Piston and Ring Part Numbering question

borkap · Aug 22, 2017

I picked up a Magnum 330 basket case that had been partially taken apart. I flip bikes pretty often but this is my first Polaris. It has the famous polaris "Purple Camo" so I am keeping this one for my daughter. I haven't gotten the engine apart yet but the rod is seized to the crank so looks like it was probably ran low on oil. In the process of its death, the cylinder took a bit of a beating, but it's not unsalvageable. The piston got pretty scarred up so I will replace that and i figure it will need to go up a size.

Anyway when I went to look up parts for it I came across The following that looked like what I want.

Set, Piston, .010 Inch/.25MM O/S [Incl. 4] - PN: 3090236

Set, Piston, Ring, .010 Inch/.25MM O/S PN: 3087245

I am assuming that the .25MM Over Stock piston requires the .25mm over stock Rings, but I wanted to make sure the .25mm O/S rings were not for a stock piston.

I will measure the crank once I get it out and cleaned up but I was thinking of using the .025 or .5 O/S Rod bearings as I am not in love with the idea of buying a new crank.

I've built motors but, again this is the first Polaris, so I am open to any and all suggestions/advice. I tend to stick with OEM hard parts and I don't mind using a generic gasket set. If there are better options, lets hear em.

borkap · Aug 24, 2017

Top of the pile!

aknawful · Aug 24, 2017

Os means over sized (over bored). So yes os .25 piston, that's millimeters or mm, needs .25 os rings.

If the stock bore is good just go with stock size.

If you buy os piston/ rings you will need a machine shop to overboard the cylinder to match the new set.

borkap · Aug 25, 2017

I measured it and its still in spec for stock pistons and rings. Thanks.

stave7119 · Aug 25, 2017

Best way I found to get the parts is a Namura top end kit. Includes the piston, rings, wristpin and circlips along with all the gaskets for the top end of the engine. You will need to get the crankshaft seals separately but aftermarket is a lot cheaper than Polaris. Polaris does not offer a rebuild gasket set. You have to get each gasket separately. $ adds up fast. The last one I got was around $120 for the kit and another $20 for the crankshaft seals. I got everything through the dealership where I work. Either Western Powersports or Parts Unlimited. The only way to get a new rod is Polaris though. No one offers an aftermarket piece that I have found.

Be prepared though. I have done a couple rebuilds on 325 and 330 engines. Same issue with them you are talking about. They share the same crankshaft and case halves but the rods are different. The undersize bearings you can get are just enough to polish the crank. If you have any scoring or material transfer, you will probably have to replace it. I have had good luck with used pieces from Ebay. A lot cheaper than OEM.

Good luck with your project!

Piston and Ring Part Numbering question

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