Modern Vespa

Hey all,

I have a stock p2 cylinder honed to the first oversized piston measurement (66.7mm). Just wondering what the piston ring gap is for this situation?

Also----any recommendation on squish band measurement?

Thanks for your input.

NM

I'd shoot for 0,40mm gap

If you're shooting for a squish band in a stock P2, it could be somewhere around 3.5~4mm! Half that would be better. But realize more compression=more heat. So adjust timing and jet accordingly.

Is there a min and max range for ring gap?

I had a 0.4mm ring gap and had a hard time sliding the cylinder over the rings. Maybe I need to file them down a bit more to more of the mid range of the allowable tolerances.

Let me know.
Just a reminder:

P200 cylinder bore to first oversized piston.

Thank you.

N

A gap is a gap. Making it slightly larger won't affect the friction between the rings and the cylinder. It needs to be there to allow the rings to expand a bit when they heat up. Too large and there would be a loss of compression through the gap. Oil them up good before inserting.

Also make sure that the rings are on the piston right way. Some pistons have wedge profile rings that do not go in their grooves all the way if fitted upside down.

If there is a gap, it won't affect how easily they slide into the bore.

Meaning that the rings could possible be upside down thus resulting in a hard time sliding them and the piston into the cylinder?

Exactly.

You should be able to compress the rings flush with piston with your bare fingers. If the rings won't go all the way in their groove, they are likely wrong way around.

Wouldn't the locating pins prevent the rings from fitting upside down ???

Depends on piston type. Some have the peg right in the center of the groove which does not prevent this.

Some on the other hand have it 50% above the groove which would indeed prevent this like here:
https://www.scooterhelp.com/tips/engine/piston.install.lf.vespa.html

My whole situation is this:

I purchased and oversized piston with appropriate rings from scootermercato. I had the cylinder honed/bored to the appropriate measurement based on the provided piston. I gapped the rings to 0.4mm.

Installed the piston with rings on the crank. Went to slide the cylinder over the rings and piston and no dice.
No movement. Cant get passed the first ring.

Took the piston off the crank. Re measured the rings and checked the gap. 0.4mm. Flipped the cylinder over (top down on bench) and tried to slide rings on piston through the bottom of the cylinder. No movement.

Side note: the piston slides through the cylinder fine without the rings.

Question: would the rings be too large?? It appears to be. Even thou they are the appropriate rings for the oversized piston.

Frustrating to say the least.
Add insult to injury——(and I can only laugh now) I snapped a ring when taking it off the piston. Great times.

Suggestions? I'm thinking of taking the piston single ring and the cylinder back to the machine shop so they can reassess and gap the rings for me to see if I'm missing something.

Thoughts?

If you can fit a ring down into the bore and square it up with the piston and measure a .4 mm gap, then it fits. If you can't get it to fit when it's on the piston then it may be your technique ?
There's a slight bevel at the bottom of the cylinder to ramp the ring into place. Sometimes it takes a but of wiggling or finger pressure to coax it in but if it is really just jamming on the edge , maybe you don't have the gap positioned properly located at the pin in the groove. You have to be careful that you don't rotate the ring in the groove and move the gap away from the pin.

I'm hoping it's technique. I was at it for a bit. Eventually it should have made its way. Once I get the new rings I'll re-gap and evaluate.

Thanks

Now that you have a snapped one, you can have a go with single ring to see if you can get that in.

Can you describe how did you measure the 0,4mm ring gap?

I did that. Lol.

Got the one in but couldn't get the piston in any further.

I basically put the ring and put the bottom of the piston on it to push it down. From there I take the feeler gauge and measure.