can you identify this piston??? polaris 94 750 says 78a

[polaris94]

jug says ec75pw, piston top says 78a. inside has a logo " s wrapped in a capital c" numbers 334 and 03/01. i guess the last part is the date.

appox. piston size is 71.8mms

let me know if you think it's standard, or oversize, or what i should look for.

want to buy another one like it and not make another mistake like i already did. thanks in advance.

[beerdart]

Welcome to the Hulk..

The jug is a 750 bored out the piston 78a is a 780 piston.

[polaris94]

can you tell if it's standard or oversize,appox. 71.8 across. or will the piston have a mark if it's oversized.
thanks again.

[xlint89]

Yep, Beer is correct.

750 cyls bored out for 780 piston.


750 = 69.72mm

780= 71.3mm

You 750 was bored out 1.5mm oversized.


Any cylinder damage done? You can go as far up as 72mm oversized.

The piston will be stamped .25, .5, .75 etc... if it were oversized.

Since your dome is shot, you'll have to go by bore size.

[polaris94]

I just scrached the carbon off with a razor blade, it's a STD. Marked not in relief but stamped in. I guess i'm gonna buy a standard one 71.3 mm, just a little concerned that the cylinder bore measures 73.46.

Thanks for the help and if anyone needs a new polaris STD 750 WSM piston 69.75 PM me. 50 bucks shipped.

[thester71]

I just scrached the carbon off with a razor blade, it's a STD. Marked not in relief but stamped in. I guess i'm gonna buy a standard one 71.3 mm, just a little concerned that the cylinder bore measures 73.46.

Thanks for the help and if anyone needs a new polaris STD 750 WSM piston 69.75 PM me. 50 bucks shipped.

What are you measuring the bore with? Your cylinder should be slightly larger than your piston however I'm not positive off hand but I believe it is in the neighborhood of .005", but don't take my word for it, look it up in the service manual!
At 2.16 mm your already a touch over .008"

[K447]

... At 2.16 mm your already a touch over .008"

Difference between 73.46 cylinder and 71.3 mm piston means clearance of 2.16mm = 0.085 inches ==> 85 thousands!

Way too much clearance

[thester71]

Difference between 73.46 cylinder and 71.3 mm piston means clearance of 2.16mm = 0.085 inches ==> 85 thousands!

Way too much clearance

Umm... Yeah that's what I meant! Lol

[xlint89]

Yeah, what he said ^^^^^.

Check your bore size again, and is it damaged???

If so, you'll want to go with a larger piston still.