Cheetah cracked skirt diagnose

[alex2218]

Hi guys,

Instead of a greatly running engine (see here viewtopic.php?f=1&t=14448&start=30" onclick="window.open(this.href);return false; )

I dismounted the engine to change the gearbox and.....can't believe what i found



Right piston is cracked on the skirt (top and left side) with something like a start of seizure. Pistons are brand new vertex ones, only one track day and some tests.
3% Motul 800V premix
Never overheat (max 70°)
Never ran lean
Clearance was 0.07mm

All the black piston treatment has gone and it seems there is lot of friction on the exhaust side piston face.
Honestly i'm thinking about a bad clearance but this engine is supposed to run 0.08 clearance as far as i know.
Engine was running very fine, starting at first kick.
I've done only one track day but it was cold, the engine never passed 62°, is it possible that it ran too cold ?

Cheetah owners, your help will be greatly appreciated
Cheers

Edit :

These are cast vertex pistons, it seems they dont need the same clearance as Wiseco forged pistons designed for this engine...

[Zombie]

These are cast vertex pistons, it seems they dont need the same clearance as Wiseco forged pistons designed for this engine...

This is why I always run forged Wisecos.
Cast pistons are "softer" and more prone to failure.
Get the proper forged pistons and enjoy your new engine!

[alex2218]

That's what i thought

Also, Roger from wicked told me it's just crazy to run cast pistons into this engine...
I use them into all my race engine with no issue, i really would understand why they are prone to problems into that one.

Maybe the clearance, expansion and forces are specific
It seems there is SPECIAL pistons designed by CPI for that kit but Wiseco blaster pistons are the standard for it

[MK]

Btw: What's the bore ?
I have a set of Woessners in 66.5 mm laying around....

[alex2218]

Don't have my notes here but clearance was 0.07mm for sure.
Pistons were 67.955mm and bore 68.02mm if i remember well

According to people, cast pistons, even vertex quality, can't handle these cylinders.

I have never seen anything like this, maybe cast pistons don't need the same clearance as forged ones ?

I'm in contact with Roger and will go for Wiseco again

[Bare]

IF by some unimaginable reason ..that engine was running Cast pistons in a bore sized for Forged..
You are V lucky that's All that happened.
PS; IMO 3% oil is simply missplaced Frugality.
More oil !

[evan_calgary]

What's the deal with the cylinders not being able to handle cast pistons? Have never heard of that issue before. Aren't almost all stock pistons cast versus forged?

PS, don't tell Rory about your oil remarks!

[T.RexRacing]

Cast pistons and not enough oil. A pre-staged disaster. Do not repeat.

[alex2218]

I'm using cast Vertex in both race bike RGV and RS 250, tolerancy mint after 10 track days and like new... that's why i'm so confused

They are just not adapted to this cylinder bore and specially so much revs and power

[T.RexRacing]

I'd venture the (lack of) piston support on the intake side of the bore is the culprit. I see evidence of hard use on a cold engine too. As a rule of thumb I run engines (no matter type) for 10 solid minutes under load before they see any real cylinder pressure.

[alex2218]

I have to admit there is no much support for pistons on the intake side, i'm going to make 2 holes on the skirt on intake side next time.
Off course i have consciousness, i never run hard a cold engine but on the only track day i have done, the engine temp was only 50° on the morning even with some tape on the radia.... could be an issue

Don't you wanted to say "without any load" instead of "under load" ?

[T.RexRacing]

Nope......under a light load,i.e. riding w/o getting into the throttle hard.Lower RPM partial throttle with occasional short bursts into the power as it warms up. It's difficult to heat the internals w/o some type of load which will only happen under cylinder pressure. I'm not referring to just the pistons I mean crank,rods,gears,ect. getting up to operating temperatures.

Don't take this wrong I'm not criticizing you just my observations of the photos.

[kenny]

Cast pistons on 0.08mm is just rattle city. Way to much clearance for cast ones thats on the outer edge for forged pistons. Piston slap was the cause of your fracture.

[(F5)]

Calvin had said they developed the Wiseco pistons to match CPI barrels after quite a bit of trial and error to get the shape right. But yeah too much clearance will snap of piston skirts with monotonous regularity.

[alex2218]

Thank you all for confirmation

Wiseco are in the way from Roger