bikes have, but later, when he wasn't around, the other old timers told me this was a pretty rare failure mode.  At this point I'm on the fence.....it's kinda like if it aint broke, why fix it....
Mike Sal

[pd]

If you might be concerned , you could pull the piston and check the ring positions and compare them to the positions you installed them in .

[Ricks]

If you might be concerned , you could pull the piston and check the ring positions and compare them to the positions you installed them in .

I have not had any problems.  Just like Mike said, Bob thought it was a big problem that should be addressed. Since I am building another motor, I thought I might give it a go.

I cleaned up the port through the crankshaft, and put a nice polished finish on it. I also did a cleanup of the cylinder ports.  Just looking to gain any free advantage in the way of power.  I may call Bob and see if I can get one of his rods, and I think he makes some that are slightly longer than stock to boost compression a little.

[Mike Sal]

Rick,
If you manage to get a rod from Bob, try to get 2 & I'll buy one from you.  When I talked to Bob, he sorta acted like he wasn't too motivated to machine any more of them.

I've wondered if a regular crowned pistion along with a longer rod would run ok in these motors.  Those contoured pistons are sorta old technology.
Mike Sal

[Ricks]

Bob told me he has to have the crank to fit the rod to the needle bearing. Why?  I have no idea.  Looking at the pictures he sent me, it looks simple.

I don't think a crown piston would clear the intake and exhaust ports. 

What I would like to see, is Wayne make some stroker cranks, with maybe another millimeter of stroke.

[kartjockey]

Here's a pic of the piston that is going back in my Sears 250.  It's a 2.5 mm wide ring and the pin is about that. This would be nearly impossible to do unless you had a mill with an end mill to drill half on the ring groove and half on the ring land. But you could drill right in the center of the groove and use a small tension pin. I have seen this method before. You would want to check the ring end gap then use a jewelers round file to file the notch in the end of the ring to go around the pin. This way you still maintain the end gap required for good compression. I would try to place the ring gaps where they don't have to pass over a port in the cylinder if possible.  So as they are less apt to catch the edge of a port.  It might be wise to do some practice runs in a scrap piece of like metal to ensure you get the fit of the pin correct before starting in on your piston.  So get some extra pins.

I have heard of guys running slightly longer rods to push the piston higher. And also about guys who have stacked gaskets to try to alter the port timing. I have never done it but sounds like a great deal of trial and error.  One thing I have learned and enjoyed with one cylinder engines is that you can see the dividend of your changes more easily. Just one piston working for you!   In terms of 4 stroke flatheads breathing is more important, whereas, I think compression is more important in a 2 stroke.