Engine -- Piston & rings

[Mike Sal]

While at the Portland meet, I spent some time talking to Bob Kerr, who has 20 plus years experience with the Simplex motor.  One of the things he told me about was a weakness in the model M engines in that the rings are not pinned (to prevent them from rotating around the piston).  The aluminum cylinders have larger ports and if the ring ends rotate around where they can get caught in one of the ports, it will damage the ring and create a groove in the cylinder wall.  He recommended modifying the pistons by drilling small holes in the grooves and adding short small pins, then grinding notches in the rings to clear the pins.  (a lot of modern 2 stroke engines are made this way).

Since I've not had my motor apart, I don't know what it's true condition is (although it has great compression).  This week I plan to pull the jug off and check things out, and make this modification to the piston.  I'll try to capture the project with some pic's & will post them.

I was curious if any of the rest of you have this modification on your machines?

[kartjockey]

I didn't pin my rings when I did my motor. As a safety measure I guess why wouldn't you. It seems like in later 2 strokes there are more port holes than there is cylinder so the pinning must have kept the end gaps where they wouldn't leave the cylinder wall and fall prey to one of the ports. Through the years I have had discussions about if rings turn that much on the piston while running and our conclusions were that they don't.  But, in concerns of the rare engines we are running,  I am all for it.  By the way Mike, I did see you at the show but I had just arrived and saw you and Rick loading your truck but didn't realize it was you until later after you had left.  Your bike looks great!

[Ricks]

I would like to try the pinining operation on my next project, and I would also like to try one of Wayne's billet crankshafts with a Bob Kerr needle bearing rod conversion.  I think with a little port work and the better engine balance provided with the larger counterweight of Wayne's crank, along will the reliability of the needle bearing rod, we could see almost 6 whole horsepower!!!

I wonder how much power a tillotson mt carb could handle?

Man, I'm getting excited just dreaming this up.

[kartjockey]

Being around the later engines and hearing them run I think Simplex made great leaps when they changed cylinder and block designs. The later ones seem to hit better and have more of the 2 cycle buzz than the earlier ones. I think I can hear the difference from one to the other pretty easily.

[Mike Sal]

Tom, sorry we missed you.  Rick & I had walked over to your tent & bike, but you were already out snooping around the grounds.  If it weren't for starting my new job the next day, I would have stayed thru Wednesday (they had a trainer in from germany & it was my only opportunity to get the training).  I was a little concerned about getting the bike home safely with that big rusty fan in the truck, but with Rick's help & strapping material, everything worked out.

I used to have a used bike / repair shop on the side years ago & worked on just about everything (antiques up to the mid-seventies).  Over the years I've seen rings that worked their way around the bore and I've seen rings that never moved.  Bob said the later cylinders had larger diameter ports & that's why they are more suceptable to catching a ring.  As you said, the motors are so scarce, I think it worth the effort to add an extra layer of protection.

I'm still on the fence about upgrading the rod.  Listening to Bob, it put the fear in me about throwing a rod & ruining the crankcase, but then later, Jim had the opinion that a thrown rod is not that common (especially in a well maintained motor).

I'd like to see one of Wayne's cranks to see the difference from a stock one.  I wonder if his new cranks are "oversized" in the port area to compensate for worn crankcases?
Mike Sal

[Ricks]

Mike, speaking with Wayne, I don't think the cranks are oversize, because how would he know how much wear on each case?  If your case is worn, it can be bored and sleeved back to stock.  Tom did this on his case, I believe he told me.
Wayne has added more counterweight to his cranks, much as Simplex did throughout the years  to their cranks for better balance.

[Ricks]

OK guys, here are some pictures Bob Kerr sent me.  He no longer makes pistons, but the pictures show how the pinning is done.  He does still make the needle bearing rods though.  He does require that you send him your crank to fit the rod.  When the conversion is done, you can still use either his needle bearing rod, or an original style rod.

[kartjockey]

Is this Wayne's supplier? I really like the rod. Nices fillets. The rod Wayne had on his table was ok but seemed to have edges that were not smooth and alot of 'square corners' which may be a place to invite a crack or offer itself a chance for failure.

[Ricks]

No, Bob does not supply Wayne with anything that I am aware of.  What do you use for a pin on the piston? Is it an allen set screw?

[kartjockey]

The ones I have seen are just a straight pin about 1/16" in diameter and pressed into the piston.  A piece of gas welding rod or TIG rod might be nice as it is nice and true material. A person could do this very easily and notch your own rings with aid of a Dremel tool.  The pin material would not have to be hardened. It would just mean finding something that works and som trial and error tests in some 'like' piston material to get the fit to be right.  And using the right drillbit.

[Ricks]

How about a small roll pin?  Do you think this would stay put?

[kartjockey]

I have seen roll pins.  Lots of pistons I have seen had a solid pin but seems like years ago I saw more roll pins. Could have just been the manufacturer of the pistons I was involved with at the time.