Metal head gasket RA specs

[drostar]

I'm putting a metal head gasket in my engine. I'd like to get an HKS head gasket (bead or stopper) but I cannot find anywhere what it officially requires for surface smoothness (ie RA). Does anyone know what RA HKS says they require? Or know where to find such specs?

I will get it as smooth as possible, but I want to know if I can order an HKS or should I just go with the Cometic gasket.

 

[suprarich]

35 is a rule of thumb with hks

 

[drostar]

I've also seen number like 9, 20, 50 thrown around. I'm hoping to get the actual specs that it would say in the manual or some kind of official hks documentation.

 

[drostar]

My machine shop has gotten me a surface finish of 20 RA. I'll be strapping it down with ARP head studs. Does everyone think I can safely use an HKS MHG with this setup?

 

[rumptis]

35 is a rule of thumb with hks

Did you miss this?

 

[IJ.]

50 is Cometic

Not many places will have the gear needed to measure RA so you're getting their "best guess" below 50 anyway.

I have my shop cut the very best/smoothest they can then Lap it using diamond paste.

 

[mkIIIman089]

^Thats pretty much how it goes at work. If a part has a surface finish requirement on sheet (LOTS do), basically just make sure it feels nice and smooth and doesn't get any smears of burs on it...

That being said, with a new insert in whatever the cutting tool is, it will make steel almost seem like glass.

 

[bowsercake]

depending on what shop you go to you can get an RA sub 20.

 

[suprarich]

Most "shops" will confuse "cuts per inch" for RA. Most shops that mill heads do not even know what RA is. The shop I use has one of the best machines money can buy and will do a head to Catapiller spec RA65 in one pass. Looks like a rainbow when it is done. But this is still a cutter wheel, and will not produce a RA needed for a MHG on our motor. Still needs to be hand lapped.

Ian - I am still old school and use crisco and graded emery flour, cut with gasoline.

 

[IJ.]

Rich: Try the diamond it's amazing

 

[drostar]

I just thought of something. RA in micrometers and RA in microinches is different. I wonder which one my machine shop was measuring.......

What would the spec for head gasket be in?

edit: microinch, right?

 

[doom26464]

I just went with cometic its good for 50 RA and it will hold pretty tight for a large amount of power.