Busted My Piston. Motron/Cimatti V1

So this happened...

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I pulled the cylinder, head, and piston off my Motron today, because it is a 39.4 mm overbore cylinder and piston and I wanted to put it on my Cimatti Town Bike. Why? In short, because my Cimatti has a 38.0 mm bore, and will only do 23-25 mph, and sucks salami up hills, but it is important to me to keep the bike completely stock. It was recommended to me by MopedLar that I could improve my power by increasing displacement slightly by putting on a 39.4 mm overbore cylinder and piston on. Then I can reduce gearing a bit, if I still need more grunt.

Back to my piston. Why'd it bust? I didn't even know it was broken until I took it apart. Still ran fine. The missing chunk is long gone, out the exhaust. No marks on the cylinder or head. This was a brand new piston and rings with ~100 miles on it; haven't even gone thru a tank of break in fuel. I rebuilt the top end not long ago because it broke a ring which destroyed the piston. It broke right at the ring gap pin, soo this is twice in a row this has happened, so something caused it. I'm guessing I had the piston in upside down or the edges of the ports/transfers was too sharp. They were pretty sharp, so I chamfered them nicely and lightly honed the cylinder. There was no arrow on the piston. I'm not 100% sure which way I had it installed, but by looking at the marks on the piston I'm pretty sure the ring pins were in the bottom half of the cylinder, straddling the exhaust port. Which way is correct? I sure don't want to screw it up... Again.

Re: Busted My Piston. Motron/Cimatti V1

Moped Lar (OFMC) /

Seth, The ring pins should be oriented toward the exhaust port.

Re: Busted My Piston. Motron/Cimatti V1

Thanks. That's what I thought I remembered reading. I had it installed correctly then. Must have been snagging a ring on the edge of a port then. Hopefully I won't have this issue again now that they're chamfered.

Re: Busted My Piston. Motron/Cimatti V1

Did you check your ring gal?

Re: Busted My Piston. Motron/Cimatti V1

Yes, I did gap the rings when I installed it.

Re: Busted My Piston. Motron/Cimatti V1

That's a chunk! I don't know if that went out your exhaust port lol , that's pretty big

Re: Busted My Piston. Motron/Cimatti V1

Treats has a new piston, but no rings for it. I'm gonna try to find a Dykes ring for this piston and run it. I think it'll be fine.

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This is the piston that was on the bike (Motron) when I got it, but broke a ring shortly afterward. I replaced it with a treats piston. I spent some time with needle files cleaning it up, and all the damage is minor, and in non-crucial areas, with the exception of the small missing chunk just above the bottom ring, but I think it will run fine. It's not a high power bike, and the top L ring will take most of the compression anyway.

Fuggit, runnit.

Re: Busted My Piston. Motron/Cimatti V1

> Rocco Taco Wrote:

> -------------------------------------------------------

> That's a chunk! I don't know if that went out your exhaust port lol ,

> that's pretty big

I know, right! It went somewhere, and it didn't beat the shit outta the head or the top of the piston. Had to go out the exhaust.

Re: Busted My Piston. Motron/Cimatti V1

It will run with a single ring . Might have a bit less compression , but ...

Just in case you can't find a top ring . ;)

Re: Busted My Piston. Motron/Cimatti V1

> P D Wrote:

> -------------------------------------------------------

> It will run with a single ring . Might have a bit less compression , but

> ...

>

> Just in case you can't find a top ring . ;)

Yeah, I know it. As a last resort I may have to. I'm waiting on a reply from Shaun at Myron's. He may have a ring or two laying around, and Jak at Treats is being awesome and looking around too.

Re: Busted My Piston. Motron/Cimatti V1

No dice on the top L ring. Put it together with just the bottom ring and checked compression. Only about 75 psi. Not great, but so what. Runnit. I fired it up on the bench. It runs. Kinda boggy and unresponsive, but it's over jetted with a 57. I'll put a 52-53 in it and see what happens. Gonna have to wait a few weeks for new rings for the new piston from treats.

Re: Busted My Piston. Motron/Cimatti V1

Did you check with Dos Cycles? Didn't they end up with the Handy Bike inventory? If so, BJ always had any piston or rings I needed.

Re: Busted My Piston. Motron/Cimatti V1

> Don Ohio Wrote:

> -------------------------------------------------------

> Did you check with Dos Cycles? Didn't they end up with the Handy Bike

> inventory? If so, BJ always had any piston or rings I needed.

Yes I did. Blake said he has a 39.2, but no 39.4. I’ll just wait for treats rings

Re: Busted My Piston. Motron/Cimatti V1

Could always just slap a polini kit on there with everything else stock. Its good for about 5-10mph increase, depending on how slow it was before and much more torque.

Re: Busted My Piston. Motron/Cimatti V1

> Daniel '' Wrote:

> -------------------------------------------------------

> Could always just slap a polini kit on there with everything else stock.

> Its good for about 5-10mph increase, depending on how slow it was before

> and much more torque.

I'm probably gonna do that to the Motron I took the 39.4 off of, since it now tops out at 25 with the smaller cylinder. If I can't get the Cimatti to climb my hill any better than 10 mph, it'll have to get a kit too. Cimatti has a crank seal leaking, I just discovered, and that's part of my problem. But any V1 kits are gonna have to wait a bit; I'm dumping a fortune into my Free Spirits now.

Re: Busted My Piston. Motron/Cimatti V1

By the way, Shaun @ Myrons has a NOS Minarelli 3rd oversize piston and rings, and a NOS Minarelli 39.4 replacement piston with the top Dykes ring. Last ones available; meaning no replacement rings available. I passed on them, due to unavailability of replacement rings, but if someone wants original...

Re: Busted My Piston. Motron/Cimatti V1

Finally got this put back together. Running much better now. 125 psi compression, and much more responsive. Hopefully I don't have any more problems breaking Pistons.

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