The mistake some make with the 57 cams is not providing enough compression. IIRC, the 57 like about 10.2 static. Which in a 98" configuration would need a dome piston in lieu of flat top.

 

Below are the calculations for Andrews 26 (186.1 PSI) and Andrews 57 (183.3 PSI).


Andrews 26 MLS 0.040"


Andrews 57 MLS 0.040"

If I use a 0.030" MLS instead, Big Boyz calculates the compression to 188.0.
Maybe I will go with the MLS 0.030".

 

Couple of things to consider. How sure are you the chamber volume is 85cc? Have you meausred? 86cc is not uncommon in the improved '06 heads. What pistons give you the 1.2cc dome: just curious. For sure run the .030" head gasket but give some thought to shaving .030" off your heads;

 

Ok, will go for the 0.030"

Chambers are measured. I did that three years ago. Front 84,8 CC and rear 85,2 CC. So I am pretty sure. I measured 192 PSI front and 194 rear last year with the 26. It was higher than calculated. I think it was because of the carbon build up, both on the pistons and in the chambers.

I will do the clay test a soon as I have put the heads together. I ended up with Manley 0.600", which will arrive from Germany soon.. They are about 5-10 % stronger than S&S 0.585", not as strong as Kibblewhite. I have replaced the exhaust valves. The stock did not have a good surface against the seat. Looked like carbon and metal melted together.

 

Sorry, forgot to answer about the pistons.The pistons are JE. The heads were shaved 0.2-0.3 mm (0.001”) three years ago. Thanks for you input.

 

Correction: 0,25 mm is 0.01”.

 

I agree with @djl ...A minimum of 10.0:1 for the 38* ivc.. I would try to return the pistons you have and go with a +3.0 cc piston.. You will notice an increase in power because of the cubic inch increase but will not really satisfy the cam of choice..

 

Thanks for the answer. These pistons are already installed since three years and I will not replace them. It was an old picture of the box, just to show what pistons I have. Thanks for the tip, anyway.

What I can do, as far as I can see, is to go for the 0.030” MLS and maybe I can see if someone can shave a little on the heads. (But I will make the clay test first to see if I can do that.) Anyway, asking Chat TGP what a 0.01 shave will do to 85 CC, it says 83.2 CC.

And when I put this in the Big Boyz it actually says 10.0:1 static, which will result in 192.3 psi with the 57 cam.

 

To get to 10:1 compression you will need to remove .020" off the cylinder head.. By what i was told .006" = 1cc but the graduation changes because of the taper in the combustion chamber..

Edited.. technically you should be able to go 30,000 without a problem and still have plenty of valve to piston clearance.. which will yield 10.2:1.. The sweet spot for that cam..

 

Oki! Sounds good. I will think about this.