Cometic Head Gaskets on Modded LQ9 and The 50RA requirement
06-22-2021, 04:48 PM
#1
Cometic Head Gaskets on Modded LQ9 and The 50RA requirement Hello,
I made some changes to a project I am working on and now I am trying to decide which head gaskets are going to best suit my needs since the project goals have changed as well. I am working on a stock bottom end LQ9 (6.0/Flat Tops/Floating Rods). I have a set of Ported 799s w/ BTR .660 springs that I am repurposing from another project I was working on. I installed a Texas Speed Torquer V4 Cam in the block. The bottom end is untouched, in good condition, with relatively low miles. I used the traditional razor blade and brake parts cleaner to prep the surface of the block. The heads were milled although I can't seem to remember how much. My PTV clearance is .105 intake and .131 exhaust. I am looking to achieve as high of a compression is possible in regard to my components and am looking to rev this thing to 7000rpm. I used the .51 LQ9 MLS gaskets to measure the PTV. It looks like I can get away with Cometic C5477-040 (.40 Compressed thickness?) gaskets. I honestly can't decide the next move. I know I can get a set of LQ9 MLS gaskets for like $60 (I have a set of LS9s but the bore seems to big) and they will seal with minimal effort. Cometic recommends a 50RA surface finish which I honestly have no idea WTF that is or how to achieve it. The car is not getting any forced induction, maybe a 100 shot of spray at most. What do you think I should do? How critical is the surface finish? Are there better options out there? I haven't built a higher compression motor yet so I generally just grab the GM MLS stuff and it works great.
Thanks!
I made some changes to a project I am working on and now I am trying to decide which head gaskets are going to best suit my needs since the project goals have changed as well. I am working on a stock bottom end LQ9 (6.0/Flat Tops/Floating Rods). I have a set of Ported 799s w/ BTR .660 springs that I am repurposing from another project I was working on. I installed a Texas Speed Torquer V4 Cam in the block. The bottom end is untouched, in good condition, with relatively low miles. I used the traditional razor blade and brake parts cleaner to prep the surface of the block. The heads were milled although I can't seem to remember how much. My PTV clearance is .105 intake and .131 exhaust. I am looking to achieve as high of a compression is possible in regard to my components and am looking to rev this thing to 7000rpm. I used the .51 LQ9 MLS gaskets to measure the PTV. It looks like I can get away with Cometic C5477-040 (.40 Compressed thickness?) gaskets. I honestly can't decide the next move. I know I can get a set of LQ9 MLS gaskets for like $60 (I have a set of LS9s but the bore seems to big) and they will seal with minimal effort. Cometic recommends a 50RA surface finish which I honestly have no idea WTF that is or how to achieve it. The car is not getting any forced induction, maybe a 100 shot of spray at most. What do you think I should do? How critical is the surface finish? Are there better options out there? I haven't built a higher compression motor yet so I generally just grab the GM MLS stuff and it works great.
Thanks!
06-23-2021, 11:35 PM
#2
50RA is a fairly fine finish; not polished by any means, but not all gnarly and rough, either. Basically if you can feel it AT ALL with your fingernail, it's rougher than 50RA.
https://www.felpro.com/technical/tec...ce-finish.html
https://www.destinytool.com/surface-finish.html
The surface finish is CRITICAL for any kind of steel-surface gaskets. Just like the old steel-shim ones on SBCs that always used to leak, the reason was always either the block or heads too rough, or the castings bent or distorted for any of a number of reasons. (over-torquing, overheating, typical sloppy factory machine work, etc.)
The various spray coatings like the old CopperKote can take up a certain amount of that, but it's hard to predict how much.
It's impossible to guess how much you'd have to take off of whatever you have, to get to a 50RA finish. If the heads were absolutely flat but just with a poor finish, like if they got machined with a dull cutter, it might only take .002" or maybe even less to clean them up. OTOH if they're just random heads found in the wild with random scratches on them, straightness unknown, it's impossible to guess how much you might have to knock off to get them both FLAT and SMOOTH.
You don't want them TOO smooth though, because then the metal may not "grip" the gasket material well enough. If your gasket spec says 50RA or better, you probably don't want it too much below 40RA for example.
Bottom line: DO IT RIGHT. Don't half-*** it.
https://www.felpro.com/technical/tec...ce-finish.html
https://www.destinytool.com/surface-finish.html
The surface finish is CRITICAL for any kind of steel-surface gaskets. Just like the old steel-shim ones on SBCs that always used to leak, the reason was always either the block or heads too rough, or the castings bent or distorted for any of a number of reasons. (over-torquing, overheating, typical sloppy factory machine work, etc.)
The various spray coatings like the old CopperKote can take up a certain amount of that, but it's hard to predict how much.
It's impossible to guess how much you'd have to take off of whatever you have, to get to a 50RA finish. If the heads were absolutely flat but just with a poor finish, like if they got machined with a dull cutter, it might only take .002" or maybe even less to clean them up. OTOH if they're just random heads found in the wild with random scratches on them, straightness unknown, it's impossible to guess how much you might have to knock off to get them both FLAT and SMOOTH.
You don't want them TOO smooth though, because then the metal may not "grip" the gasket material well enough. If your gasket spec says 50RA or better, you probably don't want it too much below 40RA for example.
Bottom line: DO IT RIGHT. Don't half-*** it.
06-25-2021, 05:38 PM
#3
I’m in the same situation as OP. I went ahead and put cometic gaskets dry on seasoned LS1 block
that I cleaned with razor blade and installed freshly decked heads.
Just looking at these bare metal gaskets, all I think of is coolant leaks. All that for .006 tighter quench.
I wish cometics had composite material or rubber inserts around coolant passages to prevent leaks.
Didn’t start the engine yet, if it leaks, I will put oem .051 gaskets.
that I cleaned with razor blade and installed freshly decked heads.
Just looking at these bare metal gaskets, all I think of is coolant leaks. All that for .006 tighter quench.
I wish cometics had composite material or rubber inserts around coolant passages to prevent leaks.
Didn’t start the engine yet, if it leaks, I will put oem .051 gaskets.
06-25-2021, 08:37 PM
#5
TECH Senior Member
06-25-2021, 08:44 PM
#6
I installed .045 cometic gaskets.
Is there a difference between WS6store .045 gaskets and .045 cometic gaskets?
Edit:
WS6store gaskets are composite .045 compressed thickness for use with notched 862 heads.
Is there a difference between WS6store .045 gaskets and .045 cometic gaskets?
Edit:
WS6store gaskets are composite .045 compressed thickness for use with notched 862 heads.
Last edited by cino; 06-25-2021 at 08:52 PM.
