cmb91rs dish piston vs flat top piston head clearence

[joseph puglia]

if you have say a 15 thou head clearence on a 91rs with a dish piston and you change piston and liner to a flat top piston, do you use a different head clearence as you are changing the combustion chamber volume or use a different head button? or use a larger head clearence to account for this?

 

[Joe Warren]

Joseph I was not aware that there was a flat top piston for a RS91 But If there is? a .002 shim needs to be added to the stack if you are changing from a dish piston to a flat top piston. The area only measures approx .1 if you are checking for total cc`s

 

[joseph puglia]

yeah the latest piston for a 91rs is a flat top piston (no dish like the old one) dont know if there is a different head button for it but i only have the old one. so you are suggesting to run about 17 thou head clearence?

 

[Joe Warren]

If you have already established a good head clearence just add .002 for the flat piston. Your Orginal squish clearence will depend on your bowl size.

 

[steve wood]

If you have already established a good head clearence just add .002 for the flat piston. Your Orginal squish clearence will depend on your bowl size.

i would not increase the clearance.set it to .014 to .015 and add bowl volume

i think all cmb's will become flat piston. the latest 67 is i know

 

[dwilfong]

If you have a way of cutting a 1.01 button to fit or can have some one do it for you it will work real nice at .012. Also a cut on the squash band at 1DEG all the way across with no flat will help.

This set up will burn all the nitro you want.

David

 

[Terry Keeley]

Hey, I thought I needed the dish to go fast and cut one in all my motors.

Oh well... :wacko:

 

[Rick Doane]

Dish or not makes no dfference. Volume is everything. Figure it out!

 

[julianconde]

LOL'

 

[Terry Keeley]

Can I fill them with JB weld?

Will it hold?

 

[Rick Doane]

H yes big T! JB weld is good for everything!

 

[joseph puglia]

what total cc's should i have for the 91rs?

 

[Rick Doane]

Try 0.89-0.91 cc for head button only.

 

[dwilfong]

Had a guy show up at the pond with a red head OPS 80 in a sprint cat he just got. was trying to run it with OD 65%. It would only run for a short time then die. opened up the Eng and there where a stack of shims in it. Pulled all the shims out and it was still at .021 clearance. Gave it one more try and it would run but not very well. So I filled him up with some 40% and it lit right up like a beast. cat walking and doing back flips at will.

Head clearance too high with high nitro dose not work.

tight head clearance burns nitro. alway adjust the volume to correct CR. not the head clearance.

This leads us to what is the correct compression ratio.Never seams to be a good answer for this question. 8, 8.5,9,9.5,10,10.5 and all the numbers in between. still trying to figure this one out. just every time you change you need to make a new button to make a big change. the clearance will only change it so much and with the big eng it will burn up the piston if you are wrong. leads to some very expensive testing.

How much pressure will the pipe charge the cylinder with? this will change the whole equation.

David