I recently baught a GSR (B18C1) head from a friend. He says its been milled but couldn't remember how much. I took it to three different local machine shops and got three different answers. One shop said its been milled .025, another said .030, and the other said .040-.045.

Taking matters in my own hands I looked up oem engine specs via "AERA" and found two head thickness measurements. They read:


-New Thickness : 5.589-5.593"
and
-Minimum Thickness: 5.581"


The "thickness" of my head reads 5.547"

So my question is: What oem measurement do I use to find the difference? New thickness or minimum thickness?

 

the third shop is correct.. you use the new thickness value..

new thickness avg. 5.591"
your head thickness 5.547"

5.591"
5.547"
-______
=0.044"

 

Thats what I figured..... Thanks for the response!

 

Congratulations on taking things into your own hands. . . most would have went with the first shop and called it a day. Potentially bending some valves down the road thinking the head was only shaved .025"

 

damn .045" is alot! better get the right gasket like a .050".

 

there is no one who can tell you exactly how much it has been milled, i would say .035-.045 though. Unless you have HUGE pistons and/or HUGE valves and cams ,you should be fine with a stock gasket

 

Thanks for all the feedback!

I've got a new problem. I've been doing some compression calculating via "zealautowerks.com" and found my compression to be over 14:1 (considering the head milled .045 and using oem gasket). I'm aiming for 11.5.

Basic setup overview:

GSR head (milled .045) w/ flat top valves
Ls block (sleeved 85mm)
Ls stroke
Supertech 11.5 pistons (1.4cc)

No big deal right? "just run a super thick head gasket".

Well idealy, I'd want around a .100 head gasket for my 11.5:1 goal. But when I spoke to Cometic they replied they didn't offer anything between .080 and .120.

-With a .080 the compression yields around 12.3 (little too high)
-With a .120 the compression yields around 10.9 (little too low)

Another option I'm highly considering is using the .120 gasket and milling my head another .010-.015 (bringing the total mill to a whopping .060).

So, what do you guys think? All opinions are appreciated.

 

I dont see how you have 14.1:1 cr. i mean assuming your head now sits at a 41cc pocket, (which it may or may not be, get it cc'd to check exactly), and even with a 0 deck piston-deck measurement, i show 12.41:1cr. what numbers did you run for: a) piston-deck; b)head cc; c) headgasket thickness ? i used a) 0; b) 41cc; c) .029 (stock)

 

Try changing the bore size to 85mm. Then I added .4 due to the flat valves*.

* When I contacted Steve @ Omnipowerusa to find out how much flat valves bump compression, he replied "the smaller B16a will get about .3 and the larger B20 will see about .4".

Of course, thats just a rule of thumb...

 

well, with a stock gsr head and 0 deck piston. it would be 12.4:1 with the head cc's at 40 (which there is no way it is there), it is 12.83
Even with a 36cc head (again, no way in hell) you are at 14.05:1. I think you are going to be right around 12.8ish. get a .040" gasket and that will drop you to a 12.01:1 with stock head, and 12.41:1 with a 40cc head and 13.54:1 with a 36 head.

 

Thanks for your research RPRacing!

As for your results, it doesn't look like your changing the bore size to 85mm. The bigger bore seems to have a huge effect on the compression (bumping it way up).

Here's the numbers I punch in the compression calculator:


Bore: 85mm
Stroke: 89
Deck: 211.84

Head cc: 41.6
Piston dome cc: 1.4 /(30mm)

Rod: 137mm
Gasket: .026 (oem)
Head milled: .040


COMPRESSION RESULTS: 13.69 : 1

* then I add .4 for the flat valves. "Steve@Omni's rule of thumb"


ESTIMATED TOTAL (after valves) : 14.09 : 1


Please review my calculation and tell me what I did wrong. I want to make sure I build this motor right the first time. Thanks!

 

well, i dont understand how that calculator can account for the head beingg milled without knowing the exact shape of the chamber. i mean .040" x pi x((.5 x bore)squared) may be what they are using (basic formula for volume of a cylinder)
and i dont think it mattters about the rod length unless they also ask you about compression height. so i dont know what other calculations it is doing. I cannot dispute Steve's claim on the .4 for flat face, but Where does that number come from? i would ask him if its .4 ccs off of the chamber volume or added to the total cr. I could seee the difference in the 16 flat valves vs the stock bowls being .4cc and therefore affecting the cr;but again, i find it hard to believe there is a universal number that can be added to every setup to a final cr number. I think you need to take your head to get cc'd to find the exact volume and eliminate all this guess work

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RPRacing &raquo;</TD></TR><TR><TD CLASS="quote">well, i dont understand how that calculator can account for the head beingg milled without knowing the exact shape of the chamber. i mean .040" x pi x((.5 x bore)squared) may be what they are using (basic formula for volume of a cylinder)
and i dont think it mattters about the rod length unless they also ask you about compression height. so i dont know what other calculations it is doing. I cannot dispute Steve's claim on the .4 for flat face, but Where does that number come from? i would ask him if its .4 ccs off of the chamber volume or added to the total cr. I could seee the difference in the 16 flat valves vs the stock bowls being .4cc and therefore affecting the cr;but again, i find it hard to believe there is a universal number that can be added to every setup to a final cr number. I think you need to take your head to get cc'd to find the exact volume and eliminate all this guess work</TD></TR></TABLE> You are to smart for your own good. If the head has had a valve job or 2 it loses compression. CC the head.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote"> You are to smart for your own good. If the head has had a valve job or 2 it loses compression. CC the head.</TD></TR></TABLE>

x2 ( it is a good thing though )

 

I totaly agree with eliminating the guesse work and getting the head CC'ed. When I enter the correct cc of the head into the compression calculator, I don't have to acount for the head being milled, right? Just leave that part 0.00?

 

RPRacing, you were right on the money!! I got the head CC'ed and the results were 39.00 cc. With the new results plugged in the calculator, my calculations read 12.65 w/ an oem gasket (.026) and 12.13 w/ a .040 gasket. Just what you had estimated. You really are too smart for your own good!

But what if I run a .060 gasket to bring my compression down to an 11.46? My plans are to eventually CC the head again with the head gasket, but I only want to buy the gasket once, you know what I mean? How does that sound?

 

bump

 

why not run a slightly lower compression piston to make up the difference? use the pistons to make the difference in CR.


Marcus

 

That would've probably been the best solution but I already have the pistons in the block. Each cylinder is specifically bored to each piston already. Thanks though.