Rotax 670 Melted Piston Why?

S

StrokedChicken

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Dec 8, 2007

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• Dec 9, 2007

• #1

WOT Melted my piston on my 95 Formula ss 670 but why?

Looks to me like a over rev but not real sure.
Sled was running around 9000rpms in a 500 yard run made it half way. What should this sled be running at stock rpms at WOT?

I will have pics up later

 

L

longhairfreak

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• Dec 9, 2007

• #2

its called detonation

 

S

StrokedChicken

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• Dec 9, 2007

• #3

Very true but what should this sled be running at WOT

 

B

bin sledin

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crabrook b.c

• Dec 9, 2007

• #4

9000rpm may be part of the problem

 

C

crazysledhead

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• Dec 9, 2007

• #5

StrokedChicken said:

WOT Melted my piston on my 95 Formula ss 670 but why?

Looks to me like a over rev but not real sure.
Sled was running around 9000rpms in a 500 yard run made it half way. What should this sled be running at stock rpms at WOT?

I will have pics up later

Click to expand...

Max rpm should be in the 7800-8000 range

 

S

StrokedChicken

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• Dec 9, 2007

• #6

well just took the jugs off found to be .20 over already.

Should i punch them .30 over with oem pistons or punch out .40 over with T-moly pistons with a mild ported head? Is the gain going to be worth the price or just go the oe route?

 

winter brew

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• Dec 9, 2007

• #7

I give up...what's a ported head?

 

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StrokedChicken

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• Dec 9, 2007

• #8

sorry how bout polished

 

H

H20SKE

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• Dec 9, 2007

• #9

StrokedChicken said:

sorry how bout polished

Click to expand...

Probabley polished because it had eaten a piston before.

That type of rpm--you are looking for a time bomb--way to high--even for a 670. It was just a matter of time at that rpm.

H20SKE...

 

snow_knight

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• Dec 9, 2007

• #10

WHY DID THE PISTON MELT?

Because you ran the motor in an rpm range that doesnt work for 670 motors... you might be better off to punch out and sleeve.. IMO

 

S

Sheepdog

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Big Sky Country

• Dec 9, 2007

• #11

If you can get away with another 10 over just do that. New pistons and re-"polish" your head and your good to go.

 

T

TravRMK

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• Dec 10, 2007

• #12

BABBITT said:

If you can get away with another 10 over just do that. New pistons and re-"polish" your head and your good to go.

Click to expand...

Yes, and change your clutching to get those revs in check.

 

B

bin sledin

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crabrook b.c

• Jan 13, 2008

• #13

clean your jug with muratic acid in sleeve carfully aluminum will fall off easily hone lightly and replace with same size piston and rings wash cylinder with soap and water as for your head shave .10-.15 thousand and you will have a rippin 670 sounds like you might want to take a look at your clutching this is a cheap easy fix that will have you runing in no time provided your jug isnt totally hooped

 

2

2zeros

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• Jan 13, 2008

• #14

I'm almost positive that all the 670's have iron liners. Hone it to the next size, or buy a new liner for it. I have a 96' 670 and my fathers 98' are iron. -00's

 

Last edited: Jan 13, 2008

D

DWHalverson

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Oct 19, 2003

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Rapid City, South Dakota

• Jan 13, 2008

• #15

9000 rpm out of 670s is not unheard of in "race" form, but in stock trim it's pointless. They tend to have a shorter life span at that rpm regardless. I doubt the rpm had anything to do with "melting" the piston. That engine should be makeing power at 7800 rpm in stock trim. The 670s are cast iron sleeved so do not use Muratic acid. Check for cylinder roundness and then bore to get the cylinders round and then use the required pistons. Have the head cleaned up if it can be done cheaper than buying a different used head. First thing is to find out what took it down to begin with.

 

catssuk

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• Jan 14, 2008

• #16

my modded 95 670 moter was dynoed up 2 10500 rpm,made the biggest hp # [162] at 9500, when i spray[60hp shot] i run at 10300-10500 all the time , never had a problem ,but his moter never stays W O T more than 15-20 secs,and i run it like this all the time. its a great moter.

 

catssuk

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Edgerton Wi.

• Jan 14, 2008

• #17

im guessing bad gas ,detonation. pull carbs off drain the float bowls. look for crap, water etc, also look at your oil inj line to the melted side could have got pluged, came off ,cracked[ i removed mine], cracked carb boot, intake.