12:1 piston question

[p26575r16]

Anyone running a 12:1 wiseco piston in their 350x? I rebuilt my engine last winter and the first time I took it out I had almost no compression. Well, that was last march. I have not looked at it since (got very busy over the summer and had another atv). I just pulled off the head and noticed that the piston appeared to have contacted the head in one little place. I am certain the compression loss was from a bad valve job, so I am going to re-do that tommoro. My question is has anyone had to modify your head/piston or is this just a fluke. My head had not been milled and neither has the jug, and I am running stock gaskets. Thanks

[Dirtcrasher]

Depends on the piston, When your running that high of a compression, clearances get real tight. And, I would imagine that the timing and or play in the cam chain and tensioner would need to be spot on or something may touch. Then there is the valve adjustment too that could have been too tight. It will all need to be checked really well.

Are you running an agressive cam?? The more agressive the cam is, the tighter all those clearances are also..

Thats why I'll probably never go above 10.5:1... I don't want to deal with race gas. But then again, if it is THAT much of a gain? I may consider it. I would just have to really take my time and make sure everything was spot on!!

Truth is I make a big deal to myself about "race gas" but regular is almost 3$ a gallon so whats the difference other than keeping a jug around for that fuel only?

QUESTION - Are there really HUGE gains with race gas and 12:1?? Maybe 11:1 would be a better choice with a WEBCAM?? Now my wheels are turning, LOL!!

I keep editing this.... How would I check interference in the head?? I guess I'd use an old gasket and a .005 thousandths brass shim on top of the piston and see whats up? I'm willing to bet that things get real close in the 4 stroke cycle. I don't know how it is truly checked....

[p26575r16]

The timing was spot on, no play in the chain. The piston and valves never contacted, the piston hit the side of the head in one little spot, did no damage to the head or piston, just scuffed it a little.

[McDerry]

I keep editing this.... How would I check interference in the head?? I guess I'd use an old gasket and a .005 thousandths brass shim on top of the piston and see whats up? I'm willing to bet that things get real close in the 4 stroke cycle. I don't know how it is truly checked....

Modeling clay (play dough).

Oil the cylinder and head up well.

You put a ball of modeling clay in the cylinder then bolt the head on etc.


With no spark plug in the head hole you turn the motor over by hand.

Assuming you didnt find a sudden stopping point turning it by hand, you can take the head off and see the actual thin spots where the clearance is tight in the clay as its peen pressed into the domed combustion chamber of the head.

[bigred1]

is a 12:1 and cam job rebuild for a 200 autox to much or is a 10:1 more resonable

[Dirtcrasher]

is a 12:1 and cam job rebuild for a 200 autox to much or is a 10:1 more resonable

All personal preferences. 12:1 needs race gas and your suspension isn't really matched for a high compression piston. 10.5:1 uses super unleaded and it would be a sleeper

[84honda200s]

i use a 10.25:1 wisco in my auto-x and it hauls ass for a 200. its got allot of other mods too. but anyway 12:1 is ok except having to hunt down race fuel and they are a bit harder to wing over. id go 10.50:1 or 10.25:1. just my opinion though.

[Dirtcrasher]

May I ask how much race gas costs? I blow threw 5 gallons of super unleaded a week at 3$ a gallon....

[NOS_350X]

You should be able to find 110 for around 35-40 for 5 gallons. I run c12 and thats $50 for 5 gal. I havent had any clearance issues with my BIG cam and 13:1 compression. Av gas is good because its cheap but jetting Av gas is a a pain, it doset lay onto a plug very well and requires HUGE jets.

Liquid at 30 hours im putting in a new piston and rings. Thats NOT when your rings are setting.