Swapping pistons (2-Stroke)

[Scotty595]

What do I need to look for when looking at suitable replacement pistons? It is an MVX250 so bloody hard to find any pistons specifically for them now. Other piston which I have identified/heard may fit is a CR80 piston.

CR80: Bore x stroke: 47.0 x 47.8 mm
MVX: Bore x stroke: 47.0 x 48 mm

I assume the 0.2mm stroke diffence won't do much?
What else do I need to look for to figure out if it is going to be a suitable replacement?

[pete376403]

gudgeon pin diameter, gudgeon height (ie bore to top of piston)

Where the piston ring pegs are in relation to the ports - you don't want the ring gaps lining up with the ports.

[Scotty595]

Cheers,

Whats the word on mixing different over size pistons?

eg 0.25mm OS for the front pistons and 0.5mm OS for the rear?

[ac3_snow]

TKRJ piston catalogue is great for measurements on some bikes. Unfortunately I just checked and couldn't see either the mvx or cr80.

However Wiseco catalogue does give you gudgeon pin size, could be a start at least. (a brief look only revealed the various cr80 years, no mvx)

[Crasherfromwayback]

Cheers,

Whats the word on mixing different over size pistons?

eg 0.25mm OS for the front pistons and 0.5mm OS for the rear?

I wouldn't purely because of the (very small admittidly) weight difference between them.

[Scotty595]

TKRJ piston catalogue is great for measurements on some bikes. Unfortunately I just checked and couldn't see either the mvx or cr80.

However Wiseco catalogue does give you gudgeon pin size, could be a start at least. (a brief look only revealed the various cr80 years, no mvx)

Thanks for that. So it looks like the CR80 is a 14mm Gudgeon. The MVX I believe is a 12mm for the front and a 14 or 18 for the rear.

So rear may be okay, how about the front though? Would it be possible to use a 12mm Gudgeon and get some sleeves machined for the piston? Appologies for the noobish questions, still learning

[Grumph]

Thanks for that. So it looks like the CR80 is a 14mm Gudgeon. The MVX I believe is a 12mm for the front and a 14 or 18 for the rear.

So rear may be okay, how about the front though? Would it be possible to use a 12mm Gudgeon and get some sleeves machined for the piston? Appologies for the noobish questions, still learning

Are you sure about the different pin sizes ? Even for Honda that's unusual...

It is possible to bush a piston to suit a smaller gudgeon pin - thousands of Villliers pistons were like that -and the bush only came out rarely....but always at the wrong moment. These days I'd only use bushes to get something very rare running for demos.
Mixing oversizes shouldn't be a problem if it's the only way you can find pistons - balance on any V formation is usually a compromise anyway.

[Scotty595]

Are you sure about the different pin sizes ? Even for Honda that's unusual...

It is possible to bush a piston to suit a smaller gudgeon pin - thousands of Villliers pistons were like that -and the bush only came out rarely....but always at the wrong moment. These days I'd only use bushes to get something very rare running for demos.
Mixing oversizes shouldn't be a problem if it's the only way you can find pistons - balance on any V formation is usually a compromise anyway.

Yup, definately different pin sizes. Because it's a V3 apparently Honda tried to balance it out somewhat by having larger parts in the rear. Okay, may look at getting a CR piston to have a play around with,

[pete376403]

Could you have the crank rebuilt with "rear" rods for the front cylinders? (to get all rods using 14mm pins)

This page http://www.tanshanomi.com/honda-mvx250f sys the rear pin is solid, standard CR pins would be hollow so you'd have a bunch of balancing to do

[Scotty595]

Could you have the crank rebuilt with "rear" rods for the front cylinders? (to get all rods using 14mm pins)

This page http://www.tanshanomi.com/honda-mvx250f sys the rear pin is solid, standard CR pins would be hollow so you'd have a bunch of balancing to do

Yup, big solid pins.
pic to show:


I'm sure the crank could be rebuild with rear rods, but then wouldn't that take the point of having a larger pin in the rear (for weight balance) away?

[Crasherfromwayback]

Mixing oversizes shouldn't be a problem if it's the only way you can find pistons - balance on any V formation is usually a compromise anyway.

I reckon Honda have tried hard to balance things as best as poss. I'd be tempted to watch the weight differences being used!

[Scotty595]

I reckon Honda have tried hard to balance things as best as poss. I'd be tempted to watch the weight differences being used!

In saying that though, I believe that honda released the 18mm pin later, making it heavier to try and improve the reliability/balance of the engine so if mine has a 14mm pin, then potentially using a bigger OS piston in the rear my be beneficial?

[Crasherfromwayback]

In saying that though, I believe that honda released the 18mm pin later, making it heavier to try and improve the reliability/balance of the engine so if mine has a 14mm pin, then potentially using a bigger OS piston in the rear my be beneficial?

That makes sense for sure then!

[imdying]

Could be worth the $300 odd dollars to have it all balanced as a unit?

[pete376403]

Yup, big solid pins.
pic to show:
I'm sure the crank could be rebuild with rear rods, but then wouldn't that take the point of having a larger pin in the rear (for weight balance) away?

What I meant was, if you used rear rods for the front cylinders, you could then put the CR pistons in all three. Use the solid pin in the rear cylinder, use the hollow CR pins (much lighter) in the front cylinders.

Would still take a shitload of balancing, but no-one ever said restoring old bikes was cheap, or easy.